U.S. patent application number 10/631742 was filed with the patent office on 2004-02-12 for finder optical system and image pickup apparatus using the same.
This patent application is currently assigned to Olympus Optical Co., Ltd.. Invention is credited to Kamo, Yuji.
Application Number | 20040027475 10/631742 |
Document ID | / |
Family ID | 17238733 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040027475 |
Kind Code |
A1 |
Kamo, Yuji |
February 12, 2004 |
Finder optical system and image pickup apparatus using the same
Abstract
A high-performance real-image finder optical system reduced in
size, particularly in thickness, includes a positive objective
optical system, an image-inverting optical system for erecting an
intermediate image formed by the objective optical system, and a
positive ocular optical system. The objective optical system has at
least two movable units moving when zooming is performed. A prism
is placed on the object side of the intermediate image. At least
one reflecting surface of the prism has a rotationally asymmetric
surface configuration. At least one reflecting surface of the
image-inverting optical system is formed from a roof surface. The
finder optical system satisfies the following condition:
1.0<d/(f.sub.W.multidot.tan .theta..sub.W.multidot.Z)<2.5
where d is the distance from the entrance surface of the objective
optical system to the first reflecting surface of the
image-inverting optical system; f.sub.W is the focal length of the
objective optical system at the wide-angle end; .theta..sub.w is
the maximum field angle of the objective optical system at the
wide-angle end; and Z is a zoom ratio.
Inventors: |
Kamo, Yuji; (Tokyo,
JP) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
Olympus Optical Co., Ltd.
Tokyo
JP
|
Family ID: |
17238733 |
Appl. No.: |
10/631742 |
Filed: |
August 1, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10631742 |
Aug 1, 2003 |
|
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09657517 |
Sep 7, 2000 |
|
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|
6643062 |
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Current U.S.
Class: |
348/335 |
Current CPC
Class: |
G02B 17/086 20130101;
G02B 23/145 20130101; G02B 17/0816 20130101; G02B 17/0848 20130101;
G02B 17/045 20130101; G02B 13/22 20130101; G02B 17/0832 20130101;
G02B 23/14 20130101 |
Class at
Publication: |
348/335 |
International
Class: |
H04N 005/225 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 7, 1999 |
JP |
11-252535 |
Claims
What we claim is:
1. A finder optical system comprising, in order from an object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including at least one reflecting surface having
a rotationally asymmetric surface configuration; said
image-inverting optical system having at least one reflecting
surface formed from a roof surface; said finder optical system
satisfying the following condition: 1.0<d/(f.sub.W.multidot.tan
.theta..sub.w.sup..multidot.Z)<2.5 (1) where d is a distance
from an entrance surface of said objective optical system to a
first reflecting surface of said image-inverting optical system;
f.sub.W is a focal length of said objective optical system at a
wide-angle end; .theta..sub.w is a maximum field angle of said
objective optical system at the wide-angle end; and Z is a zoom
ratio.
2. A finder optical system according to claim 1, wherein said roof
surface is placed on a pupil side of said intermediate image.
3. A finder optical system comprising, in order from an object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including at least one reflecting surface having
a rotationally asymmetric surface configuration; said
image-inverting optical system including a Porro prism; said finder
optical system satisfying the following conditions:
1.0<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<2.5 (3)
0.5<dp/(f.sub.W.multidot.tan .theta.w)<1.1 (4) where d is a
distance from an entrance surface of said objective optical system
to a first reflecting surface of said image-inverting optical
system; f.sub.W is a focal length of said objective optical system
at a wide-angle end; .theta..sub.w is a maximum field angle of said
objective optical system at the wide-angle end; Z is a zoom ratio;
and dp is a distance from an entrance surface of said
image-inverting optical system placed on the object side of said
intermediate image to said first reflecting surface.
4. A finder optical system according to claim 3, wherein a second
transmitting surface of said objective optical system has a
power.
5. A finder optical system according to claim 4, wherein said
second transmitting surface has a rotationally asymmetric surface
configuration.
6. A finder optical system according to claim 3, wherein a first
reflecting surface of said objective optical system has a
power.
7. A finder optical system according to claim 1 or 3, wherein the
number of reflections in said prism is two or three.
8. A finder optical system according to claim 1 or 3, wherein the
number of said movable units is two.
9. A finder optical system comprising, in order from an object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system includes: an optical system having at
least two movable units moving when zooming is performed, said
optical system having a positive composite power; and a prism
placed on a pupil side of said optical system, said prism including
an image-inverting function, and said prism including at least one
reflecting surface having a rotationally asymmetric surface
configuration; wherein at least either one of a first transmitting
surface and first reflecting surface of said prism has a negative
power, and a second transmitting surface of said prism has a
positive power.
10. A finder optical system according to claim 9, wherein either
one of the first transmitting surface and the first reflecting
surface has a positive power.
11. A finder optical system according to claim 9, wherein the
number of reflections in said prism is two or three.
12. A finder optical system according to claim 9, wherein a
composite focal length of said at least two movable units satisfies
the following condition: 0.3<f.sub.move/f.sub.W<0.9 (7) where
f.sub.move is the composite focal length of said at least two
movable units at a wide-angle end, and f.sub.W is a focal length of
said objective optical system at the wide-angle end.
13. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism having two reflecting surfaces, wherein at least
one of said reflecting surfaces has a rotationally asymmetric
surface configuration; said finder optical system satisfying the
following condition: 0.5<dp/(f.sub.W.multidot.tan
.theta..sub.w)<1.1 (9) where f.sub.W is a focal length of said
objective optical system at a wide-angle end; .theta..sub.w is a
maximum field angle of said objective optical system at the
wide-angle end; and dp is a distance from an entrance surface of
said image-inverting optical system placed on the object side of
the intermediate image to a first reflecting surface thereof.
14. A finder optical system according to claim 13, wherein a first
reflecting surface of said prism has a rotationally asymmetric
surface configuration and is formed from an independent surface
that is separate from other transmitting and reflecting
surfaces.
15. A finder optical system according to claim 13, wherein a second
transmitting surface of said prism has a power.
16. A finder optical system according to claim 15, wherein the
second transmitting surface has a rotationally asymmetric surface
configuration.
17. A finder optical system according to claim 13, wherein an axial
principal ray or a projective axial principal ray defined by
projecting the axial principal ray onto a plane containing a part
of the axial principal ray does not cross itself in said prism.
18. A finder optical system according to claim 13, wherein either
one of a first transmitting surface and first reflecting surface of
said prism has a negative power.
19. A finder optical system according to claim 13, wherein a
composite focal length of said at least two movable units satisfies
the following condition: 0.3<f.sub.move/f.sub.W<0.39 (11)
where f.sub.move is the composite focal length of said at least two
movable units at a wide-angle end, and f.sub.W is a focal length of
said objective optical system at the wide-angle end.
20. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism having three reflecting surfaces, wherein at
least one of said reflecting surfaces has a rotationally asymmetric
surface configuration.
21. A finder optical system according to claim 20, wherein at least
two of the reflecting surfaces of said prism are formed from
independent surfaces, respectively, which are separate from other
transmitting and reflecting surfaces.
22. A finder optical system according to claim 20, wherein a first
reflecting surface of said prism has a rotationally asymmetric
surface configuration and is formed from an independent surface
that is separate from other transmitting and reflecting
surfaces.
23. A finder optical system according to claim 20, wherein a third
reflecting surface of said prism has a rotationally asymmetric
surface configuration and is formed from an independent surface
that is separate from other transmitting and reflecting
surfaces.
24. A finder optical system according to claim 20, wherein a second
transmitting surface of said prism has a power.
25. A finder optical system according to claim 24, wherein the
second transmitting surface has a rotationally asymmetric surface
configuration.
26. A finder optical system according to claim 20, wherein an axial
principal ray or a projective axial principal ray defined by
projecting the axial principal ray onto a plane containing a part
of the axial principal ray does not cross itself in said prism.
27. A finder optical system according to claim 20, wherein either
one of a first transmitting surface and first reflecting surface of
said prism has a negative power.
28. A finder optical system according to claim 20, wherein a
composite focal length of said at least two movable units satisfies
the following condition: 0.3<f.sub.move/f.sub.W<0.9 (13)
where f.sub.move is the composite focal length of said at least two
movable units at a wide-angle end, and f.sub.W is a focal length of
said objective optical system at the wide-angle end.
29. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system includes, in order from an object
side thereof: a negative first unit; a positive second unit; and a
negative third unit; wherein at least the first unit and the second
unit are movable units moving when zooming is performed, and the
third unit is formed from a prism including an image-inverting
function, said prism including at least one reflecting surface
having a rotationally asymmetric surface configuration.
30. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system includes, in order from an object
side thereof: a negative first unit; a positive second unit; and a
positive third unit; wherein at least the first unit and the second
unit are movable units moving when zooming is performed, and the
third unit is formed from a prism including an image-inverting
function, said prism including at least one reflecting surface
having a rotationally asymmetric surface configuration, and said
prism including at least one transmitting surface or reflecting
surface that has a negative power, said third unit having a
principal point positioned on a pupil side of a plane where the
intermediate image is formed.
31. A finder optical system according to claim 29 or 30, wherein a
second transmitting surface of said prism has a power.
32. A finder optical system according to claim 31, wherein said
second transmitting surface has a rotationally asymmetric surface
configuration.
33. A finder optical system according to claim 29 or 30, wherein
the number of reflections in said prism is two or three.
34. A finder optical system according to claim 29 or 30, wherein a
composite focal length of said movable units satisfies the
following condition: 0.3<f.sub.move/f.sub.W<0.9 (15) where
f.sub.move is the composite focal length of said movable units at a
wide-angle end, and f.sub.W is a focal length of said objective
optical system at the wide-angle end.
35. A finder optical system according to claim 29 or 30, wherein
when zooming is performed, said third unit is stationary with
respect to a plane where the intermediate image is formed.
36. A finder optical system according to claim 29 or 30, wherein
both said movable units are formed from refracting lenses.
37. A finder optical system according to claim 29 or 30, wherein
said movable units are each formed from a single refracting
lens.
38. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including two reflecting surfaces, at least one
of the reflecting surfaces having a rotationally asymmetric surface
configuration; wherein a first transmitting surface and second
reflecting surface of said prism are formed from an identical
surface having both transmitting and reflecting actions, and said
prism has an optical path in which an axial principal ray or a
projective axial principal ray defined by projecting the axial
principal ray onto a plane containing a part of the axial principal
ray bends in different directions from each other with respect to a
travel direction of light rays at the two reflecting surfaces.
39. A finder optical system according to claim 38, wherein a first
reflecting surface of said prism has a negative power.
40. A finder optical system according to claim 38, wherein a
reflection angle at a first reflecting surface of said prism
satisfies the following condition:
15.degree.<.theta..sub.A1<40.degree. (17) where
.theta..sub.A1 is the reflection angle at the first reflecting
surface.
41. A finder optical system according to claim 38, wherein an exit
angle of said prism with respect to an optical axis entering said
objective optical system satisfies the following condition:
25.degree.<.theta..s- ub.A<75.degree. (19) where
.theta..sub.A is the exit angle of said prism with respect to the
optical axis entering said objective optical system.
42. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including two reflecting surfaces, both of said
reflecting surfaces being independent of other transmitting and
reflecting surfaces, at least one of said reflecting surfaces
having a rotationally asymmetric surface configuration, said prism
having an optical path in which an axial principal ray or a
projective axial principal ray defined by projecting the axial
principal ray onto a plane containing a part of the axial principal
ray bends in a same direction with respect to a travel direction of
light rays at said two reflecting surfaces; said finder optical
system satisfying the following condition:
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (21) where dp
is a distance from an entrance surface of said image-inverting
optical system placed on the object side of the intermediate image
to a first reflecting surface thereof; f.sub.W is a focal length of
said objective optical system at a wide-angle end; .theta..sub.w is
a maximum field angle of said objective optical system at the
wide-angle end.
43. A finder optical system according to claim 42, wherein a first
transmitting surface of said prism has a negative power.
44. A finder optical system according to claim 42, wherein a second
transmitting surface of said prism has a positive power.
45. A finder optical system according to claim 42, wherein a
reflection angle at a first reflecting surface of said prism
satisfies the following condition:
30.degree.<.theta..sub.B1<60.degree. (23) where
.theta..sub.B1 is the reflection angle at the first reflecting
surface.
46. A finder optical system according to claim 42, wherein a
reflection angle at a second reflecting surface of said prism
satisfies the following condition:
30.degree.<.theta..sub.B2<60.degree. (25) where
.theta..sub.B2 is the reflection angle at the second reflecting
surface.
47. A finder optical system according to claim 42, wherein an exit
angle of said prism with respect to an optical axis entering said
objective optical system satisfies the following condition:
150.degree.<.phi..su- b.B<210.degree. (27) where .phi..sub.B
is the exit angle of said prism with respect to the optical axis
entering said objective optical system.
48. A finder optical system according to claim 42, wherein a roof
surface is disposed on a pupil side of the intermediate image.
49. A finder optical system according to claim 42, wherein said
image-inverting optical system is a Porro prism.
50. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including three reflecting surfaces, all of said
reflecting surfaces being independent of other transmitting and
reflecting surfaces, at least one of said reflecting surfaces
having a rotationally asymmetric surface configuration, said prism
having an optical path in which an axial principal ray or a
projective axial principal ray defined by projecting the axial
principal ray onto a plane containing a part of the axial principal
ray bends in a same direction with respect to a travel direction of
light rays at two consecutive reflecting surfaces and bends in a
direction different from said same direction at the other
reflecting surface.
51. A finder optical system according to claim 50, wherein a first
transmitting surface of said prism has a negative power.
52. A finder optical system according to claim 50, wherein a
reflection angle at a first reflecting surface of said prism
satisfies the following condition:
30.degree.<.theta..sub.C1<60 .degree. (29) where
.theta..sub.C1 is the reflection angle at the first reflecting
surface.
53. A finder optical system according to claim 50, wherein a
reflection angle at a third reflecting surface of said prism
satisfies the following condition:
30.degree.<.theta..sub.C3<60.degree. (31) where
.theta..sub.C3 is the reflection angle at the third reflecting
surface.
54. A finder optical system according to claim 50, wherein an exit
angle of said prism with respect to an optical axis entering said
objective optical system satisfies the following condition:
70.degree.<.phi..sub- .C<110.degree. (33) where .phi..sub.C
is the exit angle of said prism with respect to the optical axis
entering said objective optical system.
55. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including three reflecting surfaces, at least
one of said reflecting surfaces having a rotationally asymmetric
surface configuration; wherein a first reflecting surface and third
reflecting surface of said prism are independent of other
transmitting and reflecting surfaces, and a second reflecting
surface of said prism is formed from an identical surface with a
second transmitting surface of said prism; said prism having an
optical path in which an axial principal ray or a projective axial
principal ray defined by projecting the axial principal ray onto a
plane containing a part of the axial principal ray bends in a same
direction with respect to a travel direction of light rays at two
consecutive reflecting surfaces and bends in a direction different
from said same direction at the other reflecting surface.
56. A finder optical system according to claim 55, wherein a first
transmitting surface of said prism has a negative power.
57. A finder optical system according to claim 55, wherein a
reflection angle at the first reflecting surface of said prism
satisfies the following condition:
20.degree.<.theta..sub.D1<60.degree. (35) where
.theta..sub.D1 is the reflection angle at the first reflecting
surface.
58. A finder optical system according to claim 55, wherein a
reflection angle at the third reflecting surface of said prism
satisfies the following condition:
10.degree.<.theta..sub.D3<50.degree. (37) where
.theta..sub.D3 is the reflection angle at the third reflecting
surface.
59. A finder optical system according to claim 55, wherein an exit
angle of said prism with respect to an optical axis entering said
objective optical system satisfies the following condition:
20.degree.<.phi..sub- .D<60.degree. (39) where .phi..sub.D is
the exit angle of said prism with respect to the optical axis
entering said objective optical system.
60. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including three reflecting surfaces, at least
one of said reflecting surfaces having a rotationally asymmetric
surface configuration; wherein a first reflecting surface and third
reflecting surface of said prism are independent of other
transmitting and reflecting surfaces, and a second reflecting
surface of said prism is formed from an identical surface with
first and second transmitting surfaces of said prism; said prism
having an optical path in which an axial principal ray or a
projective axial principal ray defined by projecting the axial
principal ray onto a plane containing a part of the axial principal
ray bends at each of the three reflecting surfaces in a direction
different from a direction of bending at a preceding reflecting
surface with respect to a travel direction of light rays.
61. A finder optical system according to claim 60, wherein the
first reflecting surface of said prism has a negative power.
62. A finder optical system according to claim 60, wherein a
reflection angle at the first reflecting surface of said prism
satisfies the following condition:
15.degree.<.theta..sub.E1<45.degree. (41) where
.theta..sub.E1 is the reflection angle at the first reflecting
surface.
63. A finder optical system according to claim 60, wherein a
reflection angle at the third reflecting surface of said prism
satisfies the following condition:
15.degree.<.theta..sub.E3<45.degree. (43) where
.theta..sub.E3 is the reflection angle at the third reflecting
surface.
64. A finder optical system according to claim 60, wherein an exit
angle of said prism with respect to an optical axis entering said
objective optical system satisfies the following condition:
160.degree.<.phi..su- b.E<200.degree. (45) where .phi..sub.E
is the exit angle of said prism with respect to the optical axis
entering said objective optical system.
65. A finder optical system comprising, in order from an object
side thereof: an objective optical system having a positive
refracting power; an image-inverting optical system for erecting an
intermediate image formed by said objective optical system; and an
ocular optical system having a positive refracting power; wherein
said objective optical system has: at least two movable units
moving when zooming is performed; and a prism placed on an object
side of the intermediate image formed by said objective optical
system, said prism including three reflecting surfaces, at least
one of said reflecting surfaces having a rotationally asymmetric
surface configuration, said three reflecting surfaces being all
independent of other transmitting and reflecting surfaces, said
prism having an optical path in which an axial principal ray or a
projective axial principal ray defined by projecting the axial
principal ray onto a plane containing a part of the axial principal
ray bends in a same direction with respect to a travel direction of
light rays at a first reflecting surface and a second reflecting
surface and is twisted by a third reflecting surface.
66. A finder optical system according to claim 65, wherein a
reflection angle at the first reflecting surface of said prism
satisfies the following condition:
30.degree.<.theta..sub.F1<60.degree. (47) where
.theta..sub.F1 is the reflection angle at the first reflecting
surface.
67. A finder optical system according to claim 65, wherein a
reflection angle at the third reflecting surface of said prism
satisfies the following condition:
30.degree.<.theta..sub.F3<60.degree. (49) where
.theta..sub.F3 is the reflection angle at the third reflecting
surface.
68. A finder optical system according to claim 65, wherein an exit
angle of said prism with respect to an optical axis entering said
objective optical system satisfies the following condition:
70.degree.<.phi..sub- .F<110.degree. (51) where .phi..sub.F
is the exit angle of said prism with respect to the optical axis
entering said objective optical system.
69. A finder optical system according to any one of claims 1, 3, 9,
13, 20, 38, 42, 50, 55, 60 and 65, wherein said prism is placed
closer to a pupil than any of said movable units.
70. An image pickup apparatus comprising: a photographic optical
system having an optical path for photography; and a finder optical
system having an optical path for a finder; wherein said finder
optical system according to any one of claims 1, 3, 9, 13, 20, 38,
42, 50, 55, 60 and 65 is used as said finder optical system.
Description
[0001] This application claims benefit of Japanese Application No.
Hei 11-252535 filed in Japan on Sep. 7, 1999, the contents of which
are incorporated by this reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a finder optical system
having an image-inverting optical system. More particularly, the
present invention relates to a finder optical system for use in a
camera, a video camera, etc. which uses an image-inverting optical
system whereby an inverted image of an object formed by an
objective optical system is observed as an erect image. The present
invention also relates to an image pickup apparatus using the
finder optical system.
[0003] A finder optical system used in a compact camera or the like
is constructed separately from a photographic optical system and
generally placed above the photographic optical system. Among
finder optical systems of this type, real-image finders are well
known, in which a real image formed as a first image by an
objective optical system is inverted by an image-inverting optical
system and observed as an erect image through an ocular optical
system.
[0004] Recently, it has been demanded that compact cameras should
be further reduced in size, particularly in thickness, i.e. size in
the direction of the optical axis. In a zoom lens, zooming is
performed by varying the spacing between lens units. Therefore, if
the zoom lens is arranged to attain a higher zoom ratio, the zoom
lens basically becomes large in size in the direction of the
entering optical axis. In the case of taking lenses, therefore,
schemes have been devised for the lens mount structure to attain a
reduction in thickness. That is, when not used for photography, the
taking lens is stored with the zooming spaces reduced (this system
will hereinafter be referred to as the "collapsible barrel").
[0005] In the case of finder optical systems, however, the
collapsible barrel system as used in taking lenses is unfavorable
from the viewpoint of camera design. Therefore, it is very
difficult to reduce the thickness of finder optical systems. This
is an obstacle in attaining a reduction in size of cameras.
[0006] Cameras have also been demanded to provide a higher zoom
ratio. Therefore, it is also necessary to form a finder optical
system from an increased number of optical units in order to ensure
the required performance. However, if the number of optical units
is increased, the sum total of the thicknesses of lenses increases.
Therefore, such a finder optical system arrangement is not always
favorable for attaining a reduction in thickness.
[0007] As a conventional technique, Japanese Patent Application
Unexamined Publication Number [hereinafter referred to as "JP(A)"]
Hei 5-53054 has, in order from the object side, a negative first
unit, a positive second unit, and a negative third unit. The first
and second units move for zooming. The negative third unit, which
is stationary, ensures the negative power by refraction through a
refracting lens or a prism entrance surface. The optical system
attains a reduction in overall length by disposing a negative unit
closest to the pupil.
[0008] Meanwhile, it has heretofore been common practice to use a
plane, powerless surface as a reflecting surface of an
image-inverting optical system. Accordingly, there have been made
some propositions that a reflecting surface of a prism or a mirror
constituting an image-inverting optical system is given a power so
as to have the function of an objective optical system or the
function of an ocular optical system, thereby attaining a reduction
in size.
[0009] JP(A) Hei 8-248481 used a rotationally symmetric curved
surface as a reflecting surface of a prism in a real-image finder.
Although it is stated that an aspherical surface or a toric surface
is applicable to the curved surface, the curved surface disclosed
in the specification is a rotationally symmetric aspherical
surface. In general, a toric surface is symmetric with respect to
two coordinate axes. Therefore, it is not an asymmetric surface. No
numerical values are mentioned in regard to the amounts of
displacement of reflecting surfaces in Examples.
[0010] JP(A) Hei 10-197796 uses a rotationally asymmetric curved
surface in an image-inverting optical system of a real-image finder
optical system. However, almost all Examples of this finder optical
system fail to disclose design examples. Therefore, the
performance, size, etc. of the finder optical system are unclear.
Numerical Example 5 of this finder optical system uses rotationally
asymmetric curved surfaces as refracting and reflecting surfaces of
a prism in the finder optical system. In addition, the
image-inverting optical system is arranged to have the function of
an ocular lens, thereby reducing the number of lenses used.
[0011] JP(A) Hei 11-38472 and 11-38473 use a rotationally
asymmetric surface as a reflecting surface of one of Porro prisms
of a real-image finder that is disposed on the object side of the
intermediate image formation plane, thereby attaining a reduction
in the thickness of the finder optical system.
[0012] However, these prior art optical systems suffer from various
problems as stated below.
[0013] In JP(A) Hei 5-53054, if a strong power is given to the
negative third unit, the positive power of the second unit
unavoidably needs to be increased in order to ensure the required
power, which is unfavorable from the viewpoint of performance. For
this reason, a very strong power cannot be given to the third unit.
Therefore, the effect of reducing the overall length is limited.
Moreover, an optical system having a power cannot be placed on the
pupil side of the third unit, and it is therefore necessary to
ensure a long back focus. For this reason, the basic structure of
the optical system is the retrofocus type. Accordingly, it is still
difficult to reduce the overall length of the optical system.
[0014] In JP(A) Hei 8-248481, a power is given to a reflecting
surface of a prism. However, because this reflecting surface is
tilted so as to be decentered with respect to the axial principal
ray, rotationally asymmetric decentration aberrations are produced.
The aberrations cannot be corrected by the rotationally symmetric
aspherical surface configuration. At the toric surface also,
aberration correction with respect to skew rays cannot
satisfactorily be performed. In this regard, no solution means is
disclosed for any of the arrangements of this optical system. Thus,
the disclosed optical system is unsatisfactory in terms of
performance. Furthermore, the size of the optical system is unclear
from the numerical values mentioned in the specification.
[0015] JP(A) Hei 10-197796 states a layout and arrangement of
prisms, etc. that seem to allow the finder optical system to be
reduced in size. However, no consideration is given in terms of
performance. Therefore, the disclosed technique lacks feasibility.
In Numerical Example 5, a reduction in thickness is attained by
reducing the number of lenses constituting the ocular optical
system. Accordingly, the objective optical system itself cannot be
reduced in thickness. When the optical system is arranged to attain
a higher zoom ratio, it is difficult to reduce the thickness
satisfactorily.
[0016] In JP(A) Hei 11-38472 and 11-38473, the prisms are large in
size although the spacing between the movable lens units is
reduced. Accordingly, the overall size of the finder optical
system, including the prism arrangement, has not yet been reduced
satisfactorily.
[0017] Thus, all the prior art optical systems have problems to be
solved in terms of performance or size. A compact and
high-performance finder that simultaneously satisfies the demands
for high performance and size reduction has not yet been
attained.
SUMMARY OF THE INVENTION
[0018] In view of the above-described problems with the prior art,
an object of the present invention is to provide a high-performance
real-image finder optical system reduced in size, particularly
reduced in thickness.
[0019] To attain the above-described object, the present invention
provides a first finder optical system that includes, in order from
the object side thereof: an objective optical system having a
positive refracting power; an image-inverting optical system for
erecting an intermediate image formed by the objective optical
system; and an ocular optical system having a positive refracting
power. The objective optical system has at least two movable units
moving when zooming is performed. A prism is placed on the object
side of the intermediate image formed by the objective optical
system. The prism includes at least one reflecting surface having a
rotationally asymmetric surface configuration. The image-inverting
optical system has at least one reflecting surface formed from a
roof surface. The finder optical system satisfies the following
condition:
1.0<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<2.5
(1)
[0020] where d is the distance from the entrance surface of the
objective optical system to the first reflecting surface of the
image-inverting optical system; f.sub.W is the focal length of the
objective optical system at the wide-angle end; .theta..sub.w is
the maximum field angle of the objective optical system at the
wide-angle end; and Z is a zoom ratio.
[0021] The function of the first finder optical system will be
described below.
[0022] In a real-image finder, an image-inverting optical system is
generally placed on the pupil side of an objective optical system.
Therefore, the objective optical system needs to be arranged in the
form a retrofocus type such that the image-inverting optical system
can be placed on the pupil side thereof, thereby ensuring the
required back focus. However, the retrofocus type arrangement makes
it difficult to reduce the overall length of the optical system.
Therefore, it is an unfavorable lens arrangement for the attainment
of a reduction in size. In addition, because the type of adoptable
image-inverting prisms is limited by the amount of back focus,
there are cases where the finder optical system cannot be arranged
in a compact form.
[0023] Therefore, according to the present invention, a power is
given to a reflecting surface of the image-inverting optical
system. By giving a power to a reflecting surface of the
image-inverting optical system, the function of an objective
optical system can also be imparted to the image-inverting optical
system. As a result, it becomes possible to relax the restriction
in terms of the back focus, which is an obstacle in attaining a
reduction in size.
[0024] It should be noted that if a power is simply given to a
decentered reflecting surface, rotationally asymmetric decentration
aberrations are produced, and the decentration aberrations cannot
be corrected by a rotationally symmetric aspherical surface or the
like. Therefore, it is necessary to use at least one rotationally
asymmetric surface. It is possible to correct the decentration
aberrations favorably by using at least one rotationally asymmetric
surface.
[0025] Meanwhile, when the taking lens is arranged to have a higher
zoom ratio, a larger aperture is needed in order to keep the same
F-number at the telephoto end. Consequently, the lens barrel
becomes undesirably large in diameter. Accordingly, when the taking
lens is arranged to have a higher zoom ratio, the size of the
camera in the direction of height is also likely to increase. When
laid out in a camera, the finder optical system is often placed
above the taking lens. Therefore, an effective way of preventing
the height of the camera from increasing even when the taking lens
is arranged to have a higher zoom ratio is to minimize the size of
the finder in the direction of height. For this reason, it is
desirable to use a roof surface in the image-inverting optical
system.
[0026] Thus, it becomes possible to reduce the finder optical
system in both size and thickness. However, a reduction in
thickness cannot always be attained unless careful consideration is
given to the layout and arrangement of the objective optical system
and the image-inverting optical system, which also has the function
of an objective optical system. Conventionally, the reflecting
surfaces of the image-inverting optical system are powerless and
irrespective of the optical performance. Therefore, it has
heretofore been possible to set the reflecting surface separation
and position relatively freely. However, when a power is given to a
reflecting surface of the image-inverting optical system, the
reflecting surface separation also needs to be considered.
[0027] Accordingly, it is necessary in order to reduce the
thickness of the finder optical system to reduce the space in which
the objective movable units are movable and the distance from the
entrance surface of the objective optical system to the entrance
surface of the image-inverting optical system and to reduce the
distance from the entrance surface of the image-inverting optical
system to the first reflecting surface thereof. The object of the
present invention cannot be attained unless the movable range of
the movable units and the above-described distances are reduced.
Accordingly, it is necessary to satisfy the following
condition:
1.0<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<2.5
(1)
[0028] where d is the distance from the entrance surface of the
objective optical system to the first reflecting surface of the
image-inverting optical system; f.sub.W is the focal length of the
objective optical system at the wide-angle end; .theta..sub.w is
the maximum field angle of the objective optical system at the
wide-angle end; and Z is a zoom ratio.
[0029] The definition of the focal length f in the following
description of the present invention is as follows. A light ray
which is parallel to the axial principal ray and which has a small
height h is made to enter the objective optical system from the
object side thereof. The angle that is formed between that ray and
the axial principal ray exiting from the objective optical system
is denoted by .alpha. (units:radian). The focal length f is given
by
f=h/.alpha.
[0030] In addition, d is the distance between the two surfaces as
measured at the points on the axial principal ray in parallel to
the entering optical axis.
[0031] If d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z) is not
smaller than the upper limit of the condition (1), i.e. 2.5, it
becomes impossible to attain a reduction in thickness. If
d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z) is not larger
than the lower limit, i.e. 1.0, the power of each unit becomes
excessively strong, causing the performance to be degraded.
[0032] It is preferable to satisfy the following condition:
1.2<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<1.7
(2)
[0033] In the first finder optical system, it is desirable that the
roof surface should be placed on the pupil side of the intermediate
image.
[0034] If a power is given to a roof surface, the roof edge
generally fails to be a continuous surface because of the structure
thereof. Therefore, such a roof surface cannot be produced.
Accordingly, the roof surface needs to be formed from plane
surfaces. For this reason, if the roof surface is placed closer to
the objective optical system, the finder optical system becomes
unfavorable from the viewpoint of performance, and the thickness of
the finder optical system cannot be reduced satisfactorily. For
this reason, it is desirable to place the roof surface closer to
the ocular optical system, i.e. on the pupil side of the
intermediate image.
[0035] In addition, the present invention provides a second finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism includes at least one reflecting surface having a
rotationally asymmetric surface configuration. The image-inverting
optical system is formed from a Porro prism. The finder optical
system satisfies the following conditions:
1.0<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<2.5
(3)
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (4)
[0036] where d is the distance from the entrance surface of the
objective optical system to the first reflecting surface of the
image-inverting optical system; f.sub.W is the focal length of the
objective optical system at the wide-angle end; .theta..sub.w is
the maximum field angle of the objective optical system at the
wide-angle end; Z is a zoom ratio; and dp is the distance from the
entrance surface of the image-inverting optical system placed on
the object side of the intermediate image to the first reflecting
surface.
[0037] The function of the second finder optical system will be
described below.
[0038] In general, when an image-inverting optical system is formed
by using a Porro prism, the reflection angle at the first
reflecting surface is set at approximately 45.degree.. When the
first reflecting surface has such a large reflection angle, the
effective area of the reflecting surface becomes large. In this
case, further, the size in the direction of thickness becomes large
unfavorably. For this reason, it is necessary in order to attain a
reduction in thickness to minimize at least the distance from the
entrance surface of the Porro prism to the first reflecting surface
thereof. Accordingly, it is desirable to satisfy the following
condition in addition to the already-described condition (3):
1.0<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<2.5
(3)
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (4)
[0039] It should be noted that dp is the distance between the two
surfaces as measured at the points on the axial principal ray in
parallel to the entering optical axis as in the case of the
distance d.
[0040] If d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z) is not
smaller than the upper limit of the condition (3), i.e. 2.5, it
becomes impossible to attain a reduction in thickness. If
d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z) is not larger
than the lower limit, i.e. 1.0, the power of each unit becomes
excessively strong, causing the performance to be degraded.
[0041] If dp/(f.sub.W.multidot.tan .theta..sub.w) is not smaller
than the upper limit of the condition (4), i.e. 1.1, it becomes
difficult to attain a reduction in thickness. If
dp/(f.sub.W.multidot.tan .theta..sub.w) is not larger than the
lower limit, i.e. 0.5, the effective portions of the first
transmitting surface and the first reflecting surface overlap each
other undesirably. Consequently, it becomes impossible to ensure
the required edge wall thickness.
[0042] It is preferable to satisfy the following conditions:
1.2<d/(f.sub.W.multidot.tan .theta..sub.w.multidot.Z)<1.7
(5)
0.6<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.0 (6)
[0043] In the second finder optical system, it is desirable that
the second transmitting surface of the objective optical system
should have a power.
[0044] In general, reflecting surfaces are decentered. Therefore,
if an excessively strong power is given to a reflecting surface,
decentration aberrations are produced. Therefore, to give a strong
power to a prism, a power should preferably be given to a
transmitting surface as well. In particular, giving a power to the
second transmitting surface facilitates the correction of
distortion and also makes it possible to change the position of the
exit pupil of the objective optical system. This is favorable from
the viewpoint of design and performance.
[0045] In this case, it is desirable that the second transmitting
surface should have a rotationally asymmetric surface
configuration.
[0046] In the case of a real-image finder, it is necessary to
adjust the position of the exit pupil of the objective optical
system in advance in order to relay an intermediate image formed by
the objective optical system to the ocular optical system. If a
rotationally asymmetric surface is used as a reflecting surface as
in the present invention, the exit pupil position also becomes
rotationally asymmetric. If this is corrected by the reflecting
surface itself, other aberrations are affected unfavorably.
Therefore, it is preferable to correct the exit pupil position by
using a rotationally asymmetric surface for the second transmitting
surface, at which the light beam is narrowed and which has minimal
influence on other aberrations.
[0047] In the second finder optical system, it is desirable that
the first reflecting surface of the objective optical system should
have a power.
[0048] When a Porro prism is used, the reflection angle at the
second reflecting surface is also large, i.e. about 45.degree..
Therefore, the performance is degraded unless the power
distribution is optimized. Of the reflecting surfaces, the first
reflecting surface is far from the intermediate image formation
plane. Therefore, the axial ray height is large at the first
reflecting surface. Accordingly, the first reflecting surface can
produce a strong aberration correcting effect. For this reason, it
is desirable to give a power to the first reflecting surface.
[0049] In the first or second finder optical system, it is
desirable that the number of reflections in the prism should be two
or three.
[0050] If there is only one reflection in the prism, it is
impossible to give a sufficient power for aberration correction.
Therefore, the aberration correcting effect is weak. If the number
of reflections in the prism is four or more, the prism becomes
undesirably large in size. Therefore, a reduction in size cannot be
attained. If the prism is arranged so that two or three reflections
take place therein, it is possible to attain both high performance
and size reduction with good balance.
[0051] In the first or second finder optical system, it is
desirable that the number of movable units should be two.
[0052] If the number of movable units placed in the direction of
the optical axis is increased, the sum total of the thicknesses of
lenses increases. Therefore, it is preferable to use two movable
units, which is the smallest number of optical units for
zooming.
[0053] In addition, the present invention provides a third finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system includes an optical system having at least
two movable units moving when zooming is performed. The optical
system has a positive composite power. A prism is placed on the
pupil side of the optical system having at least two movable units.
The prism includes an image-inverting function. The prism includes
at least one reflecting surface having a rotationally asymmetric
surface configuration. At least either one of the first
transmitting surface and first reflecting surface of the prism has
a negative power. The second transmitting surface of the prism has
a positive power.
[0054] The function of the third finder optical system will be
described below.
[0055] The prism power arrangement for reducing the thickness of
the finder even more effectively will be described. To attain the
object of the present invention, it is necessary to simultaneously
meet the need to reduce the thickness of the optical system and the
need to reduce the size of the prism itself. Therefore, in the
present invention, the prism is placed on the pupil side of an
optical system having a positive composite power to fold the
optical axis. In addition, a negative power is distributed to the
prism to form a telephoto type lens system, thereby effectively
reducing the thickness of the optical system. In this case, to
enhance the advantageous effect of the telephoto type lens system
and to ensure the required magnification, a negative power is given
to at least either one of the first transmitting surface and the
first reflecting surface, which are relatively close to the object.
Furthermore, to favorably correct aberrations produced by the
surface of negative power, at least one reflecting surface is
formed from a rotationally asymmetric surface.
[0056] Meanwhile, the volume of the prism itself depends on the
area of each effective surface. Therefore, it is necessary to
reduce the effective area of each reflecting surface. Accordingly,
in the present invention, a positive power is given to the second
transmitting surface, at which the light beam is narrowed and the
influence of power on aberrations weakens, thereby bringing the
entrance pupil position close to the prism while minimizing the
influence on the performance, and thus reducing the size of the
prism.
[0057] In this case, it is desirable that either one of the first
transmitting surface and the first reflecting surface should have a
positive power.
[0058] To form a telephoto type lens system to shorten the overall
length, a relatively strong negative power is needed. Consequently,
the amount of aberration produced in the optical system becomes
unfavorably large. Therefore, it is preferable to give a positive
power to either the first transmitting surface or the first
reflecting surface so that the aberrations produced by the surface
of negative power are corrected by cancellation with the surface of
positive power.
[0059] It is desirable that the number of reflections in the prism
should be two or three.
[0060] If there is only one reflection in the prism, it is
impossible to give a sufficient power for aberration correction.
Therefore, the aberration correcting effect is weak. If the number
of reflections in the prism is four or more, the prism becomes
undesirably large in size. Therefore, a reduction in size cannot be
attained. If the prism is arranged so that two or three reflections
take place therein, it is possible to attain both high performance
and size reduction with good balance.
[0061] It is desirable that the composite focal length of the
objective movable units should satisfy the following condition:
0.3<f.sub.move/f.sub.W<0.9 (7)
[0062] where f.sub.move is the composite focal length of the
objective movable units at the wide-angle end, and f.sub.W is the
focal length of the objective optical system at the wide-angle
end.
[0063] In the present invention, a telephoto type lens system is
constructed of the first and second units that have a positive
composite power and the third unit having a negative power. In this
case, because the composite power of the first and second units is
stronger than the power of the whole objective optical system, the
amount of aberration produced by the first and second units tends
to increase. Accordingly, from the viewpoint of attaining a further
reduction in size while maintaining favorable performance over the
entire zooming range, it is desirable to satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (7)
[0064] If f.sub.move/f.sub.W is not smaller than the upper limit of
the condition (7), i.e. 0.9, the advantageous effect of the
telephoto type lens system weakens, and it becomes impossible to
attain a reduction in thickness. If f.sub.move/f.sub.W is not
larger than the lower limit, i.e. 0.3, the power of each unit
becomes excessively strong, causing the performance to be
degraded.
[0065] It is preferable to satisfy the following condition:
0.4<f.sub.move/f.sub.W<0.8 (8)
[0066] In addition, the present invention provides a fourth finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism has two reflecting surfaces. At least one of the reflecting
surfaces has a rotationally asymmetric surface configuration. The
finder optical system satisfies the following condition:
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (9)
[0067] where f.sub.W is the focal length of the objective optical
system at the wide-angle end; .theta..sub.w is the maximum field
angle of the objective optical system at the wide-angle end; and dp
is the distance from the entrance surface of the image-inverting
optical system placed on the object side of the intermediate image
to the first reflecting surface thereof.
[0068] The function of the fourth finder optical system will be
described below.
[0069] The optimum arrangement of the reflecting surfaces of the
prism will be described. If an excessively strong power is given to
a reflecting surface of the image-inverting optical system, the
amount of aberration produced by the surface becomes unfavorably
large. This is unfavorable from the viewpoint of performance. As is
generally known, even when the curvature of a surface is the same,
as the reflection angle becomes larger, the amount of aberration
produced by the surface increases undesirably. Finder optical
systems are generally required to allow observation in the same
direction as the observer's viewing direction. Therefore, it is
necessary to make the entering optical axis and the exiting optical
axis approximately parallel to each other. For this reason, the
reflection angle of the image-inverting optical system is limited,
and an arrangement having an excessively small number of reflecting
surfaces is unfavorable from the viewpoint of performance.
Accordingly, it is necessary to provide at least two reflecting
surfaces on the objective optical system side in order to ensure
the required performance.
[0070] Meanwhile, it is known that reflecting surfaces are
sensitive to errors that may occur during production, assembly,
etc. in comparison to refracting surfaces. Therefore, if the number
of reflecting surfaces with power is large, the performance is
degraded severely. In view of this fact, it is preferable to use a
prism in which the number of reflecting surfaces is only two.
[0071] From the viewpoint of minimizing the size of the prism, it
is desirable to satisfy the following condition:
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (9)
[0072] If dp/(f.sub.W.multidot.tan .theta..sub.w) is not smaller
than the upper limit of the condition (9), i.e. 1.1, it becomes
difficult to attain a reduction in thickness. If
dp/(f.sub.W.multidot.tan .theta..sub.w) is not larger than the
lower limit, i.e. 0.5, the effective portions of the first
transmitting surface and the first reflecting surface overlap each
other undesirably. Consequently, it becomes impossible to ensure
the required edge wall thickness.
[0073] It is preferable to satisfy the following condition:
0.6<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.0 (10)
[0074] In the third finder optical system, it is desirable that the
first reflecting surface of the prism should have a rotationally
asymmetric surface configuration and be formed from an independent
surface that is separate from other transmitting and reflecting
surfaces.
[0075] When a reflecting surface is arranged to have a total
reflection critical angle, it can serve as both reflecting and
transmitting surfaces. That is, a transmitting surface and a
reflecting surface can be formed on the identical surface by using
a total reflection critical angle. With this arrangement, because
it is unnecessary to separate the effective portions of the
transmitting surface and the reflecting surface, the prism can be
reduced in size. However, because the prism needs to have a total
reflection critical angle as a reflection angle, the arrangement is
unfavorable from the viewpoint of performance.
[0076] In the prism of the present invention, because the axial ray
height is the highest at the first reflecting surface of the
reflecting surfaces thereof, the effect of correcting aberrations
such as spherical aberration is remarkable at the first reflecting
surface. Therefore, it is not preferable to increase the reflection
angle at the first reflecting surface. Accordingly, it is
preferable that the first reflecting surface should be formed
separately from other transmitting and reflecting surfaces.
[0077] In addition, it is desirable that the second transmitting
surface of the prism should have a power.
[0078] In general, reflecting surfaces are decentered. Therefore,
if an excessively strong power is given to a reflecting surface,
decentration aberrations are produced. Therefore, to give a strong
power to a prism, a power should preferably be given to a
transmitting surface as well. In particular, giving a power to the
second transmitting surface facilitates the correction of
distortion and also makes it possible to change the position of the
exit pupil of the objective optical system. This is favorable from
the viewpoint of design and performance.
[0079] In this case, it is desirable that the second transmitting
surface should have a rotationally asymmetric surface
configuration.
[0080] In the case of a real-image finder, it is necessary to
adjust the position of the exit pupil of the objective optical
system in advance in order to relay an intermediate image formed by
the objective optical system to the ocular optical system. If a
rotationally asymmetric surface is used as a reflecting surface as
in the present invention, the exit pupil position also becomes
rotationally asymmetric. If this is corrected by the reflecting
surface itself, other aberrations are affected unfavorably.
Therefore, it is preferable to correct the exit pupil position by
using a rotationally asymmetric surface for the second transmitting
surface, at which the light beam is narrowed and which has minimal
influence on other aberrations.
[0081] It is desirable that the axial principal ray or a projective
axial principal ray defined by projecting the axial principal ray
onto a plane containing a part of the axial principal ray should
not cross itself in the prism.
[0082] Let us describe optical paths that a prism can take. A prism
allows an optical system to be arranged in a compact form by
folding the axial principal ray (optical axis) with reflecting
surfaces. However, in order to attain a reduction in thickness,
which is the object of the present invention, it is necessary to
appropriately set the direction of travel of the optical axis
(hereinafter referred to as the "optical path").
[0083] It may be considered that if the optical path is arranged to
cross itself, the prism can be made compact in size because light
rays pass through the same portion of the prism twice. However, if
the prism is constructed by making the optical path cross itself,
as shown in FIG. 20, the separation between the surfaces A and B
depends on the size of the effective portion of the surface C or D,
and the separation between the surfaces C and D depends on the size
of the effective portion of the surface A or B. Therefore, there
are cases where the prism cannot be reduced in size when the
effective portions are large in size. Accordingly, it is preferable
in the present invention that the axial principal ray should not
cross itself.
[0084] If the axial principal ray is twisted, the optical path does
not cross itself. This is, however, technically the same as the
above. Therefore, in the present invention, a plane containing a
part of the axial principal ray is defined. The axial principal ray
is projected onto the plane, and the resulting two-dimensional
axial principal ray is defined as a projective axial principal ray.
The prism is preferably constructed so that the projective axial
principal ray does not cross itself in the prism.
[0085] In addition, it is desirable that either the first
transmitting surface or first reflecting surface of the prism
should have a negative power.
[0086] By placing a negative power on the pupil side of the
objective units having a positive composite power, a substantially
telephoto type lens system can be constructed. Thus, it is possible
to obtain an arrangement most suitable for attaining a reduction in
thickness.
[0087] In addition, it is desirable that the composite focal length
of the objective movable units should satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (11)
[0088] where f.sub.move is the composite focal length of the
objective movable units at the wide-angle end, and f.sub.W is the
focal length of the objective optical system at the wide-angle
end.
[0089] In the present invention, a telephoto type lens system is
constructed of the first and second units that have a positive
composite power and the third unit having a negative power. In this
case, because the composite power of the first and second units is
stronger than the power of the whole objective optical system, the
amount of aberration produced by the first and second units tends
to increase. Accordingly, from the viewpoint of attaining a further
reduction in size while maintaining favorable performance over the
entire zooming range, it is desirable to satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (11)
[0090] If f.sub.move/f.sub.W is not smaller than the upper limit of
the condition (11), i.e. 0.9, the advantageous effect of the
telephoto type lens system weakens, and it becomes impossible to
attain a reduction in thickness. If f.sub.move/f.sub.W is not
larger than the lower limit, i.e. 0.3, the power of each unit
becomes excessively strong, causing the performance to be
degraded.
[0091] It is preferable to satisfy the following condition:
0.4<f.sub.move/f.sub.W<0.8 (12)
[0092] In addition, the present invention provides a fifth finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism has three reflecting surfaces. At least one of the reflecting
surfaces has a rotationally asymmetric surface configuration.
[0093] The function of the fifth finder optical system will be
described below.
[0094] To attain a higher zoom ratio, aberrations produced by each
unit need to be corrected even more strictly. Meanwhile, finder
optical systems are generally required to allow observation in the
same direction as the observer's viewing direction. Therefore, it
is necessary to make the entering optical axis and the exiting
optical axis approximately parallel to each other. For this reason,
when the number of reflections taking place in the prism is two,
the position and reflection direction of each reflecting surface
are limited to a certain extent, and hence the reflection angle,
which relates to the amount of aberration produced, is limited
undesirably. Therefore, if the optical system is arranged to
provide a higher zoom ratio while maintaining a thin structure,
there are cases where aberrations cannot sufficiently be
corrected.
[0095] Accordingly, the prism is formed from three reflecting
surfaces, thereby allowing residual aberrations to be corrected
favorably even when the reflection angle is made relatively large.
Thus, it is possible to construct a finder optical system
exhibiting favorable performance even if it is arranged to provide
a higher zoom ratio while maintaining a thin structure.
[0096] In this case, it is desirable that at least two of the three
reflecting surfaces of the prism should be formed from independent
surfaces, respectively, which are separate from other transmitting
and reflecting surfaces.
[0097] When a reflecting surface is arranged to have a total
reflection critical angle, it can serve as both reflecting and
transmitting surfaces. That is, a transmitting surface and a
reflecting surface can be formed on the identical surface by using
a total reflection critical angle. However, because the reflection
angle needs to be a total reflection critical angle, the
arrangement becomes unfavorable from the viewpoint of performance.
Moreover, the exit direction of the prism is limited undesirably.
Accordingly, the use of total reflection may be unfavorable for the
reduction in thickness of the finder.
[0098] In the present invention, aberration correction is performed
even more strictly by using three reflecting surfaces in the prism.
Therefore, if an excessively large number of reflecting surfaces
are formed as the identical surfaces with other surfaces, the prism
becomes unfavorable from the viewpoint of performance, and it
becomes impossible to fully satisfy the object of the present
invention. Accordingly, it is desirable that at least two of the
three reflecting surfaces of the prism should be formed from
independent surfaces, respectively, which are separate from other
transmitting and reflecting surfaces.
[0099] In addition, it is desirable that the first reflecting
surface of the prism should have a rotationally asymmetric surface
configuration and be formed from an independent surface that is
separate from other transmitting and reflecting surfaces.
[0100] When a reflecting surface is arranged to have a total
reflection critical angle, it can serve as both reflecting and
transmitting surfaces. That is, a transmitting surface and a
reflecting surface can be formed on the identical surface by using
a total reflection critical angle. With this arrangement, because
it is unnecessary to separate the effective portions of the
transmitting surface and the reflecting surface, the prism can be
reduced in size. However, because the prism needs to have a total
reflection critical angle as a reflection angle, the arrangement is
unfavorable from the viewpoint of performance.
[0101] In the prism of the present invention, because the axial ray
height is the highest at the first reflecting surface of the
reflecting surfaces thereof, the effect of correcting aberrations
such as spherical aberration is remarkable at the first reflecting
surface. Therefore, it is not preferable to increase the reflection
angle at the first reflecting surface. Accordingly, it is
preferable that the first reflecting surface should be formed
separately from other transmitting and reflecting surfaces.
[0102] In addition, it is desirable that the third reflecting
surface of the prism should have a rotationally asymmetric surface
configuration and be formed from an independent surface that is
separate from other transmitting and reflecting surfaces.
[0103] In the prism of the present invention, because the marginal
ray height is the highest at the third reflecting surface of the
reflecting surfaces thereof, the effect of correcting aberrations
such as astigmatic and comatic aberrations is remarkable at the
third reflecting surface. Therefore, it is not preferable to
increase the reflection angle at the third reflecting surface.
Accordingly, it is preferable that the third reflecting surface
should be formed separately from other transmitting and reflecting
surfaces.
[0104] In addition, it is desirable that the second transmitting
surface of the prism should have a power.
[0105] In general, reflecting surfaces are decentered. Therefore,
if an excessively strong power is given to a reflecting surface,
decentration aberrations are produced. Therefore, to give a strong
power to a prism, a power should preferably be given to a
transmitting surface as well. In particular, giving a power to the
second transmitting surface facilitates the correction of
distortion and also makes it possible to change the position of the
exit pupil of the objective optical system. This is favorable from
the viewpoint of design and performance.
[0106] In this case, it is desirable that the second transmitting
surface should have a rotationally asymmetric surface
configuration.
[0107] In the case of a real-image finder, it is necessary to
adjust the position of the exit pupil of the objective optical
system in advance in order to relay an intermediate image formed by
the objective optical system to the ocular optical system. If a
rotationally asymmetric surface is used as a reflecting surface as
in the present invention, the exit pupil position also becomes
rotationally asymmetric. If this is corrected by the reflecting
surface itself, other aberrations are affected unfavorably.
Therefore, it is preferable to correct the exit pupil position by
using a rotationally asymmetric surface for the second transmitting
surface, at which the light beam is narrowed and which has minimal
influence on other aberrations.
[0108] In addition, it is desirable that the axial principal ray or
a projective axial principal ray defined by projecting the axial
principal ray onto a plane containing a part of the axial principal
ray should not cross itself in the prism.
[0109] Let us describe optical paths that a prism can take. A prism
allows an optical system to be arranged in a compact form by
folding the axial principal ray (optical axis) with reflecting
surfaces. However, in order to attain a reduction in thickness,
which is the object of the present invention, it is necessary to
appropriately set the direction of travel of the optical axis
(hereinafter referred to as the "optical path").
[0110] It may be considered that if the optical path is arranged to
cross itself, the prism can be made compact in size because light
rays pass through the same portion of the prism twice. However, if
the prism is constructed by making the optical path cross itself,
as shown in FIG. 20, the separation between the surfaces A and B
depends on the size of the effective portion of the surface C or D,
and the separation between the surfaces C and D depends on the size
of the effective portion of the surface A or B. Therefore, there
are cases where the prism cannot be reduced in size when the
effective portions are large in size. Accordingly, it is preferable
in the present invention that the axial principal ray should not
cross itself.
[0111] If the axial principal ray is twisted, the optical path does
not cross itself. This is, however, technically the same as the
above. Therefore, in the present invention, a plane containing a
part of the axial principal ray is defined. The axial principal ray
is projected onto the plane, and the resulting two-dimensional
axial principal ray is defined as a projective axial principal ray.
The prism is preferably constructed so that the projective axial
principal ray does not cross itself in the prism.
[0112] In addition, it is desirable that either the first
transmitting surface or first reflecting surface of the prism
should have a negative power.
[0113] By placing a negative power on the pupil side of the
objective units having a positive composite power, a substantially
telephoto type lens system can be constructed. Thus, it is possible
to obtain an arrangement most suitable for attaining a reduction in
thickness.
[0114] In addition, it is desirable that the composite focal length
of the objective movable units should satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (13)
[0115] where f.sub.move is the composite focal length of the
objective movable units at the wide-angle end, and f.sub.W is the
focal length of the objective optical system at the wide-angle
end.
[0116] In the present invention, a telephoto type lens system is
constructed of the first and second units that have a positive
composite power and the third unit having a negative power. In this
case, because the composite power of the first and second units is
stronger than the power of the whole objective optical system, the
amount of aberration produced by the first and second units tends
to increase. Accordingly, from the viewpoint of attaining a further
reduction in size while maintaining favorable performance over the
entire zooming range, it is desirable to satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (13)
[0117] If f.sub.move/f.sub.W is not smaller than the upper limit of
the condition (13), i.e. 0.9, the advantageous effect of the
telephoto type lens system weakens, and it becomes impossible to
attain a reduction in thickness. If f.sub.move/f.sub.W is not
larger than the lower limit, i.e. 0.3, the power of each unit
becomes excessively strong, causing the performance to be
degraded.
[0118] It is preferable to satisfy the following condition:
0.4<f.sub.move/f.sub.W<0.8 (14)
[0119] In addition, the present invention provides a sixth finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system includes, in order from the object side
thereof, a negative first unit, a positive second unit, and a
negative third unit. At least the first unit and the second unit
are movable units moving when zooming is performed. The third unit
is formed from a prism including an image-inverting function. The
prism includes at least one reflecting surface having a
rotationally asymmetric surface configuration.
[0120] In addition, the present invention provides a seventh finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system includes, in order from the object side
thereof, a negative first unit, a positive second unit, and a
positive third unit. At least the first unit and the second unit
are movable units moving when zooming is performed. The third unit
is formed from a prism including an image-inverting function. The
prism includes at least one reflecting surface having a
rotationally asymmetric surface configuration. The prism includes
at least one transmitting surface or reflecting surface that has a
negative power. The third unit has a principal point positioned on
the pupil side of a plane where the intermediate image is
formed.
[0121] The function of the sixth and seventh finder optical systems
will be described below.
[0122] As has been stated above, by giving a power to a reflecting
surface of the image-inverting optical system, it is possible to
relax the restriction in terms of the back focus and hence possible
to reduce the thickness of the finder optical system. However, to
attain a reduction in thickness satisfactorily, it is necessary to
optimize the arrangement of the objective units.
[0123] Although the smallest number of optical units for zooming is
two, it is necessary to increase the number of optical units in
order to attain a higher zoom ratio. It is difficult to ensure the
required performance unless the number of optical units is
increased. However, if optical units are disposed in the direction
of the entering optical axis, the sum total of the thicknesses of
optical components increases undesirably as the number of optical
units increases. Consequently, it becomes difficult to attain a
reduction in thickness.
[0124] Accordingly, in the present invention, the objective optical
system is formed from three units, in which the first and second
units are arranged in the form of moving units to have a zooming
function. The third unit is formed from an image-inverting prism
having the function of an objective optical system to fold the
optical axis, thereby preventing the optical system from increasing
in size in the direction of thickness while ensuring the required
performance.
[0125] Regarding the power distribution, the first unit is given a
negative power and the second unit is given a positive power to
ensure a certain back focus. Meanwhile, a surface of negative power
is disposed in the third unit. Thus, a substantially telephoto type
lens system is constructed of the first and second units having a
positive composite power and the third unit of negative power to
provide an arrangement that is most suitable for attaining a
reduction in thickness. Further, in the third unit, a reflecting
surface in the prism is given a power and formed from a
rotationally asymmetric surface, thereby allowing the practical
back focus to be shortened effectively.
[0126] Of the prism surfaces, a surface of low magnification that
is not very closely related to the construction of a telephoto type
lens system, e.g. the second transmitting surface, which is located
near the intermediate image plane, does not exert a very strong
influence on the focal length and the effect of shortening the
overall length even when the power of the surface is changed.
Therefore, the power of the whole prism, which constitutes the
third unit, does not always need to be made negative. Accordingly,
the prism may be arranged to have a positive power. In this case,
however, at least one transmitting surface or reflecting surface
needs to have a negative power in order to construct a telephoto
type lens system as stated above. When the composite power of the
first and second units and the power of the third unit are both
positive, the composite focal length becomes excessively short if
the principal point separation is small. Therefore, it is necessary
to increase the separation between the principal point of the first
and second units and the principal point of the third unit as shown
by the composite focal length computing expression
(1/f=1/f1+1/f2-d/(f1.multidot- .f2)). For this reason, the
principal point of the third unit needs to be positioned on the
pupil side of a plane where the intermediate image is formed.
[0127] In the sixth and seventh finder optical systems, it is
desirable that the second transmitting surface should have a
power.
[0128] In general, reflecting surfaces are decentered. Therefore,
if an excessively strong power is given to a reflecting surface,
decentration aberrations are produced. Therefore, to give a strong
power to a prism, a power should preferably be given to a
transmitting surface as well. In particular, giving a power to the
second transmitting surface facilitates the correction of
distortion and also makes it possible to change the position of the
exit pupil of the objective optical system. This is favorable from
the viewpoint of design and performance.
[0129] In this case, it is desirable that the second transmitting
surface should have a rotationally asymmetric surface
configuration.
[0130] In the case of a real-image finder, it is necessary to
adjust the position of the exit pupil of the objective optical
system in advance in order to relay an intermediate image formed by
the objective optical system to the ocular optical system. If a
rotationally asymmetric surface is used as a reflecting surface as
in the present invention, the exit pupil position also becomes
rotationally asymmetric. If this is corrected by the reflecting
surface itself, other aberrations are affected unfavorably.
Therefore, it is preferable to correct the exit pupil position by
using a rotationally asymmetric surface for the second transmitting
surface, at which the light beam is narrowed and which has minimal
influence on other aberrations.
[0131] In addition, it is desirable that the number of reflections
in the prism should be two or three.
[0132] If there is only one reflection in the prism, it is
impossible to give a sufficient power for aberration correction.
Therefore, the aberration correcting effect is weak. If the number
of reflections in the prism is four or more, the prism becomes
undesirably large in size. Therefore, a reduction in size cannot be
attained. If the prism is arranged so that two or three reflections
take place therein, it is possible to attain both high performance
and size reduction with good balance.
[0133] In addition, it is desirable that the composite focal length
of the objective movable units should satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (15)
[0134] where f.sub.move is the composite focal length of the
objective movable units at the wide-angle end, and f.sub.W is the
focal length of the objective optical system at the wide-angle
end.
[0135] In the present invention, a telephoto type lens system is
constructed of the first and second units that have a positive
composite power and the third unit having a negative power. In this
case, because the composite power of the first and second units is
stronger than the power of the whole objective optical system, the
amount of aberration produced by the first and second units tends
to increase. Accordingly, from the viewpoint of attaining a further
reduction in size while maintaining favorable performance over the
entire zooming range, it is desirable to satisfy the following
condition:
0.3<f.sub.move/f.sub.W<0.9 (15)
[0136] If f.sub.move/f.sub.W is not smaller than the upper limit of
the condition (15), i.e. 0.9, the advantageous effect of the
telephoto type lens system weakens, and it becomes impossible to
attain a reduction in thickness. If f.sub.move/f.sub.W is not
larger than the lower limit, i.e. 0.3, the power of each unit
becomes excessively strong, causing the performance to be
degraded.
[0137] It is preferable to satisfy the following condition:
0.4<f.sub.move/f.sub.W<0.8 (16)
[0138] In addition, it is desirable that when zooming is performed,
the third unit should be stationary with respect to a plane where
the intermediate image is formed.
[0139] In a case where the direction of the optical axis exiting
from the prism constituting the third unit has an angle with
respect to the optical axis entering the prism, if the prism is
arranged to be movable, the intermediate image formation plane
moves undesirably as the prism is moved for zooming. Consequently,
a complicated mechanism is needed unfavorably to transmit the image
to the ocular optical system. In addition, because the prism
becomes large in size in comparison to a refracting lens, the
mechanism for moving the prism also becomes unfavorably
complicated. Accordingly, it is desirable that the third unit
should be stationary with respect to the intermediate image
formation plane.
[0140] In addition, it is desirable that both the objective movable
units should be formed from refracting lenses.
[0141] The objective movable units can be formed from prisms as in
the case of the third unit. However, if prisms are used, the
objective movable units become large in size in comparison to
refracting lenses, making it impossible to attain a reduction in
thickness satisfactorily. Therefore, it is preferable to form the
objective movable units by using refracting lenses.
[0142] In addition, it is desirable that the objective movable
units should be each formed from a single refracting lens.
[0143] As has been stated above, as the number of refracting lenses
constituting the first and second units increases, the sum total of
the thicknesses increases, and hence it becomes impossible to
attain a reduction in thickness satisfactorily. Therefore, it is
preferable to form each of the objective movable units from a
single refracting lens.
[0144] The following is a detailed description of the arrangement
of an optical system for attaining the object of the present
invention.
[0145] An eighth finder optical system according to the present
invention includes, in order from the object side thereof: an
objective optical system having a positive refracting power; an
image-inverting optical system for erecting an intermediate image
formed by the objective optical system; and an ocular optical
system having a positive refracting power. The objective optical
system has at least two movable units moving when zooming is
performed. A prism is placed on the object side of the intermediate
image formed by the objective optical system. The prism includes
two reflecting surfaces. At least one of the reflecting surfaces
has a rotationally asymmetric surface configuration. The first
transmitting surface and second reflecting surface of the prism are
formed from the identical surface having both transmitting and
reflecting actions. The prism has an optical path in which the
axial principal ray or a projective axial principal ray defined by
projecting the axial principal ray onto a plane containing a part
of the axial principal ray bends in different directions from each
other with respect to the travel direction of light rays at the two
reflecting surfaces.
[0146] The function of the eighth finder optical system will be
described below.
[0147] By forming the first transmitting surface and second
reflecting surface of the prism from the identical surface, it
becomes unnecessary to separate the respective effective areas of
the two surfaces. Consequently, it is possible to minimize the
separation between the first transmitting surface and the first
reflecting surface and hence possible to attain a reduction in
thickness.
[0148] In this case, it is desirable that the first reflecting
surface of the prism should have a negative power.
[0149] Because it is the identical with the first transmitting
surface, the second reflecting surface needs to have a total
reflection critical angle. For this reason, giving a strong power
to the remaining reflecting surface (i.e. the first reflecting
surface) is favorable from the viewpoint of aberration correcting
performance. On the other hand, it is preferable to give a negative
power to the prism from the viewpoint of attaining a reduction in
thickness, as has been stated above. Accordingly, it is preferable
to give a negative power to the first reflecting surface.
[0150] In addition, it is desirable that the reflection angle at
the first reflecting surface of the prism should satisfy the
following condition:
15.degree.<.theta..sub.A1<40.degree. (17)
[0151] where .theta..sub.A1 is the reflection angle at the first
reflecting surface.
[0152] In Example 2 (described later), .theta..sub.A1 is
27.2.degree.. As has been stated above, it is preferable to give a
strong power to the first reflecting surface from the viewpoint of
aberration correction. Therefore, decentration aberrations are
unlikely to occur if the reflection angle at the first reflecting
surface is small. Accordingly, it is desirable for the first
reflecting surface to satisfy the condition (17). If .theta..sub.A1
is not smaller than the upper limit of the condition (17), i.e.
40.degree., decentration aberrations are produced unfavorably. If
.theta..sub.A1 is not larger than the lower limit, i.e. 15.degree.,
rays cannot be totally reflected by the second reflecting
surface.
[0153] It is preferable to satisfy the following condition:
20.degree.<.theta..sub.A1<35.degree. (18)
[0154] In addition, it is desirable that the exit angle of the
prism with respect to the optical axis entering the objective
optical system should satisfy the following condition:
25.degree.<.theta..sub.A<75.degree. (19)
[0155] where .theta..sub.A is the exit angle of the prism with
respect to the optical axis entering the objective optical
system.
[0156] In Example 2 (described later), .theta..sub.A is
55.6.degree.. As has been stated above, if the number of optical
units placed in the direction of the optical axis entering the
objective optical system is increased, it becomes difficult to
attain a reduction in thickness. Therefore, it is preferable to
change the direction of the exiting optical axis by appropriately
arranging the reflecting surfaces of the prism. However, if the
reflection angle is excessively large, the performance is degraded.
For this reason, it is necessary to set an optimum exit angle.
Accordingly, it is desirable to satisfy the condition (19). If
.theta..sub.A is not smaller than the upper limit of the condition
(19), i.e. 75.degree., the reflection angle at each reflecting
surface becomes excessively large, causing the performance to be
degraded. If .theta..sub.A is not larger than the lower limit, i.e.
25.degree., it becomes impossible to attain a reduction in
thickness.
[0157] It is preferable to satisfy the following condition:
40.degree.<.theta..sub.A<65.degree. (20)
[0158] In addition, the present invention provides a ninth finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism includes two reflecting surfaces. Both of the reflecting
surfaces are independent of other transmitting and reflecting
surfaces. At least one of the reflecting surfaces has a
rotationally asymmetric surface configuration. The prism has an
optical path in which the axial principal ray or a projective axial
principal ray defined by projecting the axial principal ray onto a
plane containing a part of the axial principal ray bends in the
same direction with respect to the travel direction of light rays
at the two reflecting surfaces. The finder optical system satisfies
the following condition:
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (21)
[0159] where dp is the distance from the entrance surface of the
image-inverting optical system placed on the object side of the
intermediate image to the first reflecting surface thereof; f.sub.W
is the focal length of the objective optical system at the
wide-angle end; and .theta..sub.w is the maximum field angle of the
objective optical system at the wide-angle end.
[0160] The function of the ninth finder optical system will be
described below.
[0161] In the ninth finder optical system, two transmitting
surfaces and two reflecting surfaces of the prism can be formed
from independent surface configurations, respectively. This is
favorable from the viewpoint of performance. A reduction in
thickness of the finder cannot be attained unless the prism itself
is reduced in size. In particular, the optical path from the first
transmitting surface to the first reflecting surface is
approximately parallel to the direction of the entering optical
axis. Therefore, it is necessary to minimize the distance between
the first transmitting surface and the first reflecting surface.
Accordingly, it is desirable to satisfy the following
condition:
0.5<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.1 (21)
[0162] where dp is the distance from the entrance surface of the
image-inverting optical system placed on the object side of the
intermediate image to the first reflecting surface thereof; f.sub.W
is the focal length of the objective optical system at the
wide-angle end; and .theta..sub.w is the maximum field angle of the
objective optical system at the wide-angle end.
[0163] If dp/(f.sub.W.multidot.tan .theta..sub.w) is not smaller
than the upper limit of the condition (21), i.e. 1.1, it becomes
difficult to attain a reduction in thickness, whereas if
dp/(f.sub.W.multidot.tan .theta..sub.w) is not larger than the
lower limit, i.e. 0.5, the effective portions of the first
transmitting surface and the first reflecting surface overlap each
other undesirably. Consequently, it becomes impossible to ensure
the required edge wall thickness.
[0164] It is preferable to satisfy the following condition:
0.6<dp/(f.sub.W.multidot.tan .theta..sub.w)<1.0 (22)
[0165] In the ninth finder optical system, it is desirable that the
first transmitting surface of the prism should have a negative
power.
[0166] As has been stated above, it is preferable to arrange the
optical system in the form of a telephoto type lens system from the
viewpoint of attaining a reduction in thickness. In this case, if
all of the negative power is given to a decentered reflecting
surface, the amount of decentration aberrations produced by this
surface becomes large. This is unfavorable from the viewpoint of
performance. In addition, of the transmitting surfaces, the first
transmitting surface, which is the entrance surface, is more away
from the intermediate image than the other transmitting surface.
Therefore, at the first transmitting surface, the axial ray height
is high, and this is effective in correcting aberrations.
Accordingly, it is preferable to give a negative power to the first
transmitting surface of the prism.
[0167] In addition, it is desirable that the second transmitting
surface of the prism should have a positive power.
[0168] The volume of the prism depends on the area of each
effective surface. Therefore, it is preferable to minimize the area
of the effective portion of each reflecting surface by bringing the
position of the entrance pupil of the objective lens system close
to the prism. Accordingly, it is desirable to bring the entrance
pupil position close to the prism by giving a positive power to the
second transmitting surface, at which the light beam is narrowed
and the influence of power on aberrations weakens.
[0169] In addition, it is preferable that the reflection angle at
the first reflecting surface of the prism should satisfy the
following condition:
30.degree.<.theta..sub.B1<60.degree. (23)
[0170] where .theta..sub.B1 is the reflection angle at the first
reflecting surface.
[0171] In Example 3 (described later), .theta..sub.B1 is
49.0.degree.. The prism in this finder optical system has an
optical path in which the axial principal ray bends in the same
direction with respect to the travel direction of light rays at the
two reflecting surfaces. Therefore, it is impossible to attain both
high performance and size reduction with good balance unless the
reflection angle at the first reflecting surface is set
appropriately. Accordingly, it is desirable to satisfy the
condition (23). If .theta..sub.B1 is not smaller than the upper
limit of the condition (23), i.e. 60.degree., decentration
aberrations are produced undesirably, and moreover, it is necessary
to position the second reflecting surface even closer to the pupil.
Consequently, it is impossible to attain a reduction in thickness.
If .theta..sub.B1 is not larger than the lower limit, i.e.
30.degree., the second reflecting surface and the objective optical
system interfere with each other unfavorably.
[0172] It is preferable to satisfy the following condition:
38.degree.<.theta..sub.B1<52.degree. (24)
[0173] In addition, it is desirable that the reflection angle at
the second reflecting surface of the prism satisfies the following
condition:
30.degree.<.theta..sub.B2<60.degree. (25)
[0174] where .theta..sub.B2 is the reflection angle at the second
reflecting surface.
[0175] In Example 3 (described later), .theta..sub.B2 is
41.0.degree.. The second reflecting surface should preferably
satisfy the condition (25) for the same reasons as stated above. If
.theta..sub.B2 is not smaller than the upper limit of the condition
(25), i.e. 60.degree., decentration aberrations are produced
undesirably, which is unfavorable from the viewpoint of
performance. If .theta..sub.B2 is not larger than the lower limit,
i.e. 30.degree., the ocular optical system and the objective
optical system interfere with each other unfavorably.
[0176] It is preferable to satisfy the following condition:
38.degree.<.theta..sub.B2<52.degree. (26)
[0177] In addition, it is desirable that the exit angle of the
prism with respect to the optical axis entering the objective
optical system should satisfy the following condition:
150.degree.<.phi..sub.B<210.degree. (27)
[0178] where .phi..sub.B is the exit angle of the prism with
respect to the optical axis entering the objective optical
system.
[0179] In Example 3 (described later), .phi..sub.B is 180.degree..
The exit angle of the prism should preferably satisfy the condition
(27). If .phi..sub.B is not smaller than the upper limit of the
condition (27), i.e. 210.degree., it becomes necessary to increase
the reflection angle at each surface. Therefore, the optical system
becomes unfavorable from the viewpoint of performance. If
.phi..sub.B is not larger than the lower limit, i.e. 150.degree.,
the ocular optical system and the objective optical system
interfere with each other unfavorably.
[0180] It is preferable to satisfy the following condition:
165.degree.<.phi..sub.B<195.degree. (28)
[0181] In addition, it is desirable that a roof surface should be
disposed on the pupil side of the intermediate image.
[0182] If a power is given to a roof surface, the roof edge
generally fails to be a continuous surface because of the structure
thereof. Therefore, such a roof surface cannot be produced.
Accordingly, the roof surface needs to be formed from plane
surfaces. For this reason, if the roof surface is placed closer to
the objective optical system, the finder optical system becomes
unfavorable from the viewpoint of performance, and the thickness of
the finder optical system cannot be reduced satisfactorily. For
this reason, it is desirable to place the roof surface closer to
the ocular optical system, i.e. on the pupil side of the
intermediate image.
[0183] In addition, it is desirable that the image-inverting
optical system should be a Porro prism.
[0184] In the case of a Porro prism, the number of reflections is
smaller than that in the case of using a roof surface by one.
Therefore, the use of a Porro prism is favorable in terms of light
quantity.
[0185] In addition, the present invention provides a tenth finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism includes three reflecting surfaces. All of the reflecting
surfaces are independent of other transmitting and reflecting
surfaces. At least one of the reflecting surfaces has a
rotationally asymmetric surface configuration. The prism has an
optical path in which the axial principal ray or a projective axial
principal ray defined by projecting the axial principal ray onto a
plane containing a part of the axial principal ray bends in the
same direction with respect to the travel direction of light rays
at two consecutive reflecting surfaces and bends in a direction
different from the above-mentioned direction at the other
reflecting surface.
[0186] The function of the tenth finder optical system will be
described below.
[0187] In the tenth finder optical system, two transmitting
surfaces and three reflecting surfaces of the prism can be formed
from independent surface configurations, respectively. This is
favorable from the viewpoint of performance. Further, performance
degradation is small even when the angle of each reflecting surface
is set relatively large. Therefore, the degree of freedom for the
exit direction is high, so that it is possible to dispose the prism
while taking into consideration the layout of the camera. In
addition, at one of the three reflecting surfaces, the optical path
bends in a different direction from the direction of bending at the
other two reflecting surfaces. Therefore, even when the reflection
angle is reduced, it is unlikely that the prism will interfere with
another optical system or the effective portions of the reflecting
surfaces will overlap each other. Accordingly, it is possible to
construct a compact and high-performance prism.
[0188] In this case, it is desirable that the first transmitting
surface of the prism should have a negative power.
[0189] As has been stated above, it is preferable to arrange the
optical system in the form of a telephoto type lens system from the
viewpoint of attaining a reduction in thickness. In this case, if
all of the negative power is given to a decentered reflecting
surface, the amount of decentration aberrations produced by this
surface becomes large. This is unfavorable from the viewpoint of
performance. In addition, of the transmitting surfaces, the first
transmitting surface, which is the entrance surface, is more away
from the intermediate image than the other transmitting surface.
Therefore, at the first transmitting surface, the axial ray height
is high, and this is effective in correcting aberrations.
Accordingly, it is preferable to give a negative power to the first
transmitting surface of the prism.
[0190] In addition, it is desirable that the reflection angle at
the first reflecting surface of the prism should satisfy the
following condition:
30.degree.<.theta..sub.C1<60.degree. (29)
[0191] where .theta..sub.C1 is the reflection angle at the first
reflecting surface.
[0192] In Example 4 (described later), 74 .sub.C1 is 43.8.degree..
Because it folds the axial principal ray first, the first
reflecting surface affects the size of the optical system and the
prism arrangement adversely unless the reflection angle at the
first reflecting surface is set appropriately. Therefore, it is
desirable to satisfy the condition (29). If .theta..sub.C1 is not
smaller than the upper limit of the condition (29), i.e.
60.degree., the second reflecting surface is placed closer to the
pupil. Consequently, it becomes impossible to attain a reduction in
thickness. If .theta..sub.C1 is not larger than the lower limit,
i.e. 30.degree., the third reflecting surface interferes with the
objective optical system undesirably.
[0193] It is preferable to satisfy the following condition:
38.degree.<.theta..sub.C1<52.degree. (30)
[0194] In addition, it is desirable that the reflection angle at
the third reflecting surface of the prism should satisfy the
following condition:
30.degree.<.theta..sub.C3<60.degree. (31)
[0195] where .theta..sub.C3 is the reflection angle at the third
reflecting surface.
[0196] In Example 4 (described later), .theta..sub.C3 is
41.7.degree.. It is desirable for the third reflecting surface to
satisfy the condition (31). If .theta..sub.C3 is not smaller than
the upper limit of the condition (31), i.e. 60.degree., the third
reflecting surface overlaps the effective portion of the second
reflecting surface. If .theta..sub.C3 is not larger than the lower
limit, i.e. 30.degree., the amount of decentration aberrations
produced by the third reflecting surface becomes large. This is
unfavorable from the viewpoint of performance.
[0197] It is preferable to satisfy the following condition:
38.degree.<.theta..sub.C3<52.degree. (32)
[0198] In addition, it is desirable that the exit angle of the
prism with respect to the optical axis entering the objective
optical system should satisfy the following condition:
70.degree.<.phi..sub.C<110.degree. (33)
[0199] where .phi..sub.C is the exit angle of the prism with
respect to the optical axis entering the objective optical
system.
[0200] In Example 4 (described later), .phi..sub.C is 90.degree..
It is desirable for the exit angle of the prism to satisfy the
condition (33). If .phi..sub.C is not smaller than the upper limit
of the condition (33), i.e. 110.degree., or not larger than the
lower limit, i.e. 70.degree., the amount to which the ocular
optical system extends in the direction of the entering optical
axis increases. Consequently, it becomes impossible to attain a
reduction in thickness.
[0201] It is preferable to satisfy the following condition:
80.degree.<.phi..sub.C<100.degree. (34)
[0202] In addition, the present invention provides an eleventh
finder optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism includes three reflecting surfaces. At least one of the
reflecting surfaces has a rotationally asymmetric surface
configuration. The first reflecting surface and third reflecting
surface of the prism are independent of other transmitting and
reflecting surfaces. The second reflecting surface of the prism is
formed from the identical surface with the second transmitting
surface of the prism. The prism has an optical path in which the
axial principal ray or a projective axial principal ray defined by
projecting the axial principal ray onto a plane containing a part
of the axial principal ray bends in the same direction with respect
to the travel direction of light rays at two consecutive reflecting
surfaces and bends in a direction different from the
above-mentioned direction at the other reflecting surface.
[0203] The function of the eleventh finder optical system will be
described below.
[0204] In the eleventh finder optical system, two transmitting
surfaces and two reflecting surfaces of the prism can be formed
from independent surface configurations, respectively. This is
favorable from the viewpoint of performance. Further, performance
degradation is small even when the angle of each reflecting surface
is set relatively large. Therefore, the degree of freedom for the
exit direction is high, so that it is possible to dispose the prism
while taking into consideration the layout of the camera. In
addition, at one of the three reflecting surfaces, the optical path
bends in a different direction from the direction of bending at the
other two reflecting surfaces. Therefore, even when the reflection
angle is reduced, it is unlikely that the prism will interfere with
another optical system or the effective portions of the reflecting
surfaces will overlap each other. Accordingly, it is possible to
construct a compact and high-performance prism. In addition,
because total reflection takes place at one surface, the prism
arrangement is favorable in terms of light quantity.
[0205] When the three reflecting surfaces of the prism are formed
independently of each other, each pair of adjacent reflecting
surfaces needs to have a sufficiently large separation therebetween
to separate the effective portions of the reflecting surfaces from
each other. Meanwhile, when a reflecting surface is arranged to
have a total reflection critical angle, it can serve as both
reflecting and transmitting surfaces. That is, a transmitting
surface and a reflecting surface can be formed on the identical
surface by using a total reflection critical angle. In the eighth
arrangement of the present invention in particular, the optical
path between the second and third reflecting surfaces is
approximately parallel to the direction of the entering optical
axis. Therefore, if these reflecting surfaces are formed from
independent surfaces, it is necessary to increase the separation
between the reflecting surfaces. This adversely affects the
attainment of a reduction in thickness. Therefore, according to the
present invention, the second reflecting surface and the second
transmitting surface are formed from the identical surface, thereby
setting a minimal separation between the second and third
reflecting surfaces, and thus attaining a further reduction in
thickness.
[0206] In this case, it is desirable that the first transmitting
surface of the prism should have a negative power.
[0207] As has been stated above, it is preferable to arrange the
optical system in the form of a telephoto type lens system from the
viewpoint of attaining a reduction in thickness. In this case, if
all of the negative power is given to a decentered reflecting
surface, the amount of decentration aberrations produced by this
surface becomes large. This is unfavorable from the viewpoint of
performance. In addition, of the transmitting surfaces, the first
transmitting surface, which is the entrance surface, is more away
from the intermediate image than the other transmitting surface.
Therefore, at the first transmitting surface, the axial ray height
is high, and this is effective in correcting aberrations.
Accordingly, it is preferable to give a negative power to the first
transmitting surface of the prism.
[0208] In the eleventh finder optical system, it is desirable that
the reflection angle at the first reflecting surface of the prism
should satisfy the following condition:
20.degree.<.theta..sub.D1<60.degree. (35)
[0209] where .theta..sub.D1 is the reflection angle at the first
reflecting surface.
[0210] In Example 1 (described later), .theta..sub.D1 is
39.4.degree.. Because it folds the axial principal ray first, the
first reflecting surface affects the size of the optical system and
the prism arrangement adversely unless the reflection angle at the
first reflecting surface is set appropriately. Therefore, it is
desirable to satisfy the condition (35). If .theta..sub.D1 is not
smaller than the upper limit of the condition (35), i.e.
60.degree., the second reflecting surface is placed closer to the
pupil. Consequently, it becomes impossible to attain a reduction in
thickness. If .theta..sub.D1 is not larger than the lower limit,
i.e. 20.degree., the third reflecting surface interferes with the
objective optical system undesirably.
[0211] It is preferable to satisfy the following condition:
30.degree.<.theta..sub.D1<50.degree. (36)
[0212] In this case, it is desirable that the reflection angle at
the third reflecting surface of the prism should satisfy the
following condition:
10.degree.<.theta..sub.D3<50.degree. (37)
[0213] where .theta..sub.D 3 is the reflection angle at the third
reflecting surface.
[0214] In Example 1 (described later), .theta..sub.D3 is
29.3.degree.. It is desirable for the third reflecting surface of
the prism to satisfy the condition (37). If .theta..sub.D3 is not
smaller than the upper limit of the condition (37), i.e.
50.degree., rays are totally reflected by the second transmitting
surface, failing to pass therethrough. If .theta..sub.D3 is not
larger than the lower limit, i.e. 10.degree., the amount of
decentration aberrations produced by the third reflecting surface
becomes large. This is unfavorable from the viewpoint of
performance.
[0215] It is preferable to satisfy the following condition:
20.degree.<.theta..sub.D3<40.degree. (38)
[0216] In addition, it is desirable that the exit angle of the
prism with respect to the optical axis entering the objective
optical system should satisfy the following condition:
20.degree.<.phi..sub.D<60.degree. (39)
[0217] where .phi..sub.D is the exit angle of the prism with
respect to the optical axis entering the objective optical
system.
[0218] In Example 1 (described later), .phi..sub.D is 40.5.degree..
It is desirable for the exit angle of the prism to satisfy the
condition (39). If .phi..sub.D is not smaller than the upper limit
of the condition (39), i.e. 60.degree., the prism and the objective
optical system interfere with each other undesirably. If
.phi..sub.D is not larger than the lower limit, i.e. 20.degree., it
becomes necessary to increase the reflection angle at each surface.
This is unfavorable from the viewpoint of performance.
[0219] It is preferable to satisfy the following condition:
30.degree.<.phi..sub.D<50.degree. (40)
[0220] In addition, the present invention provides a twelfth finder
optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism includes three reflecting surfaces. At least one of the
reflecting surfaces has a rotationally asymmetric surface
configuration. The first reflecting surface and third reflecting
surface of the prism are independent of other transmitting and
reflecting surfaces. The second reflecting surface of the prism is
formed from the identical surface with the first and second
transmitting surfaces of the prism. The prism has an optical path
in which the axial principal ray or a projective axial principal
ray defined by projecting the axial principal ray onto a plane
containing a part of the axial principal ray bends at each of the
three reflecting surfaces in a direction different from the
direction of bending at the preceding reflecting surface with
respect to the travel direction of light rays.
[0221] The function of the twelfth finder optical system will be
described below.
[0222] The prism in the twelfth finder optical system is very thin
in thickness despite the fact that it has three reflecting
surfaces. Moreover, the prism has an optical path in which the
axial principal ray bends at each of the three reflecting surfaces
in a direction different from the direction of bending at the
preceding reflecting surface. Accordingly, the optical path extends
in the lateral direction. Thus, the prism is unlikely to interfere
with the objective optical system or the ocular optical system. For
this reason, the reflection angle at each reflecting surface can be
reduced, which is favorable from the viewpoint of performance. In
addition, because total reflection takes place at one surface, the
prism arrangement is favorable in terms of light quantity.
[0223] In this case, it is desirable that the first reflecting
surface of the prism should have a negative power.
[0224] Because the second reflecting surface is the identical with
the first transmitting surface, the second reflecting surface needs
to have a total reflection critical angle. Therefore, if a strong
power is given to the second reflecting surface, the amount of
decentration aberrations produced by this surface becomes
unfavorably large. The third reflecting surface is not very
effective in correcting aberrations because the axial ray height is
not very high at this surface. Accordingly, giving a strong power
to the remaining reflecting surface (i.e. the first reflecting
surface) is favorable from the viewpoint of aberration correcting
performance. On the other hand, it is preferable to give a negative
power to a surface of the prism from the viewpoint of attaining a
reduction in thickness. Accordingly, it is preferable to give a
negative power to the first reflecting surface.
[0225] In addition, it is desirable that the reflection angle at
the first reflecting surface of the prism should satisfy the
following condition:
15.degree.<.theta..sub.E1<45.degree. (41)
[0226] where .theta..sub.E1 is the reflection angle at the first
reflecting surface.
[0227] In Example 5 (described later), .theta..sub.E1 is
30.4.degree.. It is desirable for the first reflecting surface of
the prism to satisfy the condition (41). If .theta..sub.E1 is not
smaller than the upper limit of the condition (41), i.e.
45.degree., the amount of decentration aberrations produced by the
first reflecting surface becomes large, which is unfavorable from
the viewpoint of performance. If .theta..sub.E1 is not larger than
the lower limit, i.e. 15.degree., it becomes difficult for rays to
be totally reflected by the second reflecting surface.
[0228] It is preferable to satisfy the following condition:
20.degree.<.theta..sub.E1<40.degree. (42)
[0229] In addition, it is desirable that the reflection angle at
the third reflecting surface of the prism should satisfy the
following condition:
15.degree.<.theta..sub.E3<45.degree. (43)
[0230] where .theta..sub.E3 is the reflection angle at the third
reflecting surface.
[0231] In Example 5 (described later), .theta..sub.E3 is
30.7.degree.. It is desirable for the third reflecting surface of
the prism to satisfy the condition (43). If .theta..sub.E3 is not
smaller than the upper limit of the condition (43), i.e.
45.degree., the amount of decentration aberrations produced by the
third reflecting surface becomes large, which is unfavorable from
the viewpoint of performance. If .theta..sub.E3 is not larger than
the lower limit, i.e. 15.degree., it becomes difficult for rays to
be totally reflected by the second reflecting surface.
[0232] It is preferable to satisfy the following condition:
20.degree.<.theta..sub.E3<40.degree. (44)
[0233] In addition, it is desirable that the exit angle of the
prism with respect to the optical axis entering the objective
optical system should satisfy the following condition:
160.degree.<.phi..sub.E<200.degree. (45)
[0234] where .phi..sub.E is the exit angle of the prism with
respect to the optical axis entering the objective optical
system.
[0235] In Example 5 (described later), .phi..sub.E is
180.degree..
[0236] In the prism of this finder optical system, the first and
second transmitting surfaces are the identical surface. Therefore,
if the positional relationship between the entering optical axis
and the exiting optical axis deviates from being parallel,
decentration aberrations are produced in the prism undesirably.
Accordingly, it is desirable for the exit angle of the prism to
satisfy the condition (45). If .phi..sub.E is not smaller than the
upper limit of the condition (45), i.e. 200.degree., or not larger
than the lower limit, i.e. 160.degree., the amount of decentration
aberrations produced in the prism becomes large, which is
unfavorable from the viewpoint of performance.
[0237] It is preferable to satisfy the following condition:
170.degree.<.phi..sub.E<190.degree. (46)
[0238] In addition, the present invention provides a thirteenth
finder optical system that includes, in order from the object side
thereof: an objective optical system having a positive refracting
power; an image-inverting optical system for erecting an
intermediate image formed by the objective optical system; and an
ocular optical system having a positive refracting power. The
objective optical system has at least two movable units moving when
zooming is performed. A prism is placed on the object side of the
intermediate image formed by the objective optical system. The
prism includes three reflecting surfaces. At least one of the
reflecting surfaces has a rotationally asymmetric surface
configuration. The three reflecting surfaces are all independent of
other transmitting and reflecting surfaces. The prism has an
optical path in which the axial principal ray or a projective axial
principal ray defined by projecting the axial principal ray onto a
plane containing a part of the axial principal ray bends in the
same direction with respect to the travel direction of light rays
at the first reflecting surface and the second reflecting surface
and is twisted by the third reflecting surface.
[0239] The function of the thirteenth finder optical system will be
described below.
[0240] By providing a prism incorporating one reflection as a
component of the ocular optical system, it becomes possible to
obtain an erect image without using a roof surface, which requires
relatively strict manufacturing accuracy. In addition, because two
transmitting surfaces and three reflecting surfaces of the prism
can be formed from independent surface configurations,
respectively, the prism arrangement is favorable from the viewpoint
of performance.
[0241] In the thirteenth finder optical system, it is desirable
that the reflection angle at the first reflecting surface of the
prism should satisfy the following condition:
30.degree.<.theta..sub.F1<60.degree. (47)
[0242] where .theta..sub.F1 is the reflection angle at the first
reflecting surface.
[0243] In Example 6 (described later), .theta..sub.F1 is
45.degree.. It is desirable for the first reflecting surface of the
prism to satisfy the condition (47). If .theta..sub.F1 is not
smaller than the upper limit of the condition (47), i.e.
60.degree., the second reflecting surface is placed closer to the
pupil. Consequently, it becomes impossible to attain a reduction in
thickness. If .theta..sub.F1 is not larger than the lower limit,
i.e. 30.degree., the third reflecting surface interferes with the
objective optical system undesirably.
[0244] It is preferable to satisfy the following condition:
35.degree.<.theta..sub.F1<55.degree. (48)
[0245] In addition, it is desirable that the reflection angle at
the third reflecting surface of the prism should satisfy the
following condition:
30.degree.<.theta..sub.F3<60.degree. (49)
[0246] where .theta..sub.F3 is the reflection angle at the third
reflecting surface.
[0247] In Example 6 (described later), .theta..sub.F3 is
45.degree.. It is desirable for the third reflecting surface of the
prism to satisfy the condition (49). If .theta..sub.F3 is not
smaller than the upper limit of the condition (49), i.e.
60.degree., the third reflecting surface overlaps the effective
portion of the second reflecting surface. If .theta..sub.F3 is not
larger than the lower limit, i.e. 30.degree., the amount of
decentration aberrations produced by the third reflecting surface
becomes large. This is unfavorable from the viewpoint of
performance.
[0248] It is preferable to satisfy the following condition:
35.degree.<.theta..sub.F3<55.degree. (50)
[0249] In addition, it is desirable that the exit angle of the
prism with respect to the optical axis entering the objective
optical system should satisfy the following condition:
70.degree.<.phi..sub.F<110.degree. (51)
[0250] where .phi..sub.F is the exit angle of the prism with
respect to the optical axis entering the objective optical
system.
[0251] In Example 6 (described later), .phi..sub.F is 90.degree..
It is desirable for the exit angle of the prism to satisfy the
condition (51). If .phi..sub.F is not smaller than the upper limit
of the condition (51), i.e. 110.degree., or not larger than the
lower limit, i.e. 70.degree., the amount to which the ocular
optical system extends in the direction of the entering optical
axis increases. Consequently, it becomes impossible to attain a
reduction in thickness.
[0252] It is preferable to satisfy the following condition:
80.degree.<.phi..sub.F<100.degree. (52)
[0253] In any of the above-described finder optical systems, it is
desirable that the prism should be placed closer to the pupil than
any of the movable units.
[0254] The back focus of an optical system can be shortened by
selecting an appropriate lens type. However, it is difficult to
reduce the back focus to zero. That is, there is a limit to the
reduction of the back focus. In a zoom lens system in particular,
it is difficult to shorten the back focus because the focal length
is long at the telephoto end. In the case of the objective system
of a finder optical system, a reduction in size cannot be attained
unless the space for the back focus is effectively disposed.
Therefore, it is preferable in the present invention that the
optical path should be folded by the prism to minimize the space
required for the back focus. Accordingly, it is desirable that the
prism should be placed closer to the pupil than any of the movable
units having a zooming function.
[0255] Still other objects and advantages of the invention will in
part be obvious and will in part be apparent from the
specification.
[0256] The invention accordingly comprises the features of
construction, combinations of elements, and arrangement of parts
which will be exemplified in the construction hereinafter set
forth, and the scope of the invention will be indicated in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0257] FIG. 1 is a sectional view of a finder optical system
according to Example 1 of the present invention at the wide-angle
end.
[0258] FIG. 2 is a sectional view of the finder optical system
according to Example 1 at the standard position.
[0259] FIG. 3 is a sectional view of the finder optical system
according to Example 1 at the telephoto end.
[0260] FIG. 4 is a sectional view of a finder optical system
according to Example 2 of the present invention at the wide-angle
end.
[0261] FIG. 5 is a sectional view of a finder optical system
according to Example 3 of the present invention at the wide-angle
end.
[0262] FIG. 6 is a sectional view of a finder optical system
according to Example 4 of the present invention at the wide-angle
end.
[0263] FIG. 7 is a sectional view of a finder optical system
according to Example 5 of the present invention at the wide-angle
end.
[0264] FIG. 8 is a sectional view of a finder optical system
according to Example 6 of the present invention at the wide-angle
end.
[0265] FIG. 9 is a diagram schematically showing a modification of
the finder optical system according to Example 2.
[0266] FIG. 10 is a diagram schematically showing a modification of
the finder optical system according to Example 3.
[0267] FIG. 11 is a diagram schematically showing a modification of
the finder optical system according to Example 3.
[0268] FIG. 12 is a diagram schematically showing a modification of
the finder optical system according to Example 4.
[0269] FIG. 13 is a diagram schematically showing a modification of
the finder optical system according to Example 5.
[0270] FIG. 14 is a diagram schematically showing a modification of
the finder optical system according to Example 6.
[0271] FIG. 15 is an aberrational diagram illustrating lateral
aberrations produced at the wide-angle end in Example 1.
[0272] FIG. 16 is an aberrational diagram illustrating lateral
aberrations produced at the standard position in Example 1.
[0273] FIG. 17 is an aberrational diagram illustrating lateral
aberrations produced at the telephoto end in Example 1.
[0274] FIG. 18 is a diagram for describing an electronic camera
having a finder optical system according to the present
invention.
[0275] FIG. 19 is a ray path diagram showing an optical system of
the electronic camera in FIG. 18.
[0276] FIG. 20 is a diagram for describing problems associated with
a prism in which an optical path crosses itself.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0277] Numerical Examples 1 to 6 of the finder optical system
according to the present invention will be described below. It
should be noted that constituent parameters of each example will be
shown later.
[0278] In each Example, as shown in FIGS. 1 to 8, the center of a
specific plane (hypothetic plane of surface No. 5: HRP) of the
optical system is defined as the origin of a decentered optical
system, and an axial principal ray is defined by a ray emanating
from the center of an object (not shown in the figures) and passing
through the center of a stop (eye point: EP). A Z-axis is taken in
the direction in which the axial principal ray travels from the
object center to the first surface of the optical system. A plane
containing the Z-axis and the center of an image plane formed by
the objective optical system is defined as a YZ-plane. A Y-axis is
taken in a direction in a plane in which rays are folded by the
surfaces of the optical system, perpendicularly intersecting the
Z-axis in the YZ-plane. The direction in which the Z-axis extends
from the object point toward the first surface of the optical
system is defined as a positive direction of the Z-axis. The upward
direction as viewed in the figures is defined as a positive
direction of the Y-axis. An axis that constitutes a right-handed
orthogonal coordinate system in combination with the Y- and Z-axes
is defined as an X-axis.
[0279] In Examples 1 to 5, decentration of each surface is made in
the YZ-plane, and one and only plane of symmetry of each
rotationally asymmetric free-form surface is the YZ-plane. In
Example 6, the third reflecting surface is decentered in the
XY-plane, and each rotationally asymmetric free-form surface has no
plane of symmetry.
[0280] Regarding decentered surfaces, each surface is given
displacements in the X-, Y- and Z-axis directions (X, Y and Z,
respectively) of the vertex position of the surface from the origin
of the optical system, and tilt angles (degrees) of the center axis
of the surface [the Z-axis of the following equation (a) in regard
to free-form surfaces] with respect to the X-, Y- and Z-axes
(.alpha., .beta. and .gamma., respectively). In this case, positive
.alpha. and .beta. mean counterclockwise rotation relative to the
positive directions of the corresponding axes, and positive .gamma.
means clockwise rotation relative to the positive direction of the
Z-axis.
[0281] Among optical functional surfaces constituting the optical
system in each example, a specific surface and a surface subsequent
thereto are given a surface separation when these surfaces form a
coaxial optical system. In addition, the refractive index and
Abbe's number of each medium are given according to the
conventional method.
[0282] It should be noted that those terms concerning aspherical
surfaces for which no data is shown are zero. The refractive index
is expressed by the refractive index for the spectral d-line
(wavelength: 587.56 nanometers). Lengths are given in
millimeters.
[0283] The configuration of a free-form surface used in the present
invention is defined by the following equation (a). The Z-axis of
the defining equation is the axis of the free-form surface. 1 Z =
cr 2 / [ 1 + { 1 - ( 1 + k ) c 2 r 2 } ] + j = 2 66 C j X m Y n ( a
)
[0284] In the equation (a), the first term is a spherical surface
term, and the second term is a free-form surface term.
[0285] In the spherical surface term:
[0286] c: the curvature at the vertex
[0287] k: a conic constant
[0288] r={square root}(X.sup.2+Y.sup.2)
[0289] The free-form surface term is given by 2 j = 2 66 C j X m Y
n = C 2 X + C 3 Y + C 4 X 2 + C 5 XY + C 6 Y 2 + C 7 X 3 + C 8 X 2
Y + C 9 XY 2 + C 10 Y 3 + C 11 X 4 + C 12 X 3 Y + C 13 X 2 Y 2 + C
14 XY 3 + C 15 Y 4 + C 16 X 5 + C 17 X 4 Y + C 18 X 3 Y 2 + C 19 X
2 Y 3 + C 20 XY 4 + C 21 Y 5 + C 22 X 6 + C 23 X 5 Y + C 24 X 4 Y 2
+ C 25 X 3 Y 3 + C 26 X 2 Y 4 + C 27 XY 5 + C 28 Y 6 + C 29 X 7 + C
30 X 6 Y + C 31 X 5 Y 2 + C 32 X 4 Y 3 + C 33 X 3 Y 4 + C 34 X 2 Y
5 + C 35 XY 6 + C 36 Y 7
[0290] where C.sub.j (j is an integer of 2 or higher) are
coefficients.
[0291] In general, the above-described free-form surface does not
have planes of symmetry in both the XZ- and YZ-planes. In the
present invention, however, a free-form surface having only one
plane of symmetry parallel to the YZ-plane is obtained by making
all terms of odd-numbered degrees with respect to X zero. For
example, in the above defining equation (a), the coefficients of
the terms C.sub.2, C.sub.5, C.sub.7, C.sub.9, C.sub.12, C.sub.14,
C.sub.16, C.sub.18, C.sub.20, C.sub.23, C.sub.25, C.sub.27,
C.sub.29, C.sub.31, C.sub.33, C.sub.35, . . . are set equal to
zero. By doing so, it is possible to obtain a free-form surface
having only one plane of symmetry parallel to the YZ-plane.
[0292] A free-form surface having only one plane of symmetry
parallel to the XZ-plane is obtained by making all terms of
odd-numbered degrees with respect to Y zero. For example, in the
above defining equation (a), the coefficients of the terms C.sub.3,
C.sub.5, C.sub.8, C.sub.10, C.sub.12, C.sub.14, C.sub.17, C.sub.19,
C.sub.21, C.sub.23, C.sub.25, C.sub.27, C.sub.30, C.sub.32,
C.sub.34, C.sub.36, . . . are set equal to zero. By doing so, it is
possible to obtain a free-form surface having only one plane of
symmetry parallel to the XZ-plane.
[0293] Free-form surfaces may also be defined by Zernike
polynomials. That is, the configuration of a free-form surface may
be defined by the following equation (b). The Z-axis of the
defining equation (b) is the axis of Zernike polynomial. A
rotationally asymmetric surface is defined by polar coordinates of
the height of the Z-axis with respect to the XY-plane. In the
equation (b), A is the distance from the Z-axis in the XY-plane,
and R is the azimuth angle about the Z-axis, which is expressed by
the angle of rotation measured from the Z-axis. 3 x = R .times. cos
( A ) y = R .times. sin ( A ) Z = D 2 + D 3 R cos ( A ) + D 4 R sin
( A ) + D 5 R 2 cos ( 2 A ) + D 6 ( R 2 - 1 ) + D 7 R 2 sin ( 2 A )
+ D 8 R 3 cos ( 3 A ) + D 9 ( 3 R 3 - 2 R ) cos ( A ) + D 10 ( 3 R
3 - 2 R ) sin ( A ) + D 11 R 3 sin ( 3 A ) + D 12 R 4 cos ( 4 A ) +
D 13 ( 4 R 4 - 3 R 2 ) cos ( 2 A ) + D 14 ( 6 R 4 - 6 R 2 + 1 ) + D
15 ( 4 R 4 - 3 R 2 ) sin ( 2 A ) + D 16 R 4 sin ( 4 A ) + D 17 R 5
cos ( 5 A ) + D 18 ( 5 R 5 - 4 R 3 ) cos ( 3 A ) + D 19 ( 10 R 5 -
12 R 3 + 3 R ) cos ( A ) + D 20 ( 10 R 5 - 12 R 3 + 3 R ) sin ( A )
+ D 21 ( 5 R 5 - 4 R 3 ) sin ( 3 A ) + D 22 R 5 sin ( 5 A ) + D 23
R 6 cos ( 6 A ) + D 24 ( 6 R 6 - 5 R 4 ) cos ( 4 A ) + D 25 ( 15 R
6 - 20 R 4 + 6 R 2 ) cos ( 2 A ) + D 26 ( 20 R 6 - 30 R 4 + 12 R 2
- 1 ) + D 27 ( 15 R 6 - 20 R 4 + 6 R 2 ) sin ( 2 A ) + D 28 ( 6 R 6
- 5 R 4 ) sin ( 4 A ) + D 29 R 6 sin ( 6 A ) ( b )
[0294] In the above equation, to design an optical system symmetric
with respect to the X-axis direction, D.sub.4, D.sub.5, D.sub.6,
D.sub.10, D.sub.11, D.sub.12, D.sub.13, D.sub.14, D.sub.20,
D.sub.21, D.sub.22 should be used.
[0295] Other examples of surfaces usable in the present invention
are expressed by the following defining equation (c):
Z=.SIGMA..SIGMA.C.sub.nmXY
[0296] Assuming that k=7 (polynomial of degree 7), for example, a
free-form surface is expressed by an expanded form of the above
equation as follows: 4 Z = C 2 + C 3 y + C 4 x + C 5 y 2 + C 6 y x
+ C 7 x 2 + C 8 y 3 + C 9 y 2 x + C 10 yx 2 + C 11 x 3 + C 12 y 4 +
C 13 y 3 x + C 14 y 2 x 2 + C 15 y x 3 + C 16 x 4 + C 17 y 5 + C 18
y 4 x + C 19 y 3 x 2 + C 20 y 2 x 3 + C 21 yx 4 + C 22 x 5 + C 23 y
6 + C 24 y 5 x + C 25 y 4 x 2 + C 26 y 3 x 3 + C 27 y 2 x 4 + C 28
y x 5 + C 29 x 6 + C 30 y 7 + C 31 y 6 x + C 32 y 5 x 2 + C 33 y 4
x 3 + C 34 y 3 x 4 + C 35 y 2 x 5 + C 36 yx 6 + C 37 x 7 ( c )
[0297] Although in the examples of the present invention the
surface configuration is expressed by a free-form surface using the
above equation (a), it should be noted that the same advantageous
effect can be obtained by using the above equation (b) or (c).
[0298] Aspherical surfaces are rotationally symmetric aspherical
surfaces given by the following equation: 5 Z = ( y 2 / R ) / [ 1 +
{ 1 - ( 1 + K ) y 2 / R 2 } 1 / 2 ] + Ay 4 + By 6 + Cy 8 + Dy 10 +
( d )
[0299] In the above equation, Z is an optical axis (axial principal
ray) for which the direction of travel of light is defined as a
positive direction, and y is taken in a direction perpendicular to
the optical axis. R is a paraxial curvature radius, K is a conic
constant, and A, B, C, D . . . are 4th-, 6th-, 8th- and 10th-order
aspherical coefficients, respectively. The Z-axis of this defining
equation is the axis of the rotationally symmetric aspherical
surface.
[0300] Although the prisms and refracting lenses in Examples 1 to 6
are formed by using a plastic material, it should be noted that
these optical elements may be made of glass. When a plastic
material is used, in particular, it is desirable to use a material
of low moisture absorption because performance degradation caused
by environmental changes is minimized by the use of such a
material.
[0301] Of the surfaces of the refracting lenses of the objective
and ocular optical systems, the spherical surfaces may be changed
to rotationally symmetric aspherical surfaces. Similarly, the
spherical or aspherical surfaces may be changed to rotationally
asymmetric free-form surfaces.
EXAMPLE 1
[0302] FIGS. 1 to 3 are sectional views of Example 1 taken along
the YZ-plane containing the axial principal ray. FIG. 1 is a
sectional view of Example 1 at the wide-angle end. FIG. 2 is a
sectional view of Example 1 at the standard position. FIG. 3 is a
sectional view of Example 1 at the telephoto end.
[0303] In Example 1, the horizontal half field angle is
14.19.degree., and the vertical half field angle is 21.36.degree..
The pupil diameter is 4 millimeters. The size of the intermediate
image is 2.04.times.2.96 millimeters (the diagonal image height:
3.59 millimeters). The finder magnification is 0.4 to 0.96 times.
In terms of the focal length of a rotationally symmetric optical
system, the focal length of the objective optical system is
equivalent to 8.43 millimeters to 20.26 millimeters. The focal
length of the ocular optical system is 21.0 millimeters. The
displacements of each of the surface Nos. 6 to 16 are expressed by
the amounts of displacement from the decentration reference plane
of surface No. 5 (hypothetic plane: HRP).
[0304] The finder optical system according to Example 1 has, in
order in which light passes from the object side, an objective
optical system Ob, a field mask M for defining the visual field,
and an ocular optical system Oc. The objective optical system Ob
includes a lens group having a negative first unit G1 consisting
essentially of a biconcave lens and a positive second unit G2
consisting essentially of a biconvex lens. The objective optical
system Ob further includes a first prism 10 of negative power which
has a first transmitting surface 11 of negative power, a first
reflecting surface 12 of negative power, a second reflecting
surface 13 of positive power, a third reflecting surface 14 of
negative power, and a second transmitting surface 15 of positive
power. The ocular optical system Oc includes a second prism 20
having a first transmitting surface 21 of positive power, a first
reflecting surface 22, a second reflecting surface 23 formed from a
roof surface, and a second transmitting surface 24. The ocular
optical system Oc further includes a biconvex positive lens OL.
During zooming, the first unit G1 and the second unit G2 move. The
prism 10, which forms the third unit, is stationary. It should be
noted that in FIGS. 1 to 3, the eye point is denoted by EP (the
same shall apply hereinafter).
[0305] In the first prism 10 of Example 1, the first reflecting
surface 12, the third reflecting surface 14 and the first
transmitting surface 11 are all independent optical functional
surfaces that are separate from other surfaces. The second
transmitting surface 15 and the second reflecting surface 13 are
formed from the identical optical functional surface having both
transmitting and reflecting actions. The first transmitting surface
11, the second transmitting surface 15, the first reflecting
surface 12, the second reflecting surface 13 and the third
reflecting surface 14 are all formed from rotationally asymmetric
free-form surfaces. The first prism 10 has an optical path in which
the axial principal ray bends in the same direction at the first
reflecting surface 12 and the second reflecting surface 13 with
respect to the travel direction of light and further bends in the
opposite direction to the above at the third reflecting surface
14.
[0306] Although the reflecting surfaces 22 and 23 of the second
prism 20 in this example are formed from plane surfaces, these
surfaces may also be formed from free-form surfaces. The
arrangement may be such that the second transmitting surface 24 and
the first reflecting surface 22 of the second prism 20 are formed
from independent surfaces, and a roof surface is provided on the
first reflecting surface 22.
EXAMPLE 2
[0307] FIG. 4 is a sectional view of Example 2 taken along the
YZ-plane containing the axial principal ray. FIG. 4 is a sectional
view of Example 2 at the wide-angle end. Illustration of sectional
views showing Example 2 at the standard position and the telephoto
end is omitted (the same shall apply to Examples 3 to 6).
[0308] In Example 2, the horizontal half field angle is
14.19.degree., and the vertical half field angle is 21.36.degree..
The pupil diameter is 4 millimeters. The size of the intermediate
image is 2.04.times.2.96 millimeters (the diagonal image height:
3.59 millimeters). The finder magnification is 0.4 to 0.96 times.
In terms of the focal length of a rotationally symmetric optical
system, the focal length of the objective optical system is
equivalent to 8.43 millimeters to 20.26 millimeters. The focal
length of the ocular optical system is 21.0 millimeters. The
displacements of each of the surface Nos. 6 to 10 are expressed by
the amounts of displacement from the decentration reference plane
of surface No. 5 (hypothetic plane: HRP).
[0309] The finder optical system according to Example 2 has an
objective optical system Ob, a field mask M for defining the visual
field, and an ocular optical system Oc. The objective optical
system Ob includes a lens group having a negative first unit G1
consisting essentially of a biconcave lens and a positive second
unit G2 consisting essentially of a biconvex lens. The objective
optical system Ob further includes a prism 10 of negative power
which has a first transmitting surface 11 with a positive power in
the X-direction and a negative power in the Y-direction, a first
reflecting surface 12 of negative power, a second reflecting
surface 13 with a positive power in the X-direction and a negative
power in the Y-direction, and a second transmitting surface 14 of
positive power. The ocular optical system Oc is formed from an
ideal lens OCL. During zooming, the first unit G1 and the second
unit G2 move. The prism 10, which forms the third unit, is
stationary.
[0310] In the prism 10 of Example 2, the first reflecting surface
12 and the second transmitting surface 14 are both independent
optical functional surfaces that are separate from other surfaces.
The first transmitting surface 11 and the second reflecting surface
13 are formed from the identical optical functional surface having
both transmitting and reflecting actions. The first transmitting
surface 11, the second transmitting surface 14, the first
reflecting surface 12 and the second reflecting surface 13 are all
formed from rotationally asymmetric free-form surfaces. The prism
10 has an optical path in which the axial principal ray bends in
opposite directions to each other at the first reflecting surface
12 and the second reflecting surface 13 with respect to the travel
direction of light.
[0311] It is preferable to place an image-inverting optical system
on the pupil side of the intermediate image formation plane as
shown in the schematic view of FIG. 9. By doing so, a thin finder
optical system can be attained. In this case, the second prism 20
constitutes an image-inverting optical system, and it is necessary
to provide a roof surface for inverting an image on the second
reflecting surface 23. The transmitting surfaces and reflecting
surfaces of the image-inverting optical system may also be formed
from free-form surfaces.
EXAMPLE 3
[0312] FIG. 5 is a sectional view of Example 3 taken along the
YZ-plane containing the axial principal ray. In Example 3, the
horizontal half field angle is 14.19.degree., and the vertical half
field angle is 21.36.degree.. The pupil diameter is 4 millimeters.
The size of the intermediate image is 2.04.times.2.96 millimeters
(the diagonal image height: 3.59 millimeters). The finder
magnification is 0.4 to 0.96 times. In terms of the focal length of
a rotationally symmetric optical system, the focal length of the
objective optical system is equivalent to 8.43 millimeters to 20.26
millimeters. The focal length of the ocular optical system is 21.0
millimeters. The displacements of each of the surface Nos. 6 to 10
are expressed by the amounts of displacement from the decentration
reference plane of surface No. 5 (hypothetic plane: HRP).
[0313] The finder optical system according to Example 3 has, in
order in which light passes from the object side, an objective
optical system Ob, a field mask M for defining the visual field,
and an ocular optical system Oc. The objective optical system Ob
includes a lens group having a negative first unit G1 consisting
essentially of a biconcave lens and a positive second unit G2
consisting essentially of a biconvex lens. The objective optical
system Ob further includes a prism 10 of positive power which has a
first transmitting surface 11 of negative power, a first reflecting
surface 12 of positive power, a second reflecting surface 13 with a
positive power in the X-direction and a negative power in the
Y-direction, and a second transmitting surface 14 of positive
power. The ocular optical system Oc is formed from an ideal lens
OCL. During zooming, the first unit G1 and the second unit G2 move.
The prism 10, which forms the third unit, is stationary.
[0314] In the prism 10 of Example 3, the first reflecting surface
12, the second reflecting surface 13, the first transmitting
surface 11 and the second transmitting surface 14 are all
independent optical functional surfaces that are separate from
other surfaces. The first transmitting surface 11, the second
transmitting surface 14, the first reflecting surface 12 and the
second reflecting surface 13 are all formed from rotationally
asymmetric free-form surfaces. The prism 10 has an optical path in
which the axial principal ray bends in the same direction at the
first reflecting surface 12 and the second reflecting surface 13
with respect to the travel direction of light.
[0315] It is preferable to place an image-inverting optical system
on the pupil side of the intermediate image formation plane as
shown in the schematic view of FIG. 10 or 11. By doing so, a thin
finder optical system can be attained. In the case of FIG. 10, the
second prism 20 constitutes an image-inverting optical system, and
it is necessary to provide a roof surface for inverting an image on
the first reflecting surface 22 or the third transmitting surface
24. The transmitting surfaces and reflecting surfaces of the
image-inverting optical system may also be formed from free-form
surfaces. Part (a) of FIG. 11 is a side view, and part (b) of FIG.
11 is a top plan view. The second prism 20 constitutes an
image-inverting optical system. The transmitting surfaces and
reflecting surfaces of the image-inverting optical system may also
be formed from free-form surfaces.
EXAMPLE 4
[0316] FIG. 6 is a sectional view of Example 4 taken along the
YZ-plane containing the axial principal ray. In Example 4, the
horizontal half field angle is 14.19.degree., and the vertical half
field angle is 21.36.degree.. The pupil diameter is 4 millimeters.
The size of the intermediate image is 2.04.times.2.96 millimeters
(the diagonal image height: 3.59 millimeters). The finder
magnification is 0.4 to 0.96 times. In terms of the focal length of
a rotationally symmetric optical system, the focal length of the
objective optical system is equivalent to 8.43 millimeters to 20.26
millimeters. The focal length of the ocular optical system is 21.0
millimeters. The displacements of each of the surface Nos. 6 to 11
are expressed by the amounts of displacement from the decentration
reference plane of surface No. 5 (hypothetic plane: HRP).
[0317] The finder optical system according to Example 4 has, in
order in which light passes from the object side, an objective
optical system Ob, a field mask M for defining the visual field,
and an ocular optical system Oc. The objective optical system Ob
includes a lens group having a negative first unit G1 consisting
essentially of a biconcave lens and a positive second unit G2
consisting essentially of a biconvex lens. The objective optical
system Ob further includes a prism 10 of negative power which has a
first transmitting surface 11 of negative power, a first reflecting
surface 12 of positive power, a second reflecting surface 13 of
positive power, a third reflecting surface 14 of negative power,
and a second transmitting surface 15 of positive power. The ocular
optical system Oc is formed from an ideal lens OCL. During zooming,
the first unit G1 and the second unit G2 move. The prism 10, which
forms the third unit, is stationary.
[0318] In the prism 10 of Example 4, the first reflecting surface
12, the second reflecting surface 13, the third reflecting surface
14, the first transmitting surface 11 and the second transmitting
surface 15 are all independent optical functional surfaces that are
separate from other surfaces. The first transmitting surface 11,
the second transmitting surface 15, the first reflecting surface
12, the second reflecting surface 13 and the third reflecting
surface 14 are all formed from rotationally asymmetric free-form
surfaces. The prism 10 has an optical path in which the axial
principal ray bends in the same direction at the first reflecting
surface 12 and the second reflecting surface 13 with respect to the
travel direction of light and further bends in the opposite
direction to the above at the third reflecting surface 14.
[0319] It is preferable to place an image-inverting optical system
on the pupil side of the intermediate image formation plane as
shown in the schematic view of FIG. 12. By doing so, a thin finder
optical system can be attained. In this case, the second prism 20
constitutes an image-inverting optical system, and it is necessary
to provide a roof surface for inverting an image on one of the
surfaces thereof. The transmitting surfaces and reflecting surfaces
of the image-inverting optical system may also be formed from
free-form surfaces.
EXAMPLE 5
[0320] FIG. 7 is a sectional view of Example 5 taken along the
YZ-plane containing the axial principal ray. In Example 5, the
horizontal half field angle is 14.19.degree., and the vertical half
field angle is 21.36.degree.. The pupil diameter is 4 millimeters.
The size of the intermediate image is 2.04.times.2.96 millimeters
(the diagonal image height: 3.59 millimeters). The finder
magnification is 0.4 to 0.96 times. In terms of the focal length of
a rotationally symmetric optical system, the focal length of the
objective optical system is equivalent to 8.43 millimeters to 20.26
millimeters. The focal length of the ocular optical system is 21.0
millimeters. The displacements of each of the surface Nos. 6 to 11
are expressed by the amounts of displacement from the decentration
reference plane of surface No. 5 (hypothetic plane: HRP).
[0321] The finder optical system according to Example 5 has, in
order in which light passes from the object side, an objective
optical system Ob, a field mask M for defining the visual field,
and an ocular optical system Oc. The objective optical system Ob
includes a lens group having a negative first unit G1 consisting
essentially of a biconcave lens and a positive second unit G2
consisting essentially of a biconvex lens. The objective optical
system Ob further includes a prism 10 of positive power which has a
first transmitting surface 11 of positive power, a first reflecting
surface 12 with a positive power in the X-direction and a negative
power in the Y-direction, a second reflecting surface 13 of
positive power, a third reflecting surface 14 with a negative power
in the X-direction and a positive power in the Y-direction, and a
second transmitting surface 15 of positive power. The ocular
optical system Oc is formed from an ideal lens OCL. During zooming,
the first unit G1 and the second unit G2 move. The prism 10, which
forms the third unit, is stationary.
[0322] In the prism 10 of Example 5, the first reflecting surface
12 and the third reflecting surface 14 are both independent optical
functional surfaces that are separate from other surfaces. The
first transmitting surface 11, the second transmitting surface 15
and the second reflecting surface 13 are formed from the identical
optical functional surface having both transmitting and reflecting
actions. The first transmitting surface 11, the second transmitting
surface 15, the first reflecting surface 12, the second reflecting
surface 13 and the third reflecting surface 14 are all formed from
rotationally asymmetric free-form surfaces. The prism 10 has an
optical path in which the axial principal ray bends in the opposite
directions to each other successively at the first reflecting
surface 12, the second reflecting surface 13 and the third
reflecting surface 14 with respect to the travel direction of
light.
[0323] It is preferable to place an image-inverting optical system
on the pupil side of the intermediate image formation plane as
shown in the schematic view of FIG. 13. By doing so, a thin finder
optical system can be attained. In this case, the second prism 20
constitutes an image-inverting optical system, and it is necessary
to provide a roof surface for inverting an image on one of the
surfaces thereof. The transmitting surfaces and reflecting surfaces
of the image-inverting optical system may also be formed from
free-form surfaces.
EXAMPLE 6
[0324] Part (a) of FIG. 8 is a sectional view of Example 6 taken
along the YZ-plane containing the axial principal ray. Part (b) of
FIG. 8 is a sectional view of Example 6 taken along the XZ-plane
containing the axial principal ray.
[0325] In Example 6, the horizontal half field angle is
14.19.degree., and the vertical half field angle is 21.36.degree..
The pupil diameter is 4 millimeters. The size of the intermediate
image is 2.04.times.2.96 millimeters (the diagonal image height:
3.59 millimeters). The finder magnification is 0.4 to 0.96 times.
In terms of the focal length of a rotationally symmetric optical
system, the focal length of the objective optical system is
equivalent to 8.43 millimeters to 20.26 millimeters. The focal
length of the ocular optical system is 21.0 millimeters. The
displacements of each of the surface Nos. 6 to 11 are expressed by
the amounts of displacement from the decentration reference plane
of surface No. 5 (hypothetic plane: HRP).
[0326] The finder optical system according to Example 6 has, in
order in which light passes from the object side, an objective
optical system Ob, a field mask M for defining the visual field,
and an ocular optical system Oc. The objective optical system Ob
includes a lens group having a negative first unit G1 consisting
essentially of a biconcave lens and a positive second unit G2
consisting essentially of a biconvex lens. The objective optical
system Ob further includes a prism 10 of positive power which has a
first transmitting surface 11 of negative power, a first reflecting
surface 12 with a positive power in the X-direction and a negative
power in the Y-direction, a second reflecting surface 13 with a
negative power in the X-direction and a positive power in the
Y-direction, a third reflecting surface 14 with a positive power in
the X-direction and approximately zero power in the Y-direction,
and a second transmitting surface 15 with a positive power in the
X-direction and a negative power in the Y-direction. The ocular
optical system Oc is formed from an ideal lens OCL. During zooming,
the first unit G1 and the second unit G2 move. The prism 10, which
forms the third unit, is stationary.
[0327] In the prism 10 of Example 6, the first reflecting surface
12, the second reflecting surface 13, the third reflecting surface
14, the first transmitting surface 11 and the second transmitting
surface 15 are all independent optical functional surfaces that are
separate from other surfaces. The first transmitting surface 11,
the second transmitting surface 15, the first reflecting surface
12, the second reflecting surface 13 and the third reflecting
surface 14 are all formed from rotationally asymmetric free-form
surfaces. The prism 10 has an optical path in which the axial
principal ray bends in the same direction at the first reflecting
surface 12 and the second reflecting surface 13 with respect to the
travel direction of light and which is twisted at the third
reflecting surface 14.
[0328] It is preferable to place an image-inverting optical system
on the pupil side of the intermediate image formation plane as
shown in the schematic view of FIG. 14. By doing so, a thin finder
optical system can be attained. Part (a) of FIG. 14 is a side view,
and part (b) of FIG. 14 is a top plan view. The second prism 20
constitutes an image-inverting optical system. The transmitting
surfaces and reflecting surfaces of the image-inverting optical
system may also be formed from free-form surfaces.
[0329] Commonly in all the foregoing Examples 1 to 6, the first
unit G1 moves toward the pupil side during zooming from the
wide-angle end to the intermediate position and moves toward the
object side during zooming from the intermediate position to the
telephoto end. The second unit G2 moves toward the object side
during zooming from the wide-angle end to the telephoto end.
[0330] Constituent parameters of the foregoing Examples 1 to 6 are
shown below. In the tables below: "FFS" denotes a free-form
surface; "ASS" denotes a rotationally symmetric aspherical surface;
"RS" denotes a reflecting surface; "MSK" denotes a field mask;
"OCL" denotes an ocular lens (ideal lens); "EP" denotes an eye
point (stop); and "HRP" denotes a hypothetic plane.
EXAMPLE 1
[0331]
1 Surface Radius of Surface Displacement Refractive Abbe`s No.
curvature separation and tilt index No. Object .infin. 3000.00
plane 1 -11.86 1.20 1.5842 30.5 2 8.20 d.sub.2 3 ASS{circle over
(1)} 3.00 1.4924 57.6 4 ASS{circle over (2)} d.sub.4 5 .infin.
(HRS) 6 FFS{circle over (1)} (1) 1.5254 56.2 7 FFS{circle over (2)}
(RS) (2) 1.5254 56.2 8 FFS{circle over (3)} (RS) (3) 1.5254 56.2 9
FFS{circle over (4)} (RS) (4) 1.5254 56.2 10 FFS{circle over (3)}
(3) 11 .infin. (MSK) (5) 12 7.55 (6) 1.5254 56.2 13 .infin. (RS)
(7) 1.5254 56.2 14 .infin. (RS) (8) 1.5254 56.2 15 .infin. (7) 16
ASS{circle over (3)} 2.50 (9) 1.4924 57.6 17 -30.03 17.00 18
.infin. (EP) ASS1 R 5.29 K 0.0000 A -6.4744 .times. 10.sup.-4 B
-8.5852 .times. 10.sup.-5 C 6.5776 .times. 10.sup.-8 ASS2 R -6.46 K
0.0000 A 1.4010 .times. 10.sup.-3 B -7.6860 .times. 10.sup.-5 C
1.3589 .times. 10.sup.-6 ASS3 R 16.56 K 0.0000 A -1.6574 .times.
10.sup.-4 B 4.1116 .times. 10.sup.-6 C -9.8958 .times. 10.sup.-8
FFS1 C.sub.4 -4.9131 .times. 10.sup.-2 C.sub.6 -4.9592 .times.
10.sup.-2 C.sub.8 1.7062 .times. 10.sup.-3 C.sub.10 2.6485 .times.
10.sup.-3 C.sub.11 2.1497 .times. 10.sup.-5 C.sub.13 -2.7609
.times. 10.sup.-3 C.sub.15 -9.0009 .times. 10.sup.-4 FFS2 C.sub.4
-1.0935 .times. 10.sup.-3 C.sub.6 -1.3486 .times. 10.sup.-3 C.sub.8
3.7172 .times. 10.sup.-4 C.sub.10 6.6094 .times. 10.sup.-4 C.sub.11
3.3892 .times. 10.sup.-5 C.sub.13 -1.3656 .times. 10.sup.-4
C.sub.15 1.4937 .times. 10.sup.-4 FFS3 C.sub.4 -3.8102 .times.
10.sup.-3 C.sub.6 -3.5527 .times. 10.sup.-4 C.sub.8 -1.9399 .times.
10.sup.-4 C.sub.10 4.2586 .times. 10.sup.-4 C.sub.11 8.7822 .times.
10.sup.-4 C.sub.13 2.3146 .times. 10.sup.-4 C.sub.15 -4.0294
.times. 10.sup.-5 FFS4 C.sub.4 -6.5727 .times. 10.sup.-3 C.sub.6
-4.3521 .times. 10.sup.-3 C.sub.8 -2.7068 .times. 10.sup.-4
C.sub.10 5.1724 .times. 10.sup.-4 C.sub.11 1.2600 .times. 10.sup.-3
C.sub.13 4.8722 .times. 10.sup.-4 C.sub.15 3.4087 .times. 10.sup.-6
Displacement and tilt(1) X 0.00 Y 0.00 Z 0.80 .alpha. 0.48 .beta.
0.00 .gamma. 0.00 Displacement and tilt(2) X 0.00 Y 0.01 Z 3.98
.alpha. -39.25 .beta. 0.00 .gamma. 0.00 Displacement and tilt(3) X
0.00 Y 6.08 Z 2.75 .alpha. 45.31 .beta. 0.00 .gamma. 0.00
Displacement and tilt(4) X 0.00 Y 7.05 Z -2.38 .alpha. 18.60 .beta.
0.00 .gamma. 0.00 Displacement and tilt(5) X 0.00 Y 9.80 Z 0.08
.alpha. 49.52 .beta. 0.00 .gamma. 0.00 Displacement and tilt(6) X
0.00 Y 10.18 Z 0.40 .alpha. 49.52 .beta. 0.00 .gamma. 0.00
Displacement and tilt(7) X 0.00 Y 15.72 Z 5.12 .alpha. 0.57 .beta.
0.00 .gamma. 0.00 Displacement and tilt(8) X 0.00 Y 25.39 Z -3.47
.alpha. -24.09 .beta. 0.00 .gamma. 0.00 Displacement and tilt(9) X
0.00 Y 25.42 Z 6.76 .alpha. 0.00 .beta. 0.00 .gamma. 0.00 Zooming
Spaces Wide-end Standard Tele-end d.sub.2 6.99995 3.55455 1.20000
d.sub.4 0.80000 2.89816 6.59995
EXAMPLE 2
[0332]
2 Surface Radius of Surface Displacement Refractive Abbe`s No.
curvature separation and tilt index No. Object .infin. 3000.00
plane 1 -11.14 1.24 1.5842 30.5 2 11.67 d.sub.2 3 ASS{circle over
(1)} 3.00 1.4924 57.6 4 ASS{circle over (2)} d.sub.4 5 .infin.
(HRS) 6 FFS{circle over (1)} (1) 1.5254 56.2 7 FFS{circle over (2)}
(RS) (2) 1.5254 56.2 8 FFS{circle over (1)} (RS) (1) 1.5254 56.2 9
FFS{circle over (3)} (3) 10 .infin. (MSK) 20.78 (4) 11 OCL 47.78 12
.infin. (EP) ASS1 R 5.86 K 0.0000 A -8.4991 .times. 10.sup.-4 B
-1.0411 .times. 10.sup.-5 C 1.3159 .times. 10.sup.-6 ASS2 R -7.81 K
0.0000 A 6.0656 .times. 10.sup.-4 B 4.8824 .times. 10.sup.-6 C
1.3023 .times. 10.sup.-6 FFS1 C.sub.4 2.3369 .times. 10.sup.-3
C.sub.6 -1.3528 .times. 10.sup.-4 C.sub.8 2.1349 .times. 10.sup.-4
C.sub.10 -4.1406 .times. 10.sup.-5 C.sub.11 1.0040 .times.
10.sup.-3 C.sub.13 4.8542 .times. 10.sup.-5 C.sub.15 4.0149 .times.
10.sup.-5 C.sub.17 4.4136 .times. 10.sup.-5 C.sub.19 2.7527 .times.
10.sup.-5 C.sub.21 3.8355 .times. 10.sup.-6 FFS2 C.sub.4 8.9068
.times. 10.sup.-3 C.sub.6 6.7547 .times. 10.sup.-3 C.sub.8 9.6197
.times. 10.sup.-4 C.sub.10 1.7154 .times. 10.sup.-4 C.sub.11 9.9374
.times. 10.sup.-4 C.sub.13 2.3684 .times. 10.sup.-4 C.sub.15 2.4102
.times. 10.sup.-4 C.sub.17 -7.1618 .times. 10.sup.-5 C.sub.19
8.4108 .times. 10.sup.-5 C.sub.21 5.7261 .times. 10.sup.-6 FFS3
C.sub.4 -9.5974 .times. 10.sup.-3 C.sub.6 -1.5589 .times. 10.sup.-2
C.sub.8 -1.1372 .times. 10.sup.-3 C.sub.10 -6.8569 .times.
10.sup.-4 C.sub.11 1.5767 .times. 10.sup.-4 C.sub.13 2.9766 .times.
10.sup.-4 C.sub.15 2.7340 .times. 10.sup.-4 C.sub.17 5.9875 .times.
10.sup.-4 C.sub.19 9.0116 .times. 10.sup.-5 C.sub.21 1.0921 .times.
10.sup.-4 Displacement and tilt(1) X 0.00 Y 4.11 Z 1.42 .alpha.
0.83 .beta. 0.00 .gamma. 0.00 Displacement and tilt(2) X 0.00 Y
0.02 Z 4.40 .alpha. -26.77 .beta. 0.00 .gamma. 0.00 Displacement
and tilt(3) X 0.00 Y 10.13 Z 5.54 .alpha. 55.74 .beta. 0.00 .gamma.
0.00 Displacement and tilt(4) X 0.00 Y 11.04 Z 6.17 .alpha. 55.58
.beta. 0.00 .gamma. 0.00 Zooming Spaces Wide-end Standard Tele-end
d.sub.2 7.83905 3.82890 1.20000 d.sub.4 0.80000 3.26703 7.43905
EXAMPLE 3
[0333]
3 Surface Radius of Surface Displacement Refractive Abbe`s No.
curvature separation and tilt index No. Object .infin. 3000.00
plane 1 -14.29 1.20 1.5842 30.5 2 7.24 d.sub.2 3 ASS{circle over
(1)} 2.67 1.4924 57.6 4 ASS{circle over (2)} d.sub.4 5 .infin.
(HRS) 6 FFS{circle over (1)} (1) 1.5254 56.2 7 FFS{circle over (2)}
(RS) (2) 1.5254 56.2 8 FFS{circle over (3)} (RS) (3) 1.5254 56.2 9
FFS{circle over (4)} (4) 10 .infin. (MSK) 20.78 (5) 11 OCL 29.86 12
.infin. (EP) ASS1 R 5.64 K 0.0000 A -7.0498 .times. 10.sup.-4 B
-7.5459 .times. 10.sup.-5 C 5.2691 .times. 10.sup.-6 ASS2 R -6.83 K
0.0000e+000 A 1.0789 .times. 10.sup.-3 B -7.5610 .times. 10.sup.-5
C 6.3722 .times. 10.sup.-6 FFS1 C.sub.4 -5.7553 .times. 10.sup.-2
C.sub.6 -8.1248 .times. 10.sup.-2 C.sub.8 -4.7730 .times. 10.sup.-4
C.sub.10 1.9712 .times. 10.sup.-3 C.sub.11 -6.9219 .times.
10.sup.-4 C.sub.13 -7.1482 .times. 10.sup.-4 C.sub.15 -4.9132
.times. 10.sup.-4 FFS2 C.sub.4 -1.7485 .times. 10.sup.-3 C.sub.6
-4.7051 .times. 10.sup.-3 C.sub.8 -1.5112 .times. 10.sup.-4
C.sub.10 1.7841 .times. 10.sup.-5 C.sub.11 2.2289 .times. 10.sup.-4
C.sub.13 2.3597 .times. 10.sup.-4 C.sub.15 3.6727 .times. 10.sup.-5
FFS3 C.sub.4 -8.9372 .times. 10.sup.-4 C.sub.6 1.5599 .times.
10.sup.-3 C.sub.8 -1.1534 .times. 10.sup.-4 C.sub.10 -1.6218
.times. 10.sup.-4 C.sub.11 -3.3857 .times. 10.sup.-5 C.sub.13
-3.8888 .times. 10.sup.-5 C.sub.15 3.4209 .times. 10.sup.-6 FFS4
C.sub.4 3.4173 .times. 10.sup.-2 C.sub.6 3.8829 .times. 10.sup.-2
C.sub.8 -7.2783 .times. 10.sup.-4 C.sub.10 -2.5525 .times.
10.sup.-4 C.sub.11 9.2340 .times. 10.sup.-5 C.sub.13 2.3184 .times.
10.sup.-4 C.sub.15 1.4697 .times. 10.sup.-4 Displacement and
tilt(1) X 0.00 Y 0.00 Z 1.57 .alpha. -0.01 .beta. 0.00 .gamma. 0.00
Displacement and tilt(2) X 0.00 Y 0.01 Z 4.98 .alpha. -49.01 .beta.
0.00 .gamma. 0.00 Displacement and tilt(3) X 0.00 Y 9.50 Z 6.32
.alpha. 41.01 .beta. 0.00 .gamma. 0.00 Displacement and tilt(4) X
0.00 Y 9.50 Z 0.55 .alpha. 0.13 .beta. 0.00 .gamma. 0.00
Displacement and tilt(5) X 0.00 Y 9.50 Z 0.38 .alpha. 180.00 .beta.
0.00 .gamma. 0.00 Zooming Spaces Wide-end Standard Tele-end d.sub.2
8.15555 4.39096 2.00000 d.sub.4 1.10000 3.38907 7.25555
EXAMPLE 4
[0334]
4 Surface Radius of Surface Displacement Refractive Abbe`s No.
curvature separation and tilt index No. Object .infin. 3000.00
plane 1 -11.50 1.20 1.5842 30.5 2 5.88 d.sub.2 3 ASS{circle over
(1)} 3.00 1.4924 57.6 4 ASS{circle over (2)} d.sub.4 5 .infin.
(HRS) 6 FFS{circle over (1)} (1) 1.5254 56.2 7 FFS{circle over (2)}
(RS) (2) 1.5254 56.2 8 FFS{circle over (3)} (RS) (3) 1.5254 56.2 9
FFS{circle over (4)} (RS) (4) 1.5254 56.2 10 FFS{circle over (5)}
(5) 11 .infin. (MSK) 20.78 (6) 12 OCL 36.88 13 .infin. (EP) ASS1 R
4.48 K 0.0000 A 5.5018 .times. 10.sup.-4 B -1.0835 .times.
10.sup.-3 C 2.4690 .times. 10.sup.-5 ASS2 R -5.25 K 0.0000 A 2.3163
.times. 10.sup.-3 B -2.0626 .times. 10.sup.-4 C 1.2813 .times.
10.sup.-5 FFS1 C.sub.4 -1.4743 .times. 10.sup.-1 C.sub.6 -1.4090
.times. 10.sup.-1 C.sub.8 3.8963 .times. 10.sup.-3 C.sub.10 -6.7672
.times. 10.sup.-4 C.sub.11 7.7369 .times. 10.sup.-5 C.sub.13 -9.
1587 .times. 10.sup.-5 C.sub.15 3.2032 .times. 10.sup.-3 FFS2
C.sub.4 -8.4766 .times. 10.sup.-3 C.sub.6 -4.7899 .times. 10.sup.-3
C.sub.8 5.6605 .times. 10.sup.-4 C.sub.10 -5.2847 .times. 10.sup.-4
C.sub.11 4.1384 .times. 10.sup.-4 C.sub.13 4.7154 .times. 10.sup.-4
C.sub.15 2.8820 .times. 10.sup.-4 FFS3 C.sub.4 -1.1549 .times.
10.sup.-2 C.sub.6 -2.6750 .times. 10.sup.-3 C.sub.8 -2.8887 .times.
10.sup.-4 C.sub.10 -8.8526 .times. 10.sup.-4 C.sub.11 6.8703
.times. 10.sup.-5 C.sub.13 7.2649 .times. 10.sup.-6 C.sub.15 3.4284
.times. 10.sup.-5 FFS4 C.sub.4 -6.7186 .times. 10.sup.-3 C.sub.6
-6.4824 .times. 10.sup.-4 C.sub.8 -7.7840 .times. 10.sup.-4
C.sub.10 -8.5329 .times. 10.sup.-4 C.sub.11 3.1754 .times.
10.sup.-4 C.sub.13 1.0191 .times. 10.sup.-4 C.sub.15 1.4334 .times.
10.sup.-4 FFS5 C.sub.4 -2.6315 .times. 10.sup.-2 C.sub.6 -2.1715
.times. 10.sup.-2 C.sub.8 -2.0258 .times. 10.sup.-3 C.sub.10
-3.3550 .times. 10.sup.-3 C.sub.11 7.6765 .times. 10.sup.-4
C.sub.13 3.0974 .times. 10.sup.-4 C.sub.15 1.5156 .times. 10.sup.-3
Displacement and tilt(1) X 0.00 Y 0.00 Z 1.48 .alpha. 0.68 .beta.
0.00 .gamma. 0.00 Displacement and tilt(2) X 0.00 Y 0.01 Z 4.48
.alpha. 43.59 .beta. 0.00 .gamma. 0.00 Displacement and tilt(3) X
0.00 Y 6.35 Z 4.20 .alpha. 49.03 .beta. 0.00 .gamma. 0.00
Displacement and tilt(4) X 0.00 Y 5.67 Z -2.80 .alpha. 47.16 .beta.
0.00 .gamma. 0.00 Displacement and tilt(5) X 0.00 Y 9.26 Z -2.73
.alpha. 86.65 .beta. 0.00 .gamma. 0.00 Displacement and tilt(6) X
0.00 Y 9.77 Z -2.73 .alpha. 90.00 .beta. 0.00 .gamma. 0.00 Zooming
Spaces Wide-end Standard Tele-end d.sub.2 6.11892 3.15900 1.20000
d.sub.4 0.80000 2.54034 5.71892
EXAMPLE 5
[0335]
5 Surface Radius of Surface Displacement Refractive Abbe`s No.
curvature separation and tilt index No. Object .infin. 3000.00
plane 1 -21.44 1.20 1.5842 30.5 2 10.79 d.sub.2 3 ASS{circle over
(1)} 3.00 1.4924 57.6 4 ASS{circle over (2)} d.sub.4 5 .infin.
(HRS) 6 FFS{circle over (1)} (1) 1.5254 56.2 7 FFS{circle over (2)}
(RS) (2) 1.5254 56.2 8 FFS{circle over (1)} (RS) (1) 1.5254 56.2 9
FFS{circle over (3)} (RS) (3) 1.5254 56.2 10 FFS{circle over (1)}
(1) 11 .infin. (MSK) 21.22 (4) 12 OCL 38.49 13 .infin. (EP) ASS1 R
9.47 K 0.0000 A -8.2774 .times. 10.sup.-4 B -2.6238 .times.
10.sup.-5 C -5.7408 .times. 10.sup.-6 ASS2 R -7.34 K 0.0000 A
9.5056 .times. 10.sup.-5 B -6.2435 .times. 10.sup.-5 C -1.2735
.times. 10.sup.-6 FFS1 C.sub.4 -6.6220 .times. 10.sup.-3 C.sub.6
1.4741 .times. 10.sup.-3 C.sub.8 6.9888 .times. 10.sup.-4 C.sub.10
1.1582 .times. 10.sup.-4 C.sub.11 3.0602 .times. 10.sup.-4 C.sub.13
-6.9034 .times. 10.sup.-5 C.sub.15 -6.4460 .times. 10.sup.-6 FFS2
C.sub.4 -1.7484 .times. 10.sup.-3 C.sub.6 2.8690 .times. 10.sup.-3
C.sub.8 8.5869 .times. 10.sup.-4 C.sub.10 8.0024 .times. 10.sup.-5
C.sub.11 2.2351 .times. 10.sup.-4 C.sub.13 2.6879 .times. 10.sup.-5
C.sub.15 1.6036 .times. 10.sup.-5 FFS3 C.sub.4 -7.2172 .times.
10.sup.-3 C.sub.6 -1.8257 .times. 10.sup.-3 C.sub.8 2.0960 .times.
10.sup.-4 C.sub.10 2.7943 .times. 10.sup.-4 C.sub.11 8.6439 .times.
10.sup.-5 C.sub.13 -7.7192 .times. 10.sup.-5 C.sub.15 -1.4361
.times. 10.sup.-5 Displacement and tilt(1) X 0.00 Y 5.64 Z 1.44
.alpha. 0.23 .beta. 0.00 .gamma. 0.00 Displacement and tilt(2) X
0.00 Y 0.01 Z 4.61 .alpha. -30.26 .beta. 0.00 .gamma. 0.00
Displacement and tilt(3) X 0.00 Y 10.72 Z 4.25 .alpha. 30.37 .beta.
0.00 .gamma. 0.00 Displacement and tilt(4) X 0.00 Y 10.74 Z 0.94
.alpha. 0.00 .beta. 0.00 .gamma. 0.00 Zooming Spaces Wide-end
Standard Tele-end d.sub.2 9.35799 4.37801 1.20000 d.sub.4 0.80000
3.87920 8.95799
EXAMPLE 6
[0336]
6 Surface Radius of Surface Displacement Refractive Abbe`s No.
curvature separation and tilt index No. Object .infin. 3000.00
plane 1 -13.14 1.20 1.5842 30.5 2 9.36 d.sub.2 3 ASS{circle over
(1)} 3.00 1.4924 57.6 4 ASS{circle over (2)} d.sub.4 5 .infin.
(HRS) 6 FFS{circle over (1)} (1) 1.5254 56.2 7 FFS{circle over (2)}
(RS) (2) 1.5254 56.2 8 FFS{circle over (3)} (RS) (3) 1.5254 56.2 9
FFS{circle over (4)} (RS) (4) 1.5254 56.2 10 FFS{circle over (5)}
(5) 11 .infin. (MSK) 20.78 (6) 12 OCL 36.88 13 .infin. (EP) ASS1 R
5.82 K 0.0000 A -1.3958 .times. 10.sup.-4 B -1.1048 .times.
10.sup.-4 C 3.2206 .times. 10.sup.-6 ASS2 R -7.20 K 0.0000 A 1.3132
.times. 10.sup.-3 B -9.6016 .times. 10.sup.-5 C 3.5317 .times.
10.sup.-6 FFS1 C.sub.4 -3.6931 .times. 10.sup.-2 C.sub.5 2.4034
.times. 10.sup.-3 C.sub.6 -3.8156 .times. 10.sup.-2 C.sub.7 5.0183
.times. 10.sup.-4 C.sub.8 -5.5562 .times. 10.sup.-4 C.sub.10 2.4955
.times. 10.sup.-4 C.sub.11 -7.6246 .times. 10.sup.-4 C.sub.12
-1.1638 .times. 10.sup.-4 C.sub.13 -6.3406 .times. 10.sup.-4
C.sub.14 1.5782 .times. 10.sup.-5 C.sub.15 1.1475 .times. 10.sup.-4
FFS2 C.sub.4 -1.2537 .times. 10.sup.-4 C.sub.5 7.4897 .times.
10.sup.-4 C.sub.6 1.7761 .times. 10.sup.-4 C.sub.7 1.8387 .times.
10.sup.-4 C.sub.8 -1.9272 .times. 10.sup.-4 C.sub.10 8.9267 .times.
10.sup.-5 C.sub.11 1.0683 .times. 10.sup.-4 C.sub.12 -1.2218
.times. 10.sup.-5 C.sub.13 1.6164 .times. 10.sup.-4 C.sub.14 9.2102
.times. 10.sup.-6 C.sub.15 8.2661 .times. 10.sup.-5 FFS3 C.sub.4
6.7137 .times. 10.sup.-4 C.sub.5 4.2997 .times. 10.sup.-4 C.sub.6
-1.2873 .times. 10.sup.-3 C.sub.7 1.0858 .times. 10.sup.-4 C.sub.8
3.3744 .times. 10.sup.-5 C.sub.10 1.0231 .times. 10.sup.-4 C.sub.11
-9. 6387 .times. 10.sup.-5 C.sub.12 4.0612 .times. 10.sup.-5
C.sub.13 -9.0058 .times. 10.sup.-5 C.sub.14 -7.3927 .times.
10.sup.-6 C.sub.15 -3.3187 .times. 10.sup.-5 FFS4 C.sub.4 4.8422
.times. 10.sup.-3 C.sub.5 -3.3065 .times. 10.sup.-4 C.sub.6 -4.9490
.times. 10.sup.-5 C.sub.7 -3.1687 .times. 10.sup.-4 C.sub.8 2.3077
.times. 10.sup.-4 C.sub.10 -3.8185 .times. 10.sup.-4 C.sub.11
8.6746 .times. 10.sup.-6 C.sub.12 -3.8790 .times. 10.sup.-6
C.sub.13 -8.7123 .times. 10.sup.-5 C.sub.14 8.9056 .times.
10.sup.-5 C.sub.15 2.5656 .times. 10.sup.-4 FFS5 C.sub.4 1.4423
.times. 10.sup.-3 C.sub.5 -2.7180 .times. 10.sup.-3 C.sub.6 -2.7696
.times. 10.sup.-2 C.sub.7 -4.5148 .times. 10.sup.-3 C.sub.8 1.4260
.times. 10.sup.-3 C.sub.10 -1.1338 .times. 10.sup.-3 C.sub.11
2.5537 .times. 10.sup.-4 C.sub.12 -3.0227 .times. 10.sup.-4
C.sub.13 -5.7820 .times. 10.sup.-4 C.sub.14 7.9487 .times.
10.sup.-4 C.sub.15 6.5970 .times. 10.sup.-4 Displacement and
tilt(1) X 0.00 Y 0.01 Z 1.06 .alpha. 0.01 .beta. 0.00 .gamma. 0.00
Displacement and tilt(2) X 0.00 Y 0.00 Z 4.71 .alpha. -45.00 .beta.
0.00 .gamma. 0.00 Displacement and tilt(3) X 0.00 Y 6.81 Z 4.72
.alpha. 45.00 .beta. 0.00 .gamma. 0.00 Displacement and tilt(4) X
0.00 Y 6.80 Z -1.45 .alpha. 0.00 .beta. 45.00 .gamma. 0.00
Displacement and tilt(5) X -2.59 Y 6.82 Z -1.46 .alpha. 0.00 .beta.
90.00 .gamma. 0.00 Displacement and tilt(6) X -3.49 Y 6.82 Z -1.45
.alpha. 0.00 .beta. 90.00 .gamma. 0.00 Zooming Spaces Wide-end
Standard Tele-end d.sub.2 7.61614 3.69770 1.20000 d.sub.4 0.42524
2.83762 6.84138
[0337] FIGS. 15 to 17 graphically show lateral aberrations in the
above-described Example 1 at the wide-angle end, standard position
and telephoto end, respectively. In these diagrams showing lateral
aberrations, the numerals in the parentheses denote (horizontal
(X-direction) field angle, vertical (Y-direction) field angle), and
lateral aberrations at the field angles are shown. It should be
noted that these diagrams show lateral aberrations on the
image-formation plane of an image-forming system having an ideal
lens with a focal length of 31.62 millimeters placed at a point
apart from the pupil plane (EP) by 31.62 millimeters (={square
root}1000 millimeters).
[0338] It should be noted that each aberrational diagram shows, in
order from the bottom toward the top of the diagram, lateral
aberrations in the center of the image field; lateral aberrations
at the position of minus about 70% of the image height on the
Y-axis; lateral aberrations at the position of about 70% of the
image height in the X-axis direction and minus about 70% of the
image height in the Y-axis direction; lateral aberrations at the
position of about 70% of the image height on the X-axis; lateral
aberrations at the position of about 70% of the image height in the
X-axis direction and about 70% of the image height in the Y-axis
direction; and lateral aberrations at the position of about 70% of
the image height on the Y-axis.
[0339] The values concerning the conditions (1) to (52) in the
above-described Examples 1 to 6 are shown below.
7 Conditions Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 (1), (2) 1.69 1.83
1.92 1.65 2.01 1.80 (3), (5) (4), (6) 0.81 0.76 0.87 0.76 0.81 0.94
(9), (10) (21), (22) (7), (8) 0.62 0.73 0.61 0.50 0.92 0.70 (11),
(13) (15) (35), (36) 39.4 (37), (38) 29.3 (39), (40) 40.5 (17),
(18) 27.2 (19), (20) 55.6 (23), (24) 49.0 (25), (26) 41.0 (27),
(28) 180.0 (29), (30) 43.8 (31), (32) 41.7 (33), (34) 90.0 (41),
(42) 30.4 (43), (44) 30.7 (45), (46) 180.0 (47), (48) 45.0 (49),
(50) 45.0 (51), (52) 90.0
[0340] The above-described finder optical system according to the
present invention can be used as a finder optical system 33 of an
electronic camera as shown for example in FIG. 18. In FIG. 18, part
(a) is a perspective view of the electronic camera as viewed from
the front thereof, and part (b) is a perspective view of the
electronic camera as viewed from the rear thereof. FIG. 19 is a ray
path diagram showing the optical system of the electronic camera.
The electronic camera includes a photographic optical system 31
having an optical path 32 for photography; a finder optical system
33 having an optical path 34 for a finder; a shutter 35; a flash
36; and a liquid-crystal display monitor 37. The finder optical
system 33 includes an objective optical system Ob and an ocular
optical system Oc as in Example 1 shown in FIG. 1, for example. The
finder optical system 33 is of the type which enables the visual
field to be viewed directly. It should be noted that a transparent
finder window cover 41 is placed on the entrance side of the
objective optical system Ob in the finder optical system 33.
[0341] The photographic optical system 31 includes an objective
optical system 38 for photography, a filter 39, e.g. an infrared
cutoff filter, and an electronic image pickup device 40 placed in
the image-formation plane of the objective optical system 38. A
subject image taken by the electronic image pickup device 40 or an
image recorded in a recording device is displayed on the
liquid-crystal display monitor 37.
[0342] It should be noted that the finder optical system according
to the present invention can be used as a finder optical system of
a compact camera for photography in which a photographic film is
disposed in place of the electronic image pickup device 40 to take
a picture of a subject.
[0343] The present invention makes it possible to provide a thin,
high-performance finder optical system favorably corrected for
aberrations due to decentration by appropriately disposing
rotationally asymmetric surfaces.
* * * * *