U.S. patent application number 09/866931 was filed with the patent office on 2001-11-22 for real image mode variable magnification finder.
Invention is credited to Kato, Shigeru.
Application Number | 20010043394 09/866931 |
Document ID | / |
Family ID | 15355650 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043394 |
Kind Code |
A1 |
Kato, Shigeru |
November 22, 2001 |
Real image mode variable magnification finder
Abstract
A real image mode variable magnification finder has an objective
optical system with positive refracting power, an image erecting
optical system, and an ocular optical system with positive
refracting power. The objective optical system has a first lens
unit with negative refracting power, a second lens unit with
negative refracting power, a third lens unit with positive
refracting power, and a fourth lens unit with negative refracting
power. When the magnification of the finder is changed, at least
one lens unit, namely the third lens unit is moved. In this way,
the total length of the objective optical system can be reduced,
and thus a compact finder is obtained.
Inventors: |
Kato, Shigeru;
(Tachikawa-shi, JP) |
Correspondence
Address: |
Intellectual Property Group
Pillsbury Winthrop LLP
East Tower, Ninth Floor
1100 New York Avenue, N.W.
Washington
DC
20005-3918
US
|
Family ID: |
15355650 |
Appl. No.: |
09/866931 |
Filed: |
May 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09866931 |
May 30, 2001 |
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09087964 |
Jun 1, 1998 |
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6256144 |
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Current U.S.
Class: |
359/432 ;
359/422; 359/686 |
Current CPC
Class: |
G02B 15/144505
20190801 |
Class at
Publication: |
359/432 ;
359/422; 359/686 |
International
Class: |
G02B 023/00; G02B
001/00; G02B 015/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 1997 |
JP |
HEI 9-144162 |
Claims
What is claimed is:
1. A real image mode variable magnification finder comprising: an
objective optical system with positive refracting power; an image
erecting optical system; and an ocular optical system with positive
refracting power, said objective optical system having a first lens
unit with negative refracting power, a second lens unit with
negative refracting power, a third lens unit with positive
refracting power, and a fourth lens unit with negative refracting
power, and at least one lens unit, namely said third lens unit
being moved when a magnification of said finder is changed.
2. A real image mode variable magnification finder according to
claim 1, wherein said image erecting optical system includes a
prism, and said fourth lens unit with negative refracting power is
constructed to be integral with an entrance surface of said
prism.
3. A real image mode variable magnification finder comprising: an
objective optical system with positive refracting power; an image
erecting optical system; and an ocular optical system with positive
refracting power, said image erecting optical system including a
prism and a mirror, and said ocular optical system having at least
two lenses composed of a fixed lens and a moving lens.
4. A real image mode variable magnification finder according to
claim 1, wherein when the magnification of said finder is changed,
said third lens unit Is simply moved and a distance between said
first lens unit and said second lens unit is changed.
5. A real image mode variable magnification finder according to
claim 2, satisfying the following condition:
0.5<L.sub.pr/L.sub.obj<0.7 where L.sub.pr is an optical path
length of said prism and L.sub.obj is a maximum optical path length
of said objective optical system, that is, a distance, measured
along an optical axis, from an entrance surface of said first lens
unit to an intermediate image.
6. A real image mode variable magnification finder according to
claims 2 or 3, wherein said prism has two reflecting surfaces for
shifting upward an axis of incident light and one reflecting
surface for deflecting said axis of incident light in a direction
substantially parallel thereto.
7. A real image mode variable magnification finder according to
claims 2 or 3, wherein said prism satisfies the following
condition: .nu..sub.p<50 where .nu..sub.p is an Abbe's number of
said prism.
8. A real image mode variable magnification finder according to
claims 1 or 2, wherein at least one lens unit, namely said third
lens unit has aspherical surfaces.
9. A real image mode variable magnification finder according to any
one of claims 1, 2, 4 or 5, wherein a distance between said first
lens unit and said second lens unit is changed to correct diopters
varying with variable magnification ratios.
10. A real image mode variable magnification finder according to
claim 3, wherein said ocular optical system satisfies the following
condition: .vertline.f.sub.R2/f.sub.R1<0.5 where f.sub.R1 is an
focal length of the fixed lens of said ocular optical system and
f.sub.R2 is an focal length of the moving lens of said ocular
optical system.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a real image mode variable
magnification finder for attachment which is constructed to be
independent of a photographing optical system as in a still camera
or a video camera.
[0003] 2. Description of Related Art
[0004] Real image mode variable magnification finders have been
designed so that an intermediate image is formed inside an image
erecting system to reduce the total length of an objective unit. As
an example, a design is known that a Porro prism is divided into
two pieces. When this design is used, as shown in FIG. 1A, the
entrance window of an objective optical system 2, in contrast with
FIG. 1B, can be located at a position lower than the window of an
eyepiece 4. Thus, a shift Sh of the objective optical system 2 from
the optical axis can be diminished to keep parallax with a
photographic lens 7 to a minimum. In particular, it is known that a
prism such that light from the objective optical system 2 is
reflected upward and then back minimizes interference with a film
mask 5 of a camera and is most suitable for use in reducing the
thickness of the camera. Also, in FIGS. 1A and 1B, reference
numeral 1 represents a finder unit; 3, a Porro prism; 6, a film;
and 8, a camera case.
[0005] As will be obvious from Japanese Patent Preliminary
Publication Nos. Hei 7-84184 and Hei 9-68739, it is known that, in
order to place a mechanism member for changing the size of a field
frame, the field frame is placed above the side face of an
objective optical system including a prism, after light is
reflected three times by the prism, and thereby space for the
mechanism member can be provided.
[0006] Further, as set forth in Japanese Patent Preliminary
Publication No. Hei 5-53054, it is known that, in order to increase
the optical path length of the back focus section of the objective
optical system, it is only necessary to use a prism whose entrance
surface is concave.
[0007] In addition, as disclosed in Japanese Patent Preliminary
Publication No. Hei 1-257817, a technique is known that, in a real
image mode finder using a Porro mirror in an image erecting optical
system, an eyepiece is fixed to a frame to prevent dirt particles
from penetrating into an intermediate image, providing an enclosed
structure.
[0008] However, each of Hei 7-84184, Hei 5-53054, and Hei 9-68739
which are mentioned above has the problem that the adhesion of dirt
particles to a field lens located on the pupil side of the
intermediate image cannot be prevented because the eyepiece is
movable for diopter adjustment.
[0009] Japanese Patent Preliminary Publication No. Hei 4-51108 is
capable of increasing the back focal distance of the objective
optical system, but has the problem that the total length of the
objective optical system becomes large and thus the thickness of
the camera cannot be decreased.
SUMMARY OF THE INVENTION
[0010] It is, therefore, an object of the present invention to
provide a real image mode variable magnification finder which has
an objective optical system whose back focal distance is long and
whose total length is short, rarely allows the penetration of dirt
particles although diopter-adjustable, undergoes little change in
performance, and is small in size.
[0011] In order to accomplish this object, according to the present
invention, the real image mode variable magnification finder
includes an objective optical system with positive refracting
power, an image erecting optical system, and an ocular optical
system with positive refracting power. The objective optical system
has a first lens unit with negative refracting power, a second lens
unit with negative refracting power, a third lens unit with
positive refracting power, and a fourth lens unit with negative
refracting power, and is designed so that when the magnification of
the finder is changed, at least one lens unit, namely the third
lens unit is moved.
[0012] Further, according to the present invention, the real image
mode variable magnification finder is constructed so that the image
erecting optical system includes a prism and the fourth lens unit
with negative refracting power is configured to be integral with
the entrance surface of the prism.
[0013] Still further, according to the present invention, the real
image mode variable magnification finder Includes an objective
optical system with positive refracting power, an image erecting
optical system, and an ocular optical system with positive
refracting power. The image erecting optical system is composed of
a prism and a mirror, and the ocular optical system is provided
with at least two lenses, that is, a fixed lens and a moving
lens.
[0014] This and other objects as well as the features and
advantages of the present invention will become apparent from the
following detailed description of the preferred embodiments when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIGS. 1A and 1B are views showing cases where different,
conventional real image mode variable magnification finders are
incorporated in cameras;
[0016] FIG. 2 is a view for explaining the reflection of light
caused by a prism used as a reflecting member on the ocular optical
system side;
[0017] FIGS. 3A, 3B, and 3C are sectional views showing
arrangements, each developed along the optical axis, at wide-angle,
middle, and telephoto positions, respectively, of the real image
mode variable magnification finder of a first embodiment in the
present invention;
[0018] FIG. 4 is a perspective view showing the configuration of a
Porro prism used In the real Image mode variable magnification
finder of the present invention;
[0019] FIGS. 5A, 5B, and 5C are diagrams showing aberration
characteristics at the wide-angle position of the finder in the
first embodiment;
[0020] FIGS. 6A, 6B, and 6C are diagrams showing aberration
characteristics at the middle position of the finder in the first
embodiment;
[0021] FIGS. 7A, 7B, and 7C are diagrams showing aberration
characteristics at the telephoto position of the finder in the
first embodiment;
[0022] FIGS. 8A, 8B, and 8C are sectional views showing
arrangements, each developed along the optical axis, at wide-angle,
middle, and telephoto positions, respectively, of the real image
mode variable magnification finder of a second embodiment in the
present invention; and
[0023] FIGS. 9A, 9B, and 9C are sectional views showing
arrangements, each developed along the optical axis, at wide-angle,
middle, and telephoto positions, respectively, of the real image
mode variable magnification finder of a third embodiment in the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Before undertaking the explanation of the embodiments, a
description will be given of the general function of the real image
mode variable magnification finder according to the present
invention.
[0025] For the objective optical system of the real image mode
variable magnification finder, it is necessary to improve a
retrofocus property and strengthen a forward negative power in
order to obtain a back focal distance as long as possible. In the
present invention, since the forward negative power is shared
between the first and second lens units, one lens unit need not
have a higher power than is necessary, and thus curvature of field
and distortion can be minimized. Divergent light emerging from the
second lens unit is collected by the third lend unit with positive
power. The fourth lens unit with negative power, located behind it,
has two effects. One of these is that the back focal distance can
be increased by the diverging action of the fourth lens unit. The
other is that the pupil position of the objective optical system is
shifted forward, and the diameter of a front lens can be
diminished.
[0026] The magnification of the finder is changed by chiefly moving
the third lens unit. Specifically, the third lens unit is moved
from the intermediate image side to the object side and thereby the
magnification is changed from low to high.
[0027] The fourth lens unit may be constructed with a single lens.
However, in order to further reduce the total length of the
objective optical system, it is desirable that the entrance surface
of a three-reflection prism located in the back focus section of
the objective optical system is configured as a concave surface to
thereby possess a negative power so that the number of members is
reduced. In this case, it is favorable that an Abbe's number
.nu..sub.p satisfies the following condition:
.nu..sub.p<50 (1)
[0028] When Condition (1) is satisfied, axial chromatic aberration
can be favorably corrected. Also, even though Condition (1) is not
satisfied, there is little problem in practical use.
[0029] If aspherical surfaces are used in at least one lens unit,
that is, the third lens unit, spherical aberration and coma can be
favorably corrected even when the lens unit is a single lens.
[0030] In order to correct diopters varying with variable
magnification, it is desirable to change the distance between the
first lens unit and the second lens unit. In this case, either the
first lens unit or the second lens unit, or both, may be moved. It
is, of course, favorable that the number of moving lens units is
made as small as possible, because a lens movement mechanism is
simplified. Also, if the variable magnification ratio is low, the
diopter will undergo little change, and thus there is no problem in
practical use even when only the third lens unit is moved.
[0031] Further, it is favorable that the finder satisfies the
following condition:
0.5<L.sub.pr/L.sub.obj<0.7 (2)
[0032] where L.sub.pr is the optical path length of the prism and
L.sub.obj is the maximum optical path length of the objective
optical system (a distance, measured along the optical axis, from
the entrance surface of the first lens unit to the intermediate
image).
[0033] If the lower limit of Condition (2) is passed, the total
length of the objective optical system will be increased, and hence
the thickness of the camera cannot be reduced. Beyond the upper
limit, aberration is deteriorated because the power of each lens
unit is strengthened.
[0034] Subsequently, reference is made to another function of the
present invention. The back focus section of the objective optical
system is provided with the three-reflection prism for erecting an
image. The intermediate image of the objective optical system is
formed in the vicinity of the exit surface of the prism, where a
field frame is placed. Light incident on the prism from the
objective optical system is reflected upward and then back by the
reflecting surfaces of the prism and is further reflected laterally
by the third reflecting surface. In this way, the intermediate
image Is formed in a plane nearly parallel with the axis of
incident light from the objective optical system. In order to
further erect the image, the image needs to be once-reflected
between the intermediate image and the ocular optical system. The
optical path length from the intermediate image to the ocular
optical system becomes nearly equivalent to the focal length of the
ocular optical system. If a prism is used as a reflecting member on
the ocular optical system side, it will have the total length for
more than one reflection, and a width W of the prism becomes large,
with a resulting increase in camera width (refer to FIG. 2). Thus,
the present invention uses a one-reflection mirror.
[0035] In order to prevent the adhesion of dirt particles to
optical components situated in the vicinity of the intermediate
image, notably, to the exit surface of the prism, an enclosed
structure is required. Moreover, for diopter adjustment, it is
necessary to move the eyepiece along the optical axis.
[0036] Thus, in the present invention, the ocular optical system is
constructed with at least two lenses, one fixed, lying on the
intermediate image side and the other moved for diopter adjustment,
lying on the pupil side. Consequently, the enclosed structure can
be provided, extending from the exit surface of the prism, through
the field frame and the mirror, to the fixed lens. In this case, it
is desirable that the ocular optical system satisfies the following
condition:
.vertline.f.sub.R2/f.sub.R1<0.5 (3)
[0037] where f.sub.R1 is the focal length of one lens on the
objective optical system side, of two lenses constituting the
ocular optical system and f.sub.R2 is the focal length of the other
lens on the pupil side. If the upper limit of Condition (3) is
passed, the variation of spherical aberration caused by the lens
movement for diopter adjustment becomes considerable, which is
unfavorable.
[0038] The entrance surface of the prism, as mentioned above, may
have a negative power to serve as a part of the objective optical
system. Furthermore, the exit surface of the prism may be
configured as a convex surface to play the role of a field lens.
Since the member of the field frame is located above the side face
of the objective optical system, the height of the camera is not
increased even when a mechanism member for changing the size of the
field frame is placed.
[0039] In accordance with the drawings, the embodiments of the
present invention will be explained below.
[0040] First embodiment
[0041] In FIGS. 3A, 3B, and 3C, an objective optical system 12 in
this embodiment includes a first lens unit Li with negative
refracting power, having concave surfaces on the object side and
the pupil side; a second lens unit L.sub.2 of a meniscus lens with
negative refracting power, directing concave surfaces toward the
object side; a third lens unit L.sub.3 with positive refracting
power, having convex surfaces on both sides; and a fourth lens unit
L.sub.4 with negative refracting power, having a concave surface
configured as the entrance surface of a three-reflection prism P.
An image erecting optical system 13, as shown in FIG. 4, is
constructed with the three-reflection prism P, in which a field
frame Q is placed so as to come in contact with the exit surface
thereof. A mirror R for reflecting incident light is located so
that the axis of light emerging from the mirror R becomes parallel
with that of light incident on the prism P. An ocular optical
system 14 is constructed with an eyepiece L.sub.5 with positive
refracting power, having convex surfaces on both sides.
[0042] When a change of the magnification of the finder is made
from the wide-angle position to the telephoto position, the third
lens unit L.sub.3 is simply moved toward the object side, and the
first and second lens units L.sub.1 and L.sub.2 are moved along the
optical axis for diopter adjustment involved in the change of the
magnification. Also, each of the first, second, and third lens
units is constructed with a single lens.
[0043] In the first embodiment, since the diopter adjustment is not
made with respect to an observer's eye, the ocular optical system
14 has a single fixed lens L.sub.5 to hermetically seal an
intermediate image section.
[0044] The surfaces of individual optical components, in order from
the object side, are labeled r.sub.1-r.sub.12 in FIG. 3B.
Aspherical surfaces are used for a surface r.sub.2 on the pupil
side of the first lens unit Li, a surface r.sub.3 on the object
side of the second lens unit L.sub.2, both surfaces r.sub.5 and
r.sub.6 of the third lens unit L.sub.3, and a surface r.sub.11 on
the pupil side of the eyepiece L.sub.5.
[0045] In the present invention, since the exit surface of the
three-reflection prism P is configures to be convex and is also
used as the field lens, the number of parts can be reduced. Also,
although in the present invention the entrance surface of the
three-reflection prism P is shaped into a concave form and is used
as the fourth lens unit of the objective optical system 12, it may
be constructed as an independent lens with negative refracting
power.
[0046] Subsequently, numerical data of the first embodiment are
shown below. Also, aberration characteristics of the optical system
of the finder in the first embodiment are as shown in FIGS. 5A-5C,
6A-6C, and 7A-7C.
[0047] In the numerical data, .omega. is a half angle of view of
emergence (.degree. ); EP is an eyepoint; m is a finder
magnification; r.sub.1, r.sub.2, . . . are radii of curvature (mm)
of individual lens and prism surfaces; d.sub.1, d.sub.2, are
distances (mm) between individual surfaces; n.sub.1, n.sub.2, . . .
are refractive indices of individual lenses and prisms in the d
line; .nu..sub.1, .nu..sub.2, . . . are Abbe's numbers of
individual lenses and prisms; r is a paraxial radius of curvature;
k is a conic constant; and A.sub.4, A.sub.6, A.sub.8, and A.sub.10
are aspherical coefficients of the fourth, sixth, eighth, and tenth
orders, respectively. These symbols are applied to all the
embodiments.
[0048] Also, the configuration of each of the aspherical surfaces
is given by the following equation:
x=(y.sup.2/r)/[1+{1-(1+k)(y/r).sup.2}.sup.1/2]+A.sub.4y.sup.4+A.sub.6y.sup-
.6+A.sub.8y.sup.8+A.sub.10y.sup.10
[0049] where x is the coordinate in the direction of the optical
axis and y is the coordinate in the direction normal to the optical
axis.
1 Magnification 0.43x (wide-angle)-0.64x (middle)-1.00x (telephoto)
(m) Half angle of 26.0.degree. (wide-angle)-17.1.degree.
(middle)-10.8.degree. (telephoto) view (.omega.) Pupil diameter .o
slashed. 4 mm r.sub.1 = -22.013 d.sub.1 = 0.800 n.sub.1 = 1.58423
.nu..sub.1 = 30.49 r.sub.2 = 12.273 d.sub.2 = 6.150 (wide-angle),
3.482 (middle), 1.231 (telephoto) r.sub.3 = -3.597 d.sub.3 = 1.147
n.sub.3 = 1.58423 .nu..sub.3 = 30.49 r.sub.4 = -9.743 d.sub.4 =
1.844 (wide-angle), 0.924 (middle), 0.200 (telephoto) r.sub.5 =
7.775 d.sub.5 = 2.043 n.sub.5 = 1.52542 .nu..sub.5 = 55.78 r.sub.6
= -4.613 d.sub.6 = 1.716 (wide-angle), 4.190 (middle), 8.278
(telephoto) r.sub.7 = -17.007 d.sub.7 = 23.900 n.sub.7 = 1.52542
.nu..sub.7 = 55.78 r.sub.8 = -14.423 d.sub.8 = 0.000 r.sub.9 =
.infin. d.sub.9 = 18.000 r.sub.10 = 28.036 d.sub.10 = 2.600
n.sub.10 = 1.49241 .nu..sub.10 = 57.66 r.sub.11 = -14.005 d.sub.11
= 18.500 r.sub.12 = (EP) Aspherical coefficients Second surface r =
12.275, k = -1.02222 A4 = -2.89549 .times. 10.sup.-4, A6 = -5.06179
.times. 10.sup.-5, A8 = 3.39356 .times. 10.sup.-6, A10 = 0.00000
Third surface r = -3.597, k = -1.19309 A4 = -2.50359 .times.
10.sup.-3, A6 = -3.43509 .times. 10.sup.-4, A8 = 2.66297 .times.
10.sup.-5, A10 = -3.31788 .times. 10.sup.-6 Fifth surface r =
7.775, k = -13.22244 A4 = 9.85901 .times. 10.sup.-4, A6 = -6.07753
.times. 10.sup.-5, A8 = -1.42148 .times. 10.sup.-6, A10 = 5.49069
.times. 10.sup.-8 Sixth surface r = -4.613, k = -0.28412 A4 =
5.47072 .times. 10.sup.-4, A6 = 6.94534 .times. 10.sup.-5, A8 =
-4.73049 .times. 10.sup.-6, A10 = 0.00000 Eleventh surface r =
-14.005, k = -3.60103 A4 = -4.86880 .times. 10.sup.-5, A6 =
-2.05766 .times. 10.sup.-6, A8 = 1.08973 .times. 10.sup.-7, A10 =
-1.73226 .times. 10.sup.-9 Values of parameters shown in Conditions
(1) and (2) Condition (1): .nu.p = 55.78 Condition (2): Lobj = 37.6
mm, Lpr = 23.9 mm, Lpr / Lobj = 0.636
[0050] Second embodiment
[0051] The second embodiment is explained with reference to FIGS.
8A, 8B, and 8C. This embodiment has the same arrangement as the
first embodiment with the exception that when the magnification is
changed, the second and third lens units are moved. Since the first
lens unit is fixed and the number of moving lens units is reduced,
a variable magnification mechanism can be simplified.
[0052] Subsequently, numerical data of the second embodiment are
shown below.
2 Magnification 0.43x (wide-angle)-0.64x (middle)-1.00x (telephoto)
(m) Half angle of 25.6.degree. (wide-angle)-16.7.degree.
(middle)-10.4.degree. (telephoto) view (.omega.) Pupil diameter .o
slashed. 4 mm r.sub.1 = -68.520 d.sub.1 = 0.800 n.sub.1 = 1.58423
.nu..sub.1 = 30.49 r.sub.2 = 15.062 d.sub.2 = 3.232 (wide-angle),
3.555 (middle), 1.069 (telephoto) r.sub.3 = -6.164 d.sub.3 = 1.041
n.sub.3 = 1.58423 .nu..sub.3 = 30.49 r.sub.4 = -18.904 d.sub.4 =
5.729 (wide-angle), 2.449 (middle), 0.800 (telephoto) r.sub.5 =
9.585 d.sub.5 = 2.097 n.sub.5 = 1.52542 .nu..sub.5 = 55.78 r.sub.6
= -6.269 d.sub.6 = 0.800 (wide-angle), 3.757 (middle), 7.893
(telephoto) r.sub.7 = -27.823 d.sub.7 = 23.900 n.sub.7 = 1.52542
.nu..sub.7 = 55.78 r.sub.8 = -17.083 d.sub.8 = 0.000 r.sub.9 =
.infin. d.sub.9 = 18.000 r.sub.10 = 217.193 d.sub.10 = 2.600
n.sub.10 = 1.49241 .nu..sub.10 = 57.66 r.sub.11 = -10.197 d.sub.11
= 18.500 r.sub.12 = (EP) Aspherical coefficients Second surface r =
15.062, k = -0.64061 A4 = -3.00125 .times. 10.sup.-4, A6 = -5.16751
.times. 10.sup.-5, A8 = 2.18066 .times. 10.sup.-6, A10 = 0.00000
Third surface r = -6.164, k = -1.21250 A4 = -6.73166 .times.
10.sup.-4, A6 = -2.42596 .times. 10.sup.-5, A8 = -1.39647 .times.
10.sup.-5, A10 = 9.10282 .times. 10.sup.-7 Fifth surface r = 9.585,
k = -9.47791 A4 = 4.12329 .times. 10.sup.-4, A6 = -5.34777 .times.
10.sup.-5, A8 = 2.91441 .times. 10.sup.-6, A10 = -6.22966 .times.
10.sup.-8 Sixth surface r = -6.269, k = -0.07796 A4 = 4.64493
.times. 10.sup.-4, A6 = -1.57600 .times. 10.sup.-5, A8 = 6.72748
.times. 10.sup.-7, A10 = 0.00000 Eleventh surface r = -10.197, k =
-1.66633 A4 = -8.32536 .times. 10.sup.-5, A6 = 1.55257 .times.
10.sup.-6, A8 = -4.32080 .times. 10.sup.-8, A10 = 3.99884 .times.
10.sup.-10 Values of parameters shown in Conditions (1) and (2)
Condition (1): .nu.p = 55.78 Condition (2): Lobj = 37.599 mm, Lpr =
23.9 mm, Lpr / Lobj = 0.636
[0053] Third embodiment
[0054] The third embodiment is explained with reference to FIGS.
9A, 9B, and 9C. The objective optical system 12 in this embodiment,
unlike those in the first and second embodiments, includes the
first lens unit L.sub.1 with negative refracting power, having a
convex surface on the object side and a concave surface on the
pupil side; the second lens unit L.sub.2 of a meniscus lens with
negative refracting power, having a concave surface on the object
side and a convex surface on the pupil side; the third lens unit
L.sub.3 with positive refracting power, having convex surfaces on
both sides; and the fourth lens unit L.sub.4 having a concave
surface directed toward the object side, configured as the entrance
surface of the three-reflection prism P. The image erecting optical
system 13 has the same arrangement as in the first embodiment. The
ocular optical system 14, unlike those of the first and second
embodiments, is constructed with a fixed lens L.sub.6 having a
concave surface on the object side and a moving lens L.sub.7 having
convex surfaces on both sides. The fixed lens L.sub.6 is designed
so that the intermediate image section is hermetically sealed to
prevent the adhesion of dirt particles to the exit surface of the
prism P. In the third embodiment, aspherical surfaces are used for
a surface r.sub.2 on the pupil side of the first lens unit Li, a
surface r.sub.3 on the object side of the second lens unit L.sub.2,
both surfaces r.sub.5 and r.sub.6 of the third lens unit L.sub.3,
and a surface r.sub.13 on the pupil side of the moving lens
L.sub.7.
[0055] In the third embodiment, materials that satisfy Condition
(1) are used and thus axial chromatic aberration is favorably
corrected.
[0056] Subsequently, numerical data of the third embodiment are
shown below.
3 Magnification 0.43x (wide-angle)-0.64x (middle)-0.99x (telephoto)
(m) Half angle of 25.8.degree. (wide-angle)-16.9.degree.
(middle)-10.8.degree. (telephoto) view (.omega.) Pupil diameter .o
slashed. 4 mm r.sub.1 = 9.543 d.sub.1 = 0.800 n.sub.1 = 1.58423
.nu..sub.1 = 30.49 r.sub.2 = 4.794 d.sub.2 = 1.578 (wide-angle),
2.622 (middle), 1.816 (telephoto) r.sub.3 = -7.169 d.sub.3 = 0.800
n.sub.3 = 1.58423 .nu..sub.3 = 30.49 r.sub.4 = -16.728 d.sub.4 =
7.512 (wide-angle), 3.193 (middle), 0.800 (telephoto) r.sub.5 =
6.681 d.sub.5 = 2.710 n.sub.5 = 1.49241 .nu..sub.5 = 57.66 r.sub.6
= -7.381 d.sub.6 = 0.800 (wide-angle), 3.374 (mIddle), 7.273
(telephoto) r.sub.7 = -24.314 d.sub.7 = 22.900 n.sub.7 = 1.58423
.nu..sub.7 = 30.49 r.sub.8 = -17.977 d.sub.8 = 0.000 r.sub.9 =
.infin. d.sub.9 = 15.200 r.sub.10 = -42.007 d.sub.10 = 0.800
n.sub.10 = 1.58423 .nu..sub.10 = 30.49 r.sub.11 = .infin. d.sub.11
= variable (fixed in magnification change) r.sub.12 = 34.828
d.sub.12 = 2.600 n.sub.12 = 1.49241 .nu..sub.12 = 57.66 r.sub.13 =
-9.980 d.sub.13 = variable (fixed in magnification change) r.sub.14
= (EP) Aspherical coefficients Second surface r = 4.794, k =
1.22930 A4 = -6.62879 .times. 10.sup.-4, A6 = -9.53119 .times.
10.sup.-5, A8 = 3.35102 .times. 10.sup.-5, A10 = 0.00000 Third
surface r = -7.169, k = 1.43673 A4 = 1.68122 .times. 10.sup.-3, A6
= 1.66187 .times. 10.sup.-4, A8 = -1.92425 .times. 10.sup.-5, A10 =
6.11812 .times. 10.sup.-6 Fifth surface r = 6.681, k = -0.29920 A4
= -2.41385 .times. 10.sup.-4, A6 = 1.06721 .times. 10.sup.-4, A8 =
3.58275 .times. 10.sup.-6, A10 = -1.74078 .times. 10.sup.-7 Sixth
surface r = -7.381, k = -1.52416 A4 = 8.10006 .times. 10.sup.-4, A6
= 5.81412 .times. 10.sup.-5, A8 = 9.43870 .times. 10.sup.-6, A10 =
0.00000 Thirteenth surface r = -9.980, k = -5.25419 A4 = -5.08607
.times. 10.sup.-4, A6 = 1.10983 .times. 10.sup.-5, A8 = -2.33686
.times. 10.sup.-7, A10 = 2.69264 .times. 10.sup.-9 Values of
parameters shown in Conditions (1), (2), and (3) Condition (1):
.nu.p = 30.49 Condition (2): Lobj = 37.1 mm, Lpr = 22.9 mm, Lpr /
Lobj = 0.617 Condition (3): fR1 = -71.9, fR2 = 16.0, .vertline. fR2
/ fR1 .vertline. = 0.223
* * * * *