U.S. patent application number 10/994092 was filed with the patent office on 2005-12-08 for imaging lens.
This patent application is currently assigned to Milestone Co., Ltd.. Invention is credited to Do, Satoshi.
Application Number | 20050270665 10/994092 |
Document ID | / |
Family ID | 33509288 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050270665 |
Kind Code |
A1 |
Do, Satoshi |
December 8, 2005 |
IMAGING LENS
Abstract
The present invention is an imaging lens in which various
aberrations are favorably corrected, the optical length is short,
and a sufficient back focus is secured. The imaging lens is
constituted by arranging a first lens L1, an aperture diaphragm S1,
a second lens L2, and a third lens L3 in succession from the object
side to the image side. The first lens L1 is a lens having a
positive refractive power and a meniscus shape in which the convex
surface faces the object side, the second lens L2 is a lens having
a negative refractive power and a meniscus shape in which the
convex surface faces the image side, and the third lens L3 is a
lens in which the convex surface faces the object side. The imaging
lens satisfies the following conditions:
0.35<r.sub.1/r.sub.2<0.45 (1) 0.07<D.sub.2/f<0.1 (2)
0.01<D.sub.4/f<0.04 (3) 1.00<d/f<1.30 (4)
0.3<b.sub.f/f<0.5 (5) where f is the combined focal length of
the imaging lens, r.sub.1 is the radius of curvature (axial radius
of curvature) of the object-side surface of the first lens L1 in
the vicinity of the optical axis, r.sub.2 is the radius of
curvature (axial radius of curvature) of the image-side surface of
the first lens L1 in the vicinity of the optical axis, D.sub.2 is
the distance between the first lens L1 and second lens L2, D.sub.4
is the distance between the second lens L2 and third lens L3, d is
the distance (atmospheric) from the object-side surface of the
first lens L1 to the imaging surface, and b.sub.f is the distance
(atmospheric) from the image-side surface of the third lens to the
imaging surface.
Inventors: |
Do, Satoshi; (Saitama,
JP) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20045-9998
US
|
Assignee: |
Milestone Co., Ltd.
Tokyo
JP
|
Family ID: |
33509288 |
Appl. No.: |
10/994092 |
Filed: |
November 22, 2004 |
Current U.S.
Class: |
359/716 |
Current CPC
Class: |
G02B 13/0035 20130101;
G02B 13/16 20130101; G02B 9/16 20130101 |
Class at
Publication: |
359/716 |
International
Class: |
G02B 003/02; G02B
021/02; G02B 009/12; G02B 013/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 2004 |
JP |
2004-164599 |
Claims
What is claimed is:
1. An imaging lens comprising a first lens L1, an aperture
diaphragm S1, a second lens L2, and a third lens L3, and
constituted such that said first lens L1, aperture diaphragm S1,
second lens L2, and third lens L3 are arranged in succession from
the object side to the image side, wherein said first lens L1 is a
lens having a positive refractive power and a meniscus shape in
which the convex surface faces the object side, said second lens L2
is a lens having a negative refractive power and a meniscus shape
in which the convex surface faces the image side, and said third
lens L3 is a lens in which the convex surface faces the object
side, both surfaces of said first lens L1 and said second lens L2
being aspheric, and at least one surface of said third lens L3
being aspheric, and said imaging lens satisfying the following
conditions. 0.35<r.sub.1/r.sub.2<0.45 (1)
0.07<D.sub.2/f<0.1 (2) 0.01<D.sub.4/f<0.04 (3)
1.00<d/f<1.30 (4) 0.3<b.sub.f/f<0.5 (5) where f is the
combined focal length of the imaging lens, r.sub.1 is the radius of
curvature (axial radius of curvature) of the object-side surface of
the first lens L1 in the vicinity of the optical axis, r.sub.2 is
the radius of curvature (axial radius of curvature) of the
image-side surface of the first lens L1 in the vicinity of the
optical axis, D.sub.2 is the distance between the first lens L1 and
second lens L2, D.sub.4 is the distance between the second lens L2
and the third lens L3, d is the distance (atmospheric) from the
object-side surface of the first lens L1 to the imaging surface,
and b.sub.f is the distance (atmospheric) from the image-side
surface of the third lens L3 to the imaging surface.
2. The imaging lens according to claim 1, wherein said first lens
L1, said second lens L2, and said third lens L3 constituting said
imaging lens are formed from a material having an Abbe number
within a range of thirty to sixty.
3. The imaging lens according to claim 1, wherein said first lens
L1, said second lens L2, and said third lens L3 constituting said
imaging lens are formed using cycloolefin plastics as a
material.
4. The imaging lens according to claim 1, wherein said first lens
L1 and said third lens L3 constituting said imaging lens are formed
using cycloolefin plastics as a material, and said second lens L2
is formed using polycarbonate as a material.
5. The imaging lens according to claim 1, wherein said first lens
L1 and said third lens L3 constituting said imaging lens are formed
using cycloolefin plastics as a material, said second lens L2 is
formed using polycarbonate as a material, and said third lens L3 is
a lens having a negative refractive power and a meniscus shape in
which the convex surface faces the object side.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an imaging lens, and more
particularly to an imaging lens which is suitable for installation
into an image input device of a portable telephone or personal
computer, a digital camera, a CCD (charge-coupled device) camera
used for monitoring purposes, a surveying device, or similar which
uses a CCD or CMOS (complementary metal-oxide semiconductor) as an
imaging device.
[0003] 2. Description of Related Art
[0004] In such an imaging lens, the optical length, which is
defined as the distance from the entrance surface on the object
side of the imaging lens to the imaging surface (the image-forming
surface of a CCD or the like), must be short. In other words,
during design of the lens, a method of reducing the ratio of the
optical length to the combined focal length of the imaging lens is
required. An imaging lens having a short optical length and a small
optical length to focal length ratio will occasionally be referred
to as a compact lens below.
[0005] Taking a portable telephone as an example, the optical
length must at least be shorter than the thickness of the portable
telephone body. Meanwhile, the back focus, which is defined as the
distance from the exit surface on the image side of the imaging
lens to the imaging surface, is preferably as long as possible. In
other words, during design of the lens, a method of increasing the
ratio of the back focus to the focal length as much as possible is
required. This is due to the need to insert components such as a
filter or cover glass between the imaging lens and the imaging
surface.
[0006] As well as the points described above, there is also demand
for imaging lenses in which various types of aberration have been
corrected to such a small degree that image warping is not visually
recognizable and that is required by the integration density of
imaging elements (also known as "pixels"). In other words,
favorable aberration correction is required, and images in which
such aberration correction has been performed favorably will
occasionally be referred to as "favorable images" below.
[0007] As will be described below, imaging lenses with a
three-layer structure which are suitable for use in imaging devices
such as portable computers, video telephones, or similar using a
solid-state imaging device such as a CCD or CMOS have been
disclosed. These lenses all secure a wide viewing angle, and are
compact and lightweight.
[0008] Of these lenses, an imaging lens capable of obtaining images
with favorably corrected aberration while securing a wide viewing
angle has been disclosed as a first three-layer lens (see, for
example, Japanese Unexamined Patent Application Publication
2001-075006).
[0009] However, the refractive power of these three lenses, which
are constituted by first, second, and third lenses arrayed in
succession from the object side, is positive in the first lens,
negative in the second lens, and positive in the third lens, and
hence the distance (optical length) from the surface of the first
lens on the object side to the imaging surface is too long.
Further, a diaphragm is disposed on the object-side surface of the
first lens, and hence the effective diameter of the third lens
cannot be reduced. As a result, a compact lens cannot be
produced.
[0010] Imaging lenses in which aberration is favorably corrected
and a short focus is realized while securing a wide viewing angle
have been respectively disclosed as second through fourth
three-layer lenses (see, for example, Japanese Unexamined Patent
Application Publication 2003-149548, Japanese Unexamined Patent
Application Publication 2002-221659, and Japanese Unexamined Patent
Application Publication 2002-244030).
[0011] However, similarly to the imaging lens described above, the
refractive power of the three lenses of these imaging lenses,
constituted by first, second, and third lenses arranged in
succession from the object side, is positive in the first lens,
negative in the second lens, and positive in the third lens. Hence,
although these imaging lenses are set with a short combined imaging
lens focal length, the back focus is long, and thus the optical
length is too long. In addition, these lenses use glass materials,
and are therefore expensive.
[0012] An imaging lens which uses aspheric lenses and is reduced in
size by appropriately setting power distribution and surface shape
has been disclosed as a fifth three-layer lens (see, for example,
Japanese Unexamined Patent Application Publication
2003-149545).
[0013] However, the refractive power of the three lenses of this
imaging lens, constituted by first, second, and third lenses
arranged in succession from the object side, is negative in the
first lens, positive in the second lens, and negative in the third
lens. As a result, the imaging lens has a long optical length. In
addition, the lenses use glass materials, and are therefore
expensive.
[0014] A lens in which a pair of meniscus lenses whose concave
surfaces face each other are constituted by plastic lenses each
having at least one aspheric surface, and in which the entire lens
system has a three-layer structure, has been disclosed as a sixth
three-layer lens (see, for example, Japanese Unexamined patent
Application Publication H10-301022). This lens achieves compactness
and low cost, and is capable of suppressing focus movement due to
temperature change with ease.
[0015] However, the refractive power of the three lenses in this
imaging lens, which are arranged as first, second, and third lenses
in succession from the object side, is weak in the first lens, weak
in the second lens, and positive in the third lens, and hence the
refractive power of the first lens and second lens cannot be fully
compensated for by the third lens alone. As a result, the back
focus lengthens, causing an increase in the optical length.
Furthermore, the third lens uses a glass material, and hence cost
reduction is incomplete.
[0016] A low-cost lens system with a short optical length which has
a telephoto-type lens constitution in which the entire lens system
is divided into a front group and a rear group, the front group
having a positive refractive power and the rear group having a
negative refractive power, has been disclosed as a seventh
three-layer lens (see, for example, Japanese Unexamined Patent
Application Publication H10-301021).
[0017] However, the refractive power of the three lenses in this
lens system, which are arranged as first, second, and third lenses
in succession from the object side, is negative in the first lens,
positive in the second lens, and negative in the third lens, and
the distance between the second lens and third lens is wide. As a
result, the optical length is long, and the aperture of the third
lens widens. This is unsuitable for installation in image input
devices of portable telephones or personal computers, digital
cameras, CCD cameras used for monitoring purposes, surveying
devices, and so on.
[0018] An imaging lens comprising, in succession from the object
side, two positive lenses, and a negative lens whose concave
surface faces the image side, both surfaces of which are aspheric
and the negative power of which gradually weakens from the center
of the lens toward the periphery so as to have a positive power on
the periphery, has been disclosed as an eighth three-layer lens
(see, for example, Japanese Unexamined Patent Application
Publication 2003-322792).
[0019] In this lens system, however, the lens corresponding to a
third lens L3 gradually weakens in negative power from the center
of the lens toward the periphery, and the position where the
negative power turns into positive power exists within a range of
between 0.7 times and 1.0 times the effective diameter of the lens
from the center of the lens. In the lens disclosed in the
embodiments of the invention, the positions where the negative
power turns into positive power are set respectively at 0.96 and
0.97 times the effective diameter of the lens from the center of
the lens, i.e. substantially at the periphery of the lens.
[0020] By setting the position where negative power turns into
positive power at the peripheral portion of the lens, light
entering the vicinity of the intersecting point between the optical
axis of the lens and the imaging surface and the periphery of the
lens has an almost right-angled angle of incidence onto the imaging
device, whereas in an intermediate position between the
intersecting point of the optical axis of the lens and the imaging
surface and the periphery of the lens, the angle of incidence onto
the imaging device deviates greatly from a right angle. Since the
angle of incidence of the light entering an intermediate position
from the peripheral portion of the lens, which forms an important
part of an image, deviates greatly from a right angle, the light
enters the imaging device in a diagonal direction to the imaging
device, thereby increasing the amount of reflection on the entrance
surface such that the light reaching a photoelectric conversion
surface of the imaging device is low in energy. As a result, this
part of the image becomes dark.
[0021] It is therefore an object of the present invention to
provide an imaging lens which is suitable for installation in a
camera using a CCD or CMOS as an imaging device, which has a short
optical length (a small optical length to focal length ratio), a
back focus which is as long as possible (a back focus to focal
length ratio which is as large as possible), and which is thus
capable of obtaining favorable images.
[0022] A further object of the present invention is to provide an
imaging lens in which all of the (three) lenses constituting the
imaging lens of the invention are made of plastic materials to
thereby reduce cost and weight. Here, "plastic materials" refers to
high polymeric substances which are transparent to visible light,
and may be molded by being subjected to plastic deformation through
application of heat, pressure, or both, and thereby formed into
lenses.
SUMMARY OF THE INVENTION
[0023] In order to achieve the objects described above, an imaging
lens according to the present invention is constituted by arranging
a first lens L1, an aperture diaphragm S1, a second lens L2, and a
third lens L3 in succession from the object side to the image side.
The first lens L1 is a lens having a positive refractive power and
a meniscus shape in which the convex surface faces the object side.
The second lens L2 is a lens having a negative refractive power and
a meniscus shape in which the convex surface faces the image side.
The third lens L3 is a lens in which the convex surface faces the
object side.
[0024] Further, both surfaces of the first lens L1, both surfaces
of the second lens L2, and at least one surface of the third lens
L3 are constituted by aspheric surfaces.
[0025] According to constitutional examples of the present
invention, this imaging lens satisfies the following conditions (1)
through (5).
0.35<r.sub.1/r.sub.2<0.45 (1)
0.07<D.sub.2/f<0.1 (2)
0.01<D.sub.4/f<0.04 (3)
1.00<d/f<1.30 (4)
0.3<b.sub.f/f<0.5 (5)
[0026] where
[0027] f is the combined focal length of the imaging lens,
[0028] r.sub.1 is the radius of curvature (axial radius of
curvature) of the object-side surface of the first lens L1 in the
vicinity of the optical axis,
[0029] r.sub.2 is the radius of curvature (axial radius of
curvature) of the image-side surface of the first lens L1 in the
vicinity of the optical axis,
[0030] D.sub.2 is the distance from the first lens L1 to the second
lens L2,
[0031] D.sub.4 is the distance from the second lens L2 to the third
lens L3,
[0032] d is the distance (atmospheric) from the object-side surface
of the first lens L1 to the imaging surface, and
[0033] b.sub.f is the distance (atmospheric) from the image-side
surface of the third lens L3 to the imaging surface.
[0034] The back focus b.sub.f, which is defined as the distance
from the exit surface on the image side of the imaging lens to the
imaging surface, is defined here as the distance from the
image-side surface r.sub.8 of the third lens L3 to the imaging
surface r.sub.11.
[0035] Further, the first lens L1, second lens L2, and third lens
L3 are preferably constituted by lenses formed from a material
having an Abbe number within a range of thirty to sixty. It is also
preferable that the first lens L1, second lens L2, and third lens
L3 be constituted by lenses formed using a cycloolefin plastics or
a polycarbonate as a material.
[0036] Further, the first lens L1 and third lens L3 are preferably
constituted as lenses formed using cycloolefin plastics as a
material, the second lens L2 is preferably constituted as a lens
formed using a polycarbonate as a material, and the third lens L3
is preferably constituted by a lens having a negative refractive
power and a meniscus shape in which the convex surface faces the
object side.
[0037] It was clarified through simulation that by constituting the
first lens L1 by a lens having a positive refractive power and a
meniscus shape in which the convex surface faces the object side,
constituting the second lens L2 by a lens having a negative
refractive power and a meniscus shape in which the convex surface
faces the image side, and constituting the third lens L3 by a lens
in which the convex surface faces the object side, an optical
length d can be shortened. It was also learned through simulation
that by forming the second lens L2 using a material having a higher
refractivity than the refractivity of the material of the first
lens L1 and a smaller Abbe number than the Abbe number of the
material of the first lens L1, chromatic and spherical aberration
can be reduced effectively.
[0038] The effects on the imaging lens of the present invention
exhibited by the conditional expressions (1) through (5) are as
follows.
[0039] The conditional expression (1) mentioned above is a
condition for determining the ratio r.sub.1/r.sub.2 of the axial
radius of curvature r.sub.1 of the first surface of the first lens
L1 and the axial radius of curvature r.sub.2 of the second surface
of the first lens L1. If the ratio r.sub.1/r.sub.2 is larger than
the lower limit provided by the conditional expression (1), then
the back focus of the imaging lens is sufficient for inserting a
component such as a cover glass or filter between the imaging lens
and the imaging surface, and thus the back focus can be set within
a range which does not impair the compactness of the device into
which the imaging lens is to be installed. Moreover, distortion can
be reduced sufficiently, and hence manufacturing of the first
surface of the first lens L1 is facilitated.
[0040] If the ratio r.sub.1/r.sub.2 is smaller than the upper limit
provided by the conditional expression (1), then the absolute
distortion value is sufficiently small. Furthermore, in this case,
distortion can be reduced sufficiently without increasing the
number of aspheric elements.
[0041] The conditional expression (2) mentioned above is for
defining the allowable range of the distance D.sub.2 between the
first lens L1 and second lens L2 by D.sub.2/f, which is
standardized by the combined focal length f of the imaging lens. If
D.sub.2/f is larger than the lower limit provided by the
conditional expression (2), then the distance between the
image-side surface r.sub.2 of the first lens L1 and the object-side
surface r.sub.5 of the second lens L2 can be secured as a
sufficient distance for inserting the aperture diaphragm S1. In
other words, the outer forms of the first lens L1 and second lens
L2 do not have to be reduced to the extent that manufacture becomes
difficult, and a sufficient space for inserting the aperture
diaphragm S1 can be ensured.
[0042] If D.sub.2/f is smaller than the upper limit provided by the
conditional expression (2), then there is no need to increase the
outer form of the first lens L1 and second lens L2, and hence the
imaging lens can be made compact. Further, imaging surface
distortion does not increase, and hence favorable images are
obtained.
[0043] The conditional expression (3) mentioned above is for
defining the allowable range of the distance D.sub.4 between the
second lens L2 and the third lens L3 by D.sub.4/f, which is
standardized by the combined focal length f of the imaging lens. If
D.sub.4/f is larger than the lower limit provided by the
conditional expression (3), then the gradient of the light rays
entering the imaging surface in relation to the optical axis can be
reduced, and hence the shading phenomenon whereby light is
obstructed around the periphery of the lens such that the
peripheral parts of the image become dark can be avoided.
[0044] If D.sub.4/f is smaller than the upper limit provided by the
conditional expression (3), then distortion does not increase, and
hence favorable images are obtained. Moreover, the effective
diameter of the third lens L3 can be reduced, and hence the imaging
lens can be made compact.
[0045] The conditional expression (4) illustrated above is for
defining the allowable range of the distance (atmospheric) d from
the object-side surface of the first lens L1 to the imaging surface
by d/f, which is standardized by the combined focal length f of the
imaging lens. The notation "distance (atmospheric) d" used in
reference to the distance d from the object-side surface of the
first lens L1 to the imaging surface signifies the distance from
the object-side surface of the first lens L1 to the imaging surface
measured on the condition that no transparent object (a cover glass
or the like) other than air be inserted between the object-side
surface of the first lens L1 and the imaging surface.
[0046] If d/f is larger than the lower limit provided by the
conditional expression (4), then there is no need to reduce the
thickness of the first lens L1, second lens L2, and third lens L3,
and hence it does not become difficult to distribute resin over a
die during formation of the resin lenses. If d/f is smaller than
the upper limit provided by the conditional expression (4), then
the problem of the amount of light on the periphery of the lens
being less than that in the central portion of the lens does not
arise. Thus the amount of light on the periphery of the lenses can
be increased without increasing the size of the outer forms of the
first lens L1, second lens L2, and third lens L3, which are the
constituent lenses of the imaging lens. As a result, the imaging
lens can be made compact.
[0047] The conditional expression (5) mentioned above is for
defining the length of the back focus b.sub.f in relation to the
combined focal length f of the imaging lens. If the length of the
back focus b.sub.f is within the range provided by the conditional
expression (5), then a component such as a filter, which is often
required in image input devices of portable telephones and the
like, can be inserted between the imaging lens and the imaging
surface.
[0048] By providing a lens constitution which satisfies the five
conditions in the conditional expressions (1) to (5) mentioned
above, the problems described above can be solved, and a compact
imaging lens which is small yet capable of obtaining favorable
images can be provided.
[0049] Further, by constituting the first lens L1, second lens L2,
and third lens L3 by lenses formed from a material having an Abbe
number within a range of thirty to sixty, more favorable images are
obtained more easily than when the lenses are manufactured using a
material with an Abbe number outside of this range. The Abbe number
of cycloolefin plastics is 56.2, and the Abbe number of
polycarbonate is 30.0, and hence these materials may be used for
the imaging lens of the present invention. It is known that
cycloolefin plastics or polycarbonate material is suitable for
forming lenses using a well-established injection molding method.
Needless to say, the present invention is not limited to a specific
plastic material, and any plastic material or molded glass material
having an Abbe number of between thirty and sixty may be used.
[0050] Polycarbonate has a higher refractivity and a smaller Abbe
number than cycloolefin plastics. Hence, by forming the first lens
L1 and third lens L3 from cycloolefin plastics and forming the
second lens L2 from polycarbonate, the optical length can be
shortened, and the resolution can be further increased.
[0051] Moreover, by constituting the first lens L1 and third lens
L3 by lenses formed using cycloolefin plastics, constituting the
second lens L2 by a lens formed using polycarbonate, and
constituting the third lens L3 by a lens having a negative
refractive power and a meniscus shape in which the convex surface
faces the object side, the optical length can be shortened more
reliably, and an imaging lens having a high resolution can be
realized. This will be described in further detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] The foregoing and other objects, features and advantages of
the present invention will be better understood from the following
description taken in connection with the accompanying drawings, in
which:
[0053] FIG. 1 is a sectional view of an imaging lens according to
the present invention;
[0054] FIG. 2 is a sectional view of an imaging lens of Embodiment
1;
[0055] FIG. 3 is a view of distortion in the imaging lens of the
Embodiment 1;
[0056] FIG. 4 is a view of astigmatism in the imaging lens of the
Embodiment 1;
[0057] FIG. 5 is a view of chromatic and spherical aberration in
the imaging lens of the Embodiment 1;
[0058] FIG. 6 is a sectional view of an imaging lens of Embodiment
2;
[0059] FIG. 7 is a view of distortion in the imaging lens of the
Embodiment 2;
[0060] FIG. 8 is a view of astigmatism in the imaging lens of the
Embodiment 2;
[0061] FIG. 9 is a view of chromatic and spherical aberration in
the imaging lens of the Embodiment 2;
[0062] FIG. 10 is a sectional view of an imaging lens of Embodiment
3;
[0063] FIG. 11 is a view of distortion in the imaging lens of the
Embodiment 3;
[0064] FIG. 12 is a view of astigmatism in the imaging lens of the
Embodiment 3;
[0065] FIG. 13 is a view of chromatic and spherical aberration in
the imaging lens of the Embodiment 3;
[0066] FIG. 14 is a sectional view of an imaging lens of Embodiment
4;
[0067] FIG. 15 is a view of distortion in the imaging lens of the
Embodiment 4;
[0068] FIG. 16 is a view of astigmatism in the imaging lens of the
Embodiment 4;
[0069] FIG. 17 is a view of chromatic and spherical aberration in
the imaging lens of the Embodiment 4;
[0070] FIG. 18 is a sectional view of an imaging lens of Embodiment
5;
[0071] FIG. 19 is a view of distortion in the imaging lens of the
Embodiment 5;
[0072] FIG. 20 is a view of astigmatism in the imaging lens of the
Embodiment 5;
[0073] FIG. 21 is a view of chromatic and spherical aberration in
the imaging lens of the Embodiment 5;
[0074] FIG. 22 is a sectional view of an imaging lens of Embodiment
6;
[0075] FIG. 23 is a view of distortion in the imaging lens of the
Embodiment 6;
[0076] FIG. 24 is a view of astigmatism in the imaging lens of the
Embodiment 6;
[0077] FIG. 25 is a view of chromatic and spherical aberration in
the imaging lens of the Embodiment 6; and
[0078] FIG. 26 is a view showing the MTF of the imaging lenses of
the Embodiments 1 through 6 of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0079] Embodiments of the present invention will be described below
with reference to the drawings. Note that in the drawings, the
form, magnitude, and positional relationships of each
constitutional element are merely illustrated schematically in
order to facilitate understanding of the invention, and the
numerical conditions and other conditions to be described below are
merely preferred examples thereof. Accordingly, the present
invention is in no way limited to or by the embodiments of the
invention.
[0080] FIG. 1 is a constitutional diagram of an imaging lens
according to the present invention. The reference symbols defined
in FIG. 1, which indicate surface numbers, surface distances, and
so on, are also used in FIGS. 2, 6, 10, 14, 18, and 22.
[0081] From the object side, first, second, and third lenses are
indicated by the reference symbols L1, L2, and L3 respectively. An
imaging device constituting the imaging surface is indicated by the
numeral 10, a cover glass separating the imaging surface and lens
system is indicated by the numeral 12, and an aperture diaphragm is
indicated by the symbol S1. The surfaces of the aperture diaphragm
S1 are indicated by r.sub.3 and r.sub.4. The symbol r.sub.i (i=1,
2, 3, . . . , 11) is used as both a variable representing an axial
radius of curvature value and a symbol identifying a lens, cover
glass, or imaging surface (for example, r.sub.1 is used to
represent the object-side surface of the first lens and so on)
provided that no confusion is caused thereby.
[0082] Parameters shown in the drawing such as r.sub.i (where i=1,
2, 3, . . . , 11) and d.sub.i (where i=1, 2, 3, . . . , 10) are
provided as specific numerical values in the following Tables 1
through 6. The suffix i corresponds to the surface numbers of each
lens, the lens thickness, the lens distance, or similar, in
succession from the object side to the image side.
[0083] More specifically:
[0084] r.sub.i is the axial radius of curvature of the i.sup.th
surface;
[0085] d.sub.i is the distance from the ith surface to the
(i+1).sup.th surface;
[0086] N.sub.i is the refractive index of the lens material
constituted by the i.sup.th surface and the (i+1).sup.th surface;
and
[0087] v.sub.i is the Abbe number of the lens material constituted
by the i.sup.th surface and the (i+1).sup.th surface.
[0088] The optical length d is a value obtained by adding together
the distances from d.sub.1 through d.sub.7, and further adding the
back focus b.sub.f thereto. The back focus b.sub.f is the distance
from the image-side surface of the third lens L3 to the imaging
surface on the optical axis. It is assumed that the back focus
b.sub.f is measured with the cover glass 12 that is inserted
between the third lens L3 and the imaging surface removed. More
specifically, since the cover glass has a refractive index of more
than one, the geometrical distance from the image-side surface of
the third lens L3 to the imaging surface is longer when the cover
glass is inserted than when the cover glass is removed. The extent
to which the distance increases is determined by the refractive
index and thickness of the inserted cover glass. Hence, in order to
define the back focus b.sub.f as a value which is unique to the
imaging lens and does not depend on the presence or absence of a
cover glass, a value measured with the cover glass removed is used.
The distance D.sub.2 between the first lens L1 and second lens L2
is defined as D.sub.2=d.sub.2+d.sub.3+d.sub.4, and the distance
D.sub.4 between the second lens L2 and third lens L3 is defined as
D.sub.4=d.sub.6.
[0089] Aspheric surface data are illustrated together with the
surface numbers in the respective columns of Tables 1 through 6.
The surfaces r.sub.3 and r.sub.4 of the aperture diaphragm S1, the
two surfaces r.sub.9, r.sub.10 of the cover glass, and the imaging
surface r.sub.11 are flat, and hence the radius of curvature
thereof is displayed as .infin..
[0090] The aspheric surfaces used in the present invention are
obtained according to the following equation.
Z=ch.sup.2/[1+[1-(1+k)c.sup.2h.sup.2].sup.+1/2]+A.sub.0h.sup.4+B.sub.0h.su-
p.6+C.sub.0h.sup.8+D.sub.0h.sup.10
[0091] where
[0092] Z is the depth from the tangential plane to the surface
apex,
[0093] c is the paraxial curvature of the surface,
[0094] h is the height from the optical axis,
[0095] k is the conic constant,
[0096] A.sub.0 is the quartic aspheric coefficient,
[0097] B.sub.0 is the sextic aspheric coefficient,
[0098] C.sub.0 is the eighth-order ashperic coefficient, and
[0099] D.sub.0 is the tenth-order aspheric coefficient.
[0100] Numerical values indicating the aspheric coefficients are
displayed as indices in Tables 1 through 6 in this specification.
For example, "e-1" signifies "10.sup.-1". Further, the value
illustrated as the focal length f is the combined focal length of
the lens system constituted by the first through third lenses.
[0101] Embodiments 1 through 6 will now be described with reference
to FIGS. 2 through 25. FIGS. 2, 6, 10, 14, 18, and 22 are schematic
diagrams showing lens constitutions. FIGS. 3, 7, 11, 15, 19, and 23
show distortion curves, FIGS. 4, 8, 12, 16, 20, and 24 show
astigmatism curves, and FIGS. 5, 9, 13, 17, 21, and 25 show
chromatic and spherical aberration curves.
[0102] The distortion curve shows aberration (the abscissa shows as
a percentage the degree to which the tangent condition is
unsatisfied) in relation to the distance from the optical axis (the
ordinate shows a percentage with the maximum distance from the
optical axis on the imaging surface set to 100). Similarly to the
distortion curve, the astigmatism curve shows the amount of
aberration (in units of mm) in relation to the distance from the
optical axis along the abscissa, and shows the amount of aberration
(in units of mm) on the meridional plane and the sagittal plane.
The chromatic and spherical aberration curve shows the amount of
aberration (in units of mm) along the abscissa in relation to the
distance of incidence h (F number) along the ordinate.
[0103] The chromatic and spherical aberration curve also shows
aberration values for the C line (light with a wavelength of 656.3
nm), the d line (light with a wavelength of 587.6 nm), the e line
(light with a wavelength of 546.1 nm), the F line (light with a
wavelength of 486.1 nm), and the g line (light with a wavelength of
435.8 nm). The refractive index is the refractive index on the d
line (587.6 nm light).
[0104] The radius of curvature (mm units), lens surface distance
(mm units), refractive index of the lens material, Abbe number of
the lens material, focal length, numerical aperture, and aspheric
coefficient of the lenses used in the Embodiments 1 through 6 are
listed below. In the Embodiments 1 through 6, the focal lengths of
the first lens L1, second lens L2, and third lens L3 are indicated
as f.sub.1, f.sub.2, and f.sub.3 respectively.
1TABLE 1 First Embodiment Radius of Abbe Curvature Refractivity
Number Aspheric Coefficient (ri) Distance (di) (Ni) (.nu. i) K
A.sub.0 B.sub.0 C.sub.0 D.sub.0 r1 = 0.296 d1 = 0.2704 N1 = 1.525
.nu. 1 = 56.2 3.005e-1 -8.640e-1 1.398e+1 -3.892e+2 5.465e+3 r2 =
0.720 d2 = 0.0148 N5 = 1.525 .nu. 5 = 56.2 9.062 4.608 -1.552e+2
4.615e+4 -1.069e+6 r3 = .infin. d3 = 0.0123 N7 = 1.525 .nu. 7 =
56.2 1.593e-1 2.511 6.815e+2 -3.653e+4 6.896e+5 r4 = .infin. d4 =
0.0492 N9 = 1.500 .nu. 9 = 56.0 -2.442e-1 7.378 6.553e+1 -9.128e+2
1.577e+3 r5 = -0.310 d5 = 0.2459 -1.211 -1.240e-1 5.397e-1 3.175e+1
-3.943e+1 r6 = -0.446 d6 = 0.0369 -2.361 -5.596 1.749e+1 -7.991e+1
4.312e+2 r7 = 1.416 d7 = 0.2007 r8 = 1.808 d8 = 0.0608 r9 = .infin.
d9 = 0.1140 r10 = .infin. d10 = 0.2280 r11 = .infin. Focal Length f
= 1.0 mm Numerical Aperture Fno = 3.4 f.sub.1 = 0.79 mm f.sub.2 =
-5.14 mm f.sub.3 = 10.56 mm
[0105]
2TABLE 2 Second Embodiment Radius of Abbe Curvature Refractivity
Number Aspheric Coefficient (ri) Distance (di) (Ni) (.nu. i) K
A.sub.0 B.sub.0 C.sub.0 D.sub.0 r1 = 0.300 d1 = 0.2752 N1 = 1.525
.nu. 1 = 56.2 2.936e-1 -7.636e-1 1.153e+1 -3.543e+2 5.309e+3 r2 =
0.748 d2 = 0.0150 N5 = 1.525 .nu. 5 = 56.2 9.804 4.541 -1.340e+2
4.140e+4 -9.523e+5 r3 = .infin. d3 = 0.0125 N7 = 1.525 .nu. 7 =
56.2 1.070e-1 2.635 6.477e+2 -3.272e+4 5.628e+5 r4 = .infin. d4 =
0.0500 N9 = 1.500 .nu. 9 = 56.0 -1.867e-1 6.981 5.846e+1 -8.195e+2
1.463e+3 r5 = -0.315 d5 = 0.2502 -1.931 -1.458e-1 2.760e-1 2.766e+1
-3.098e+1 r6 = -0.483 d6 = 0.0232 -1.517 -5.334 1.586e+1 -7.114e+1
3.705e+2 r7 = 1.183 d7 = 0.2158 r8 = 1.784 d8 = 0.0523 r9 = .infin.
d9 = 0.1160 r10 = .infin. d10 = 0.2322 r11 = .infin. Focal Length f
= 1.0 mm Numerical Aperture Fno = 3.4 f.sub.1 = 0.79 mm f.sub.2 =
-3.57 mm f.sub.3 = 5.96 mm
[0106]
3TABLE 3 Third Embodiment Radius of Abbe Curvature Refractivity
Number Aspheric Coefficient (ri) Distance (di) (Ni) (.nu. i) K
A.sub.0 B.sub.0 C.sub.0 D.sub.0 r1 = 0.296 d1 = 0.2790 N1 = 1.525
.nu. 1 = 56.2 1.085e-1 5.752e-1 -9.986e-1 7.337e+1 2.948e+3 r2 =
0.738 d2 = 0.0129 N5 = 1.525 .nu. 5 = 56.2 1.041e+1 4.517 -3.163e+2
-3.377e+3 3.605e+5 r3 = .infin. d3 = 0.0107 N7 = 1.525 .nu. 7 =
56.2 5.540e-1 5.756 -5.728e+2 4.876e+4 -1.973e+6 r4 = .infin. d4 =
0.0665 N9 = 1.500 .nu. 9 = 56.0 -2.358e-1 8.365 7.370e-1 1.006e+2
-1.699e+3 r5 = -0.257 d5 = 0.2253 2.837e+1 8.284e-2 -8.029e-1 3.318
-2.980e+1 r6 = -0.363 d6 = 0.0215 2.513e+1 -5.310 1.391e+1
-2.237e+1 4.114e+1 r7 = 2.596 d7 = 0.2682 r8 = 5.926 d8 = 0.0509 r9
= .infin. d9 = 0.1073 r10 = .infin. d10 = 0.2146 r11 = .infin.
Focal Length f = 1.0 mm Numerical Aperture Fno = 3.4 f.sub.1 = 0.78
mm f.sub.2 = -6.27 mm f.sub.3 = 8.56 mm
[0107]
4TABLE 4 Fourth Embodiment Radius of Abbe Curvature Refractivity
Number Aspheric Coefficient (ri) Distance (di) (Ni) (.nu. i) K
A.sub.0 B.sub.0 C.sub.0 D.sub.0 r1 = 0.295 d1 = 0.2820 N1 = 1.525
.nu. 1 = 56.2 1.154e-1 5.139e-1 -1.710 7.632e+1 2.940e+3 r2 = 0.724
d2 = 0.0125 N5 = 1.525 .nu. 5 = 56.2 1.029e+1 4.678 -3.379e+2
-3.139e+3 4.502e+5 r3 = .infin. d3 = 0.0104 N7 = 1.525 .nu. 1 =
56.2 5.483e-1 6.432 -6.597e+2 6.007e+4 -2.437e+6 r4 = .infin. d4 =
0.0647 N9 = 1.500 .nu. 9 = 56.0 -2.113e-1 8.971 -1.252 1.048e+2
-2.112e+3 r5 = -0.256 d5 = 0.2297 3.134e+1 1.331e-1 -4.695e-1 4.197
-4.096e+1 r6 = -0.366 d6 = 0.0209 5.570e+1 -5.629 1.633e+1
-2.746e+1 6.040e+1 r7 = 2.636 d7 = 0.2506 r8 = 9.313 d8 = 0.0695 r9
= .infin. d9 = 0.1044 r10 = .infin. d10 = 0.2090 r11 = .infin.
Focal Length f = 1.0 mm Numerical Aperture Fno = 3.4 f.sub.1 = 0.77
mm f.sub.2 = -5.72 mm f.sub.3 = 6.91 mm
[0108]
5TABLE 5 Fifth Embodiment Radius of Abbe Curvature Refractivity
Number Aspheric Coefficient (ri) Distance (di) (Ni) (.nu. i) K
A.sub.0 B.sub.0 C.sub.0 D.sub.0 r1 = 0.299 d1 = 0.2733 N1 = 1.525
.nu. 1 = 56.2 1.493e-1 4.282e-1 6.175e-1 -1.305e+1 4.431e+3 r2 =
0.748 d2 = 0.0131 N5 = 1.525 .nu. 5 = 56.2 9.585 2.922 -1.409e+2
7.790e+3 -3.274e+5 r3 = .infin. d3 = 0.0109 N7 = 1.525 .nu. 7 =
56.2 4.477e-1 5.388 -3.525e+2 4.469e+4 -1.992e+6 r4 = .infin. d4 =
0.0656 N9 = 1.500 .nu. 9 = 56.0 -3.852e-1 9.149 3.840 7.229e+1
-2.955e+3 r5 = -0.262 d5 = 0.2186 2.723e+1 4.776e-1 -2.570 6.475
-3.627e+1 r6 = -0.362 d6 = 0.0219 -1.491e+2 -5.198 1.266e+1 -6.554
-9.668 r7 = 2.626 d7 = 0.2733 r8 = 4.922 d8 = 0.0084 r9 = .infin.
d9 = 0.1271 r10 = .infin. d10 = 0.2542 r11 = .infin. Focal Length f
= 1.0 mm Numerical Aperture Fno = 3.4 f.sub.1 = 0.79 mm f.sub.2 =
-7.42 mm f.sub.3 = 10.30 mm
[0109]
6TABLE 6 Sixth Embodiment Radius of Abbe Curvature Refractivity
Number Aspheric Coefficient (ri) Distance (di) (Ni) (.nu. i) K
A.sub.0 B.sub.0 C.sub.0 D.sub.0 r1 = 0.290 d1 = 0.2730 N1 = 1.525
.nu. 1 = 56.2 1.784e-1 -3.272e-1 1.338e+1 -3.702e+2 5.659e+3 r2 =
0.777 d2 = 0.0147 N5 = 1.583 .nu. 5 = 30.0 1.098e+1 2.983 -2.162e+1
7.146e+3 -3.417e+5 r3 = .infin. d3 = 0.0082 N7 = 1.525 .nu. 7 =
56.2 4.747e-1 4.033 -7.681e+2 5.775e+4 -2.042e+6 r4 = .infin. d4 =
0.0539 N9 = 1.500 .nu. 9 = 56.0 -2.898e-1 8.951 -2.204e+1 5.668
-8.195e+2 r5 = -0.330 d5 = 0.2484 2.893e+1 5.449e-1 9.807e-1 -9.957
1.731 r6 = -0.448 d6 = 0.0360 -1.498e+1 -5.362 2.138e+1 -4.449e+1
7.215e+1 r7 = 2.623 d7 = 0.1969 r8 = 1.602 d8 = 0.0769 r9 = .infin.
d9 = 0.0985 r10 = .infin. d10 = 0.1969 r11 = .infin. Focal Length f
= 1.0 mm Numerical Aperture Fno = 3.4 f.sub.1 = 0.74 mm f.sub.2 =
-9.52 mm f.sub.3 = -8.39 mm
[0110] The features of the lenses used in each of the embodiments
are described below. In the Embodiments 1 through 5, ZEONEX 480R
("ZEONEX" is a registered trademark of Zeon Corporation, and 480R
is the series number), which is cycloolefin plastics, is used as
the material for the first lens L1, second lens L2, and third lens
L3. In the Embodiment 6, polycarbonate is used instead of ZEONEX
480R as the material for the second lens L2.
[0111] The refractivity on the d line of ZEONEX 480R is 1.525, and
the refractivity on the d line of polycarbonate is 1.583. The Abbe
number of ZEONEX 480R is 56.2, and the Abbe number of polycarbonate
is 30.0.
[0112] Both surfaces of the first lens L1, second lens L2, and
third lens L3 respectively are aspheric surfaces. Hence in each of
the Embodiments and comparative examples, the number of aspheric
surfaces is six, and thus the condition that at least one surface
of the third lens L3 be an aspheric surface is satisfied.
[0113] It was learned through simulation that if the Abbe number of
the material of the lenses is within a range of thirty to sixty,
substantially no discrepancies appear in lens performance qualities
such as aberration. In other words, it was learned that as long as
the Abbe number is a value within this range, the object of the
present invention, i.e. the favorable correction of various
aberrations in an imaging lens in comparison to aberration
correction in a conventional imaging lens, can be realized.
[0114] A cover glass 12 which also serves as an infrared cut filter
is inserted between the lens system and the imaging surface in each
of Embodiments 1 through 6. Glass (with a refractive index on the d
line of 1.50) is used as the material for this filter. The various
aberrations to be described below are calculated on the premise
that the filter is present. The focal length of the entire imaging
lens system disclosed in the following Embodiments 1 through 6, or
in other words the combined focal length f, is set to 11.0 mm.
Embodiment 1
[0115] (A) The object-side radius of curvature r.sub.1 of the first
lens L1 is r.sub.1=0.296 mm.
[0116] (B) The image-side radius of curvature r.sub.2 of the first
lens L1 is r.sub.2=0.720 mm.
[0117] (C) The back focus b.sub.f is b.sub.f=0.365 mm.
[0118] (D) The distance through the atmosphere from the object-side
surface of the first lens L1 to the imaging surface, or in other
words the optical length d, is
d=d.sub.1+d.sub.2+d.sub.3+d.sub.4+d.sub.5+d.sub.-
6+d.sub.7+b.sub.f=1.195 mm.
[0119] (E) The distance D.sub.2 between the first lens L1 and
second lens L2 is D.sub.2=d.sub.2+d.sub.3+d.sub.4=0.076 mm.
[0120] (F) The distance D.sub.4 between the second lens L2 and
third lens L3 is D.sub.4=d.sub.6=0.0369 mm.
[0121] (G) The focal length f.sub.1 of the first lens L1 is
f.sub.1=0.79 mm.
[0122] (H) The focal length f.sub.2 of the second lens L2 is
f.sub.2=-5.14 mm.
[0123] (I) The focal length f.sub.3 of the third lens L3 is
f.sub.3=10.56 mm.
[0124] Hence
[0125] (1) r.sub.1/r.sub.2=0.296/0.720=0.4111
[0126] (2) D.sub.2/f=0.076/1.00=0.076
[0127] (3) D.sub.4/f=0.0369/1.00=0.0369
[0128] (4) d/f=1.195/1.00=1.195, and
[0129] (5) b.sub.f/f=0.365/1.00=0.365.
[0130] Thus the lens system of the Embodiment 1 satisfies all of
the following conditional expressions (1) through (5).
0.35<r.sub.1/r.sub.2<0.45 (1)
0.07<D.sub.2/f<0.1 (2)
0.01<D.sub.4/f<0.04 (3)
1.00<d/f<1.30 (4)
0.3<b.sub.f/f<0.5 (5)
[0131] Hereafter, the term "conditional expressions" will be used
to indicate these five expressions (1) through (5).
[0132] As shown in Table 1, the aperture diaphragm S1 is provided
in a position 0.0148 mm (d.sub.2=0.0148 mm) rearward of the second
surface (the image-side surface) of the first lens L1. The
numerical aperture (F number) is 3.4.
[0133] A sectional view of the imaging lens of the Embodiment 1 is
shown in FIG. 2. The back focus in relation to a focal length of
1.00 mm is 0.365 mm, and hence a sufficient length is secured.
[0134] The distortion curve 20 shown in FIG. 3, the astigmatism
curve (the aberration curve 22 relating to the meridional plane and
the aberration curve 24 relating to the sagittal plane) shown in
FIG. 4, and the chromatic and spherical aberration curve (the
aberration curve 26 relating to the C line, the aberration curve 28
relating to the d line, the aberration curve 30 relating to the e
line, the aberration curve 32 relating to the F line, and the
aberration curve 34 relating to the g line) shown in FIG. 5 are
respectively illustrated by graphs.
[0135] The ordinate of the aberration curves in FIGS. 3 and 4
illustrate the image height as a percentage of the distance from
the optical axis. In FIGS. 3 and 4, 100%, 80%, 70%, and 60%
correspond to 0.534 mm, 0.427 mm, 0.374 mm, and 0.320 mm
respectively. The ordinate of the aberration curve in FIG. 5
indicates the distance of incidence h (F number), corresponding at
its maximum to F3.4. The abscissa in FIG. 5 shows the magnitude of
the aberration.
[0136] As regards distortion, the absolute value of the amount of
aberration reaches a maximum of 4.5919% in an image height position
of 80% (image height 0.427 mm), and hence within a range of image
height 0.534 mm and below, the absolute value of the aberration
amount is held within 4.5919%.
[0137] As for astigmatism, the absolute value of the aberration
amount on the meridional plane reaches a maximum of 0.0273 mm in an
image height position of 100% (image height 0.534 mm), and hence
within a range of image height 0.534 mm and below, the absolute
value of the aberration amount is held within 0.0273 mm.
[0138] As for chromatic and spherical aberration, the absolute
value of the aberration curve 34 relating to the g line reaches a
maximum of 0.0235 mm at a distance of incidence h of 85%, and hence
the absolute value of the aberration amount is held within 0.0235
mm.
Embodiment 2
[0139] (A) The object-side radius of curvature r.sub.1 of the first
lens L1 is r.sub.1=0.300 mm.
[0140] (B) The image-side radius of curvature r.sub.2 of the first
lens L1 is r.sub.2=0.748 mm.
[0141] (C) The back focus b.sub.f is b.sub.f=0.362 mm.
[0142] (D) The distance through the atmosphere from the object-side
surface of the first lens L1 to the imaging surface, or in other
words the optical length d, is
d=d.sub.1+d.sub.2+d.sub.3+d.sub.4+d.sub.5+d.sub.-
6+d.sub.7+b.sub.f=1.204 mm.
[0143] (E) The distance D.sub.2 between the first lens L1 and
second lens L2 is D.sub.2=d.sub.2+d.sub.3+d.sub.4=0.078 mm.
[0144] (F) The distance D.sub.4 between the second lens L2 and
third lens L3 is D.sub.4=d.sub.6=0.0232 mm.
[0145] (G) The focal length f.sub.1 of the first lens L1 is
f.sub.1=0.79 mm.
[0146] (H) The focal length f.sub.2 of the second lens L2 is
f.sub.2=-3.57 mm.
[0147] (I) The focal length f.sub.3 of the third lens L3 is
f.sub.3=5.96 mm.
[0148] Hence
[0149] (1) r.sub.1/r.sub.2=0.300/0.748=0.4011
[0150] (2) D.sub.2/f=0.078/1.00=0.078
[0151] (3) D.sub.4/f=0.0232/1.00=0.0232
[0152] (4) d/f=1.204/1.00=1.204, and
[0153] (5) b.sub.f/f=0.362/1.00=0.362.
[0154] Thus the lens system of the Embodiment 2 satisfies the
conditional expressions.
[0155] As shown in Table 2, the aperture diaphragm S1 is provided
in a position 0.015 mm (d.sub.2=0.015 mm) rearward of the second
surface (the image-side surface) of the first lens L1. The
numerical aperture (F number) is 3.4.
[0156] A sectional view of the imaging lens of the Embodiment is
shown in FIG. 6. The back focus in relation to a focal length of
1.00 mm is 0.362 mm, and hence a sufficient length is secured.
[0157] The distortion curve 36 shown in FIG. 7, the astigmatism
curve (the aberration curve 38 relating to the meridional plane and
the aberration curve 40 relating to the sagittal plane) shown in
FIG. 8, and the chromatic and spherical aberration curve (the
aberration curve 42 relating to the C line, the aberration curve 44
relating to the d line, the aberration curve 46 relating to the e
line, the aberration curve 48 relating to the F line, and the
aberration curve 50 relating to the g line) shown in FIG. 9 are
respectively illustrated by graphs.
[0158] The ordinate of the aberration curves in FIGS. 7 and 8
illustrate the image height as a percentage of the distance from
the optical axis. In FIGS. 7 and 8, 100%, 80%, 70%, and 60%
correspond to 0.543 mm, 0.434 mm, 0.380 mm, and 0.326 mm
respectively. The ordinate of the aberration curve in FIG. 9
indicates the distance of incidence h (F number), corresponding at
its maximum to F3.4. The abscissa in FIG. 9 shows the magnitude of
the aberration.
[0159] As regards distortion, the absolute value of the amount of
aberration reaches a maximum of 4.526% in an image height position
of 80% (image height 0.434 mm), and hence within a range of image
height 0.543 mm and below, the absolute value of the aberration
amount is held within 4.526%.
[0160] As for astigmatism, the absolute value of the aberration
amount on the meridional plane reaches a maximum of 0.0175 mm in an
image height position of 80% (image height 0.434 mm), and hence
within a range of image height 0.543 mm and below, the absolute
value of the aberration amount is held within 0.0175 mm.
[0161] As for chromatic and spherical aberration, the absolute
value of the aberration curve 50 relating to the g line reaches a
maximum of 0.0230 mm at a distance of incidence h of 85%, and hence
the absolute value of the aberration amount is held within 0.0230
mm.
Embodiment 3
[0162] (A) The object-side radius of curvature r.sub.1 of the first
lens L1 is r.sub.1=0.296 mm.
[0163] (B) The image-side radius of curvature r.sub.2 of the first
lens L1 is r.sub.2=0.738 mm.
[0164] (C) The back focus b.sub.f is b.sub.f=0.337 mm.
[0165] (D) The distance through the atmosphere from the object-side
surface of the first lens L1 to the imaging surface, or in other
words the optical length d, is
d=d.sub.1+d.sub.2+d.sub.3+d.sub.4+d.sub.5+d.sub.-
6+d.sub.7+b.sub.f=1.221 mm.
[0166] (E) The distance D.sub.2 between the first lens L1 and
second lens L2 is D.sub.2=d.sub.2+d.sub.3+d.sub.4=0.09 mm.
[0167] (F) The distance D.sub.4 between the second lens L2 and
third lens L3 is D.sub.4=d.sub.6=0.0215 mm.
[0168] (G) The focal length f.sub.1 of the first lens L1 is
f.sub.1=0.78 mm.
[0169] (H) The focal length f.sub.2 of the second lens L2 is
f.sub.2=-6.27 mm.
[0170] (I) The focal length f.sub.3 of the third lens L3 is
f.sub.3=8.56 mm.
[0171] Hence
[0172] (1) r.sub.1/r.sub.2=0.296/0.738=0.4011
[0173] (2) D.sub.2/f=0.09/1.00=0.09
[0174] (3) D.sub.4/f=0.0215/1.00=0.0215
[0175] (4) d/f=1.221/1.00=1.221, and
[0176] (5) b.sub.f/f=0.337/1.00=0.337.
[0177] Thus the lens system of the Embodiment 3 satisfies the
conditional expressions.
[0178] As shown in Table 3, the aperture diaphragm S1 is provided
in a position 0.0129 mm (d.sub.2=0.0129 mm) rearward of the second
surface (the image-side surface) of the first lens L1. The
numerical aperture (F number) is 3.4.
[0179] A sectional view of the imaging lens of the Embodiment 3 is
shown in FIG. 10. The back focus in relation to a focal length of
1.00 mm is 0.337 mm, and hence a sufficient length is secured.
[0180] The distortion curve 52 shown in FIG. 11, the astigmatism
curve (the aberration curve 54 relating to the meridional plane and
the aberration curve 56 relating to the sagittal plane) shown in
FIG. 12, and the chromatic and spherical aberration curve (the
aberration curve 58 relating to the C line, the aberration curve 60
relating to the d line, the aberration curve 62 relating to the e
line, the aberration curve 64 relating to the F line, and the
aberration curve 66 relating to the g line) shown in FIG. 13 are
respectively illustrated by graphs.
[0181] The ordinate of the aberration curves in FIGS. 11 and 12
illustrate the image height as a percentage of the distance from
the optical axis. In FIGS. 11 and 12, 100%, 80%, 70%, and 60%
correspond to 0.580 mm, 0.464 mm, 0.406 mm, and 0.348 mm
respectively. The ordinate in the aberration curve of FIG. 13
indicates the distance of incidence h (F number), corresponding at
its maximum to F3.4. The abscissa in FIG. 13 shows the magnitude of
the aberration.
[0182] As regards distortion, the absolute value of the amount of
aberration reaches a maximum of 3.8995% in an image height position
of 80% (image height 0.464 mm), and hence within a range of image
height 0.580 mm and below, the absolute value of the aberration
amount is held within 3.8995%.
[0183] As for astigmatism, the absolute value of the aberration
amount on the meridional plane reaches a maximum of 0.0192 mm in an
image height position of 100% (image height 0.580 mm), and hence
within a range of image height 0.580 mm and below, the absolute
value of the aberration amount is held within 0.0192 mm.
[0184] As for chromatic and spherical aberration, the absolute
value of the aberration curve 66 relating to the g line reaches a
maximum of 0.0284 mm at a distance of incidence h of 100%, and
hence the absolute value of the aberration amount is held within
0.0284 mm.
Embodiment 4
[0185] (A) The object-side radius of curvature r.sub.1 of the first
lens L1 is r.sub.1=0.295 mm.
[0186] (B) The image-side radius of curvature r.sub.2 of the first
lens L1 is r.sub.2=0.724 mm.
[0187] (C) The back focus b.sub.f is b.sub.f=0.348 mm.
[0188] (D) The distance through the atmosphere from the object-side
surface of the first lens L1 to the imaging surface, or in other
words the optical length d, is
d=d.sub.1+d.sub.2+d.sub.3+d.sub.4+d.sub.5+d.sub.-
6+d.sub.7+b.sub.f=1.219 mm.
[0189] (E) The distance D.sub.2 between the first lens L1 and
second lens L2 is D.sub.2=d.sub.2+d.sub.3+d.sub.4=0.088 mm.
[0190] (F) The distance D.sub.4 between the second lens L2 and
third lens L3 is D.sub.4=d.sub.6=0.0209 mm.
[0191] (G) The focal length f.sub.1 of the first lens L1 is
f.sub.1=0.77 mm.
[0192] (H) The focal length f.sub.2 of the second lens L2 is
f.sub.2=-5.72 mm.
[0193] (I) The focal length f.sub.3 of the third lens L3 is
f.sub.3=6.91 mm.
[0194] Hence
[0195] (1) r.sub.1/r.sub.2=0.295/0.724=0.4075
[0196] (2) D.sub.2/f=0.088/1.00=0.088
[0197] (3) D.sub.4/f=0.0209/1.00=0.0209
[0198] (4) d/f=1.219/1.00=1.219, and
[0199] (5) b.sub.f/f=0.348/1.00=0.348.
[0200] Thus the lens system of the Embodiment 4 satisfies the
conditional expressions.
[0201] As shown in Table 4, the aperture diaphragm S1 is provided
in a position 0.0125 mm (d.sub.2=0.0125 mm) rearward of the second
surface (the image-side surface) of the first lens L1. The
numerical aperture (F number) is 3.4.
[0202] A sectional view of the imaging lens of the Embodiment 4 is
shown in FIG. 14. The back focus in relation to a focal length of
1.00 mm is 0.348 mm, and hence a sufficient length is secured.
[0203] The distortion curve 68 shown in FIG. 15, the astigmatism
curve (the aberration curve 70 relating to the meridional plane and
the aberration curve 72 relating to the sagittal plane) shown in
FIG. 16, and the chromatic and spherical aberration curve (the
aberration curve 74 relating to the C line, the aberration curve 76
relating to the d line, the aberration curve 78 relating to the e
line, the aberration curve 80 relating to the F line, and the
aberration curve 82 relating to the g line) shown in FIG. 17 are
respectively illustrated by graphs.
[0204] The ordinate of the aberration curves in FIGS. 15 and 16
illustrate the image height as a percentage of the distance from
the optical axis. In FIGS. 15 and 16, 100%, 80%, 70%, and 60%
correspond to 0.564 mm, 0.451 mm, 0.395 mm, and 0.338 mm
respectively. The ordinate of the aberration curve in FIG. 17
indicates the distance of incidence h (F number), corresponding at
its maximum to F3.4. The abscissa in FIG. 17 shows the magnitude of
the aberration.
[0205] As regards distortion, the absolute value of the amount of
aberration reaches a maximum of 3.6086% in an image height position
of 80% (image height 0.451 mm), and hence within a range of image
height 0.564 mm and below, the absolute value of the aberration
amount is held within 3.6086%.
[0206] As for astigmatism, the absolute value of the aberration
amount on the meridional plane reaches a maximum of 0.0148 mm in an
image height position of 100% (image height 0.564 mm), and hence
within a range of image height 0.564 mm and below, the absolute
value of the aberration amount is held within 0.0148 mm.
[0207] As for chromatic and spherical aberration, the absolute
value of the aberration curve 82 relating to the g line reaches a
maximum of 0.0289 mm at a distance of incidence h of 100%, and
hence the absolute value of the aberration amount is held within
0.0289 mm.
Embodiment 5
[0208] (A) The object-side radius of curvature r.sub.1 of the first
lens L1 is r.sub.1=0.299 mm.
[0209] (B) The image-side radius of curvature r.sub.2 of the first
lens L1 is r.sub.2=0.748 mm.
[0210] (C) The back focus b.sub.f is b.sub.f=0.347 mm.
[0211] (D) The distance through the atmosphere from the object-side
surface of the first lens L1 to the imaging surface, or in other
words the optical length d, is
d=d.sub.1+d.sub.2+d.sub.3+d.sub.4+d.sub.5+d.sub.-
6+d.sub.7+b.sub.f=1.224 mm.
[0212] (E) The distance D.sub.2 between the first lens L1 and
second lens L2 is D.sub.2=d.sub.2+d.sub.3+d.sub.4=0.0896 mm.
[0213] (F) The distance D.sub.4 between the second lens L2 and
third lens L3 is D.sub.4=d.sub.6=0.0219 mm.
[0214] (G) The focal length f.sub.1 of the first lens L1 is
f.sub.1=0.79 mm.
[0215] (H) The focal length f.sub.2 of the second lens L2 is
f.sub.2=-7.42 mm.
[0216] (I) The focal length f.sub.3 of the third lens L3 is
f.sub.3=10.30 mm.
[0217] Hence
[0218] (1) r.sub.1/r.sub.2=0.299/0.748=0.3997
[0219] (2) D.sub.2/f=0.0896/1.00=0.0896
[0220] (3) D.sub.4/f=0.0219/1.00=0.0219
[0221] (4) d/f=1.224/1.00=1.224, and
[0222] (5) b.sub.f/f=0.347/1.00=0.347.
[0223] Thus the lens system of the Embodiment 5 satisfies the
conditional expressions.
[0224] As shown in Table 5, the aperture diaphragm S1 is provided
in a position 0.0131 mm (d.sub.2=0.0131 mm) rearward of the second
surface (the image-side surface) of the first lens L1. The
numerical aperture (F number) is 3.4.
[0225] A sectional view of the imaging lens of the Embodiment 5 is
shown in FIG. 18. The back focus in relation to a focal length of
1.00 mm is 0.347 mm, and hence a sufficient length is secured.
[0226] The distortion curve 84 shown in FIG. 19, the astigmatism
curve (the aberration curve 86 relating to the meridional plane and
the aberration curve 88 relating to the sagittal plane) shown in
FIG. 20, and the chromatic and spherical aberration curve (the
aberration curve 90 relating to the C line, the aberration curve 92
relating to the d line, the aberration curve 94 relating to the e
line, the aberration curve 96 relating to the F line, and the
aberration curve 98 relating to the g line) shown in FIG. 21 are
respectively illustrated by graphs.
[0227] The ordinate of the aberration curves in FIGS. 19 and 20
illustrate the image height as a percentage of the distance from
the optical axis. In FIGS. 19 and 20, 100%, 80%, 70%, and 60%
correspond to 0.585 mm, 0.468 mm, 0.409 mm, and 0.351 mm
respectively. The ordinate of the aberration curve in FIG. 21
indicates the distance of incidence h (F number), corresponding at
its maximum to F3.4. The abscissa in FIG. 21 shows the magnitude of
the aberration.
[0228] As regards distortion, the absolute value of the amount of
aberration reaches a maximum of 4.4431% in an image height position
of 80% (image height 0.468 mm), and hence within a range of image
height 0.585 mm and below, the absolute value of the aberration
amount is held within 4.4431%.
[0229] As for astigmatism, the absolute value of the aberration
amount on the meridional plane reaches a maximum of 0.0195 mm in an
image height position of 100% (image height 0.585 mm), and hence
within a range of image height 0.585 mm and below, the absolute
value of the aberration amount is held within 0.0195 mm.
[0230] As for chromatic and spherical aberration, the absolute
value of the aberration curve 98 relating to the g line reaches a
maximum of 0.0266 mm at a distance of incidence h of 100%, and
hence the absolute value of the aberration amount is held within
0.0266 mm.
Embodiment 6
[0231] (A) The object-side radius of curvature r.sub.1 of the first
lens L1 is r.sub.1=0.290 mm.
[0232] (B) The image-side radius of curvature r.sub.2 of the first
lens L1 is r.sub.2=0.777 mm.
[0233] (C) The back focus b.sub.f is b.sub.f=0.34 mm.
[0234] (D) The distance through the atmosphere from the object-side
surface of the first lens L1 to the imaging surface, or in other
words the optical length d, is
d=d.sub.1+d.sub.2+d.sub.3+d.sub.4+d.sub.5+d.sub.-
6+d.sub.7+b.sub.f=1.17 mm.
[0235] (E) The distance D.sub.2 between the first lens L1 and
second lens L2 is D.sub.2=d.sub.2+d.sub.3+d.sub.4=0.077 mm.
[0236] (F) The distance D.sub.4 between the second lens L2 and
third lens L3 is D.sub.4=d.sub.6=0.036 mm.
[0237] (G) The focal length f.sub.1 of the first lens L1 is
f.sub.1=0.74 mm.
[0238] (H) The focal length f.sub.2 of the second lens L2 is
f.sub.2=-9.52 mm.
[0239] (I) The focal length f.sub.3 of the third lens L3 is
f.sub.3=-8.39 mm.
[0240] Hence
[0241] (1) r.sub.1/r.sub.2=0.290/0.777=0.3732
[0242] (2) D.sub.2/f=0.077/1.00=0.077
[0243] (3) D.sub.4/f=0.036/1.00=0.036
[0244] (4) d/f=1.17/1.00=1.17, and
[0245] (5) b.sub.f/f=0.34/1.00=0.34.
[0246] Thus the lens system of the Embodiment 6 satisfies the
conditional expressions.
[0247] As shown in Table 6, the aperture diaphragm S1 is provided
in a position 0.0147 mm (d.sub.2=0.0147 mm) rearward of the second
surface (the image-side surface) of the first lens L1. The
numerical aperture (F number) is 3.4.
[0248] A sectional view of the imaging lens of the Embodiment 6 is
shown in FIG. 22. The back focus in relation to a focal length of
1.00 mm is 0.34 mm, and hence a sufficient length is secured.
[0249] The distortion curve 100 shown in FIG. 23, the astigmatism
curve (the aberration curve 102 relating to the meridional plane
and the aberration curve 104 relating to the sagittal plane) shown
in FIG. 24, and the chromatic and spherical aberration curve (the
aberration curve 106 relating to the C line, the aberration curve
108 relating to the d line, the aberration curve 110 relating to
the e line, the aberration curve 112 relating to the F line, and
the aberration curve 114 relating to the g line) shown in FIG. 25
are respectively illustrated by graphs.
[0250] The ordinate of the aberration curves in FIGS. 23 and 24
illustrate the image height as a percentage of the distance from
the optical axis. In FIGS. 23 and 24, 100%, 80%, 70%, and 60%
correspond to 0.585 mm, 0.468 mm, 0.409 mm, and 0.351 mm
respectively. The ordinate of the aberration curve in FIG. 25
indicates the distance of incidence h (F number), corresponding at
its maximum to F3.4. The abscissa in FIG. 25 shows the magnitude of
the aberration.
[0251] As regards distortion, the absolute value of the amount of
aberration reaches a maximum of 2.2562% in an image height position
of 80% (image height 0.468 mm), and hence within a range of image
height 0.585 mm and below, the absolute value of the aberration
amount is held within 2.2562%.
[0252] As for astigmatism, the absolute value of the aberration
amount on the meridional plane reaches a maximum of 0.0104 mm in an
image height position of 100% (image height 0.585 mm), and hence
within a range of image height 0.585 mm and below, the absolute
value of the aberration amount is held within 0.0104 mm.
[0253] As for chromatic and spherical aberration, the absolute
value of the aberration curve 114 relating to the g line reaches a
maximum of 0.0176 mm at a distance of incidence h of 50%, and hence
the absolute value of the aberration amount is held within 0.0176
mm.
[0254] The material constituting the second lens L2 in the
Embodiments 1 through 5 is ZEONEX 480R, which is a cycloolefin
plastics. In the Embodiment 6, however, the material constituting
the second lens L2 is polycarbonate. The refractivity on the d line
of ZEONEX 480R is 1.525, whereas the refractivity on the d line of
polycarbonate is 1.583, and hence polycarbonate has a higher
refractivity.
[0255] Thus in the imaging lens of the Embodiments 1 through 5,
where the second lens L2 is constituted using a lens made of
low-refractivity ZEONEX 480R, the value of f.sub.3 is a positive
value, and hence the third lens L3 has a positive refractive power.
On the other hand, in the imaging lens of the Embodiment 6, where
the second lens L2 is constituted using a lens made of a
high-refractivity polycarbonate material, the value of f.sub.3 is a
negative value, and hence the third lens L3 has a negative
refractive power.
[0256] The first lens L1 of the imaging lens of the present
invention mainly serves to determine the combined focal length as
an imaging lens. The second lens L2 mainly serves to determine the
resolution of the imaging lens, and hence by increasing the
refractivity of the material constituting the second lens L2, the
resolution can be increased. The third lens L3 serves to reduce the
gradient of the light rays entering the image forming surface in
relation to the optical axis. By reducing the gradient of the light
rays entering the image forming surface in relation to the optical
axis, the shading phenomenon whereby light is obstructed around the
periphery of the lens such that the peripheral parts of the image
become dark can be avoided.
[0257] When the refractivity of the material constituting the
second lens L2 is increased, the refractive power of the third lens
L3, or in other words the optimum value of f.sub.3, changes. This
is the reason why the refractive power of the third lens L3 is set
to be negative in the Embodiment 6 of the present invention.
[0258] The reason why the resolution can be increased by increasing
the refractivity of the material constituting the second lens L2 is
that the Abbe number tends to decrease as the refractivity of the
material increases. When comparing cycloolefin plastics and
polycarbonate, the Abbe number of polycarbonate, which has a high
refractivity, is smaller than that of cycloolefin plastics, which
has a low refractivity. Hence the chromatic aberration generated by
the first lens L1 having positive refractive power and the
chromatic aberration generated by the second lens L2 having
negative refractive power cancel each other out, as a result of
which chromatic aberration can be reduced. When chromatic
aberration is reduced, the resolution increases.
[0259] In a comparison of the chromatic and spherical aberration
characteristics of the Embodiments 1 through 6, it can be seen by
referring to FIGS. 5, 9, 13, 17, 21, and 25 that the curves
illustrating the aberration values on the C line (light with a
wavelength of 656.3 nm), d line (light with a wavelength of 587.6
nm), e line (light with a wavelength of 546.1 nm), F line (light
with a wavelength of 486.1 nm), and g line (light with a wavelength
of 435.8 nm) are gathered around zero, particularly in the
Embodiment 6. It can also be seen that the aberration curves
illustrating the chromatic and spherical aberration characteristics
of the Embodiments 1 through 5 are not as densely gathered as the
aberration curves illustrating the chromatic and spherical
aberration characteristics of the Embodiment 6 in relation to light
within the range of the C line to the g line, and have wider
distances therebetween.
[0260] When the aberration curves indicating aberration in light
within the range of the C line to the g line gather around zero,
this signifies that the chromatic aberration of the imaging lens is
small. Accordingly, it also signifies that the resolution of the
imaging lens is high.
[0261] To describe with a specific example the fact that the
resolution of the imaging lens of the Embodiment 6, in which the
aberration curves indicating the chromatic and spherical aberration
characteristics gather most closely around zero, is higher than
that of the imaging lenses in the Embodiments 1 through 5, a
comparison using an MTF (Modulation Transfer Function) is
performed. MTF is a function for illustrating the absolute value of
the OTF (Optical Transfer Function), which illustrates the spatial
filter characteristic from the object to the image in an optical
system. In other words, MTF is a function for providing the
contrast between the object and the image.
[0262] As the MTF of an imaging lens increases, the imaging lens is
able to exhibit a better resolution capability. FIG. 26 shows the
MTF in the center of images captured by the imaging lenses of the
Embodiments 1 through 6 of the present invention. The abscissa
shows the spatial frequency scale (unit: lines per mm), and the
ordinate shows the MTF value as a percentage.
[0263] In FIG. 26, the curves denoted by A, B, C, D, E, and F show
the MTF corresponding to the imaging lenses of the Embodiments 1
through 6, respectively. It can be seen that the curve denoted by F
exists in the highest position in FIG. 26, and hence that the MTF
is greatest in the Embodiment 6. As for the MTF on the meridional
plane and saggital plane in the parts removed from the center of
the image captured by the imaging lens, the curve denoted by F,
which illustrates the MTF characteristic in the center of the image
as described above, likewise exists in the highest position in FIG.
26. Hence this is not illustrated in FIG. 26.
[0264] As can be seen from the above description, the imaging lens
of the Embodiment 6, in which the material constituting the second
lens L2 is polycarbonate, has the highest resolution.
[0265] Moreover, as a result of an investigation performed by the
inventor of this imaging lens into the optimum combination of the
first, second, and third lenses constituting the imaging lens, it
was discovered that when the second lens L2 is constituted by a
material having a higher refractivity (and a smaller Abbe number)
than the first lens L1, as in the Embodiment 6, the optical length
can be shortened. Incidentally, when comparing the value of d/f in
the Embodiments 1 through 6, the Embodiment 6 has a d/f value of
1.17, which is smaller than all of the Embodiments 1 through 5.
[0266] In other words, by constructing a lens system such as that
of the Embodiment 6, the resolution of the lenses can be increased
beyond that of lens systems such as those in the Embodiments 1
through 5, and the optical length can be shortened. However,
polycarbonate is less resistant to heat than cycloolefin plastics.
Therefore, a determination should be made as to whether to employ a
lens system such as that of the Embodiments 1 through 5 or a lens
system such as that of the Embodiment 6 according to the object
with which the imaging lens of the present invention is to be
used.
[0267] It was thus learned that in all of the imaging lenses of the
Embodiments 1 through 6, a sufficient performance for installation
in a small camera using a CCD or CMOS as an imaging device is
secured.
[0268] As is clear from the above description of the imaging lens
of the present invention, by designing each of the lenses
constituting the imaging lens so as to satisfy the conditional
expressions (1) through (5), the problems to be solved by the
present invention are solved. In other words, various aberrations
are favorably corrected, and an imaging lens having a sufficient
back focus and a short optical length is obtained.
[0269] Note that in the embodiments described above, the plastic
material ZEONEX 480R or polycarbonate is used for the first lens
L1, second lens L2, and third lens L3, but it goes without saying
that plastic materials other than that cited in the embodiments,
and also non-plastic materials such as molded glass or the like,
for example, may be employed as long as the various conditions
described in the embodiments and so on are satisfied.
[0270] Incidentally, in a portable telephone or the like, the cover
glass 12, which serves as an infrared cut filter or the like, is
inserted between the third lens L3 and the imaging surface
r.sub.11. According to current technology, as long as a gap of at
least 0.95 mm is secured between the third lens L3 and the imaging
surface r.sub.11, the cover glass 12 may be inserted.
[0271] Further, in order to install an imaging lens in a current
portable telephone or the like, the optical length of the imaging
lens is preferably no more than 5 mm. According to the imaging
lenses disclosed in the Embodiments 1 through 6 of the present
invention, the optical length is no more than 1.30 times the focal
length, as can be seen from the conditional expression
1.00<d/f<1.30 (4). Hence, assuming that the imaging lens is
designed such that the optical length is 1.30 times the focal
length, an optical length of 5 mm provides a focal length of 3.85
mm. As for the back focus, when f=3.85, then
1.15<b.sub.f<1.92 according to the conditional expression
0.3<b.sub.f/f<0.5 (5), and hence a minimum length of 1.15 mm
can be secured.
[0272] When installing an imaging lens into a current portable
telephone or the like, the distance from the image side surface of
the third lens L3 to the imaging surface must be no less than 0.95
mm. According to the imaging lens of the present invention, a
minimum back focus of 1.15 mm can be secured, and hence the
required distance from the image side surface of the third lens L3
to the imaging surface can be secured to a satisfactory extent.
[0273] Further, in the imaging lens of the Embodiments 1 through 6,
the height position from the optical axis of the point on the
aspheric surface of the image side of the third lens L3, which is
an aspheric surface, where the gradient of the tangential plane to
the tangential plane of the surface apex (the perpendicular plane
to the optical axis) reaches zero, or in other words the height
position from the optical axis of the point on the aspheric lens
where the negative power of the lens, which weakens gradually from
the center of the lens toward the periphery, turns into positive
power, is as follows. When the effective diameter of the lens is
standardized to 1, this point exists in positions from the center
of the lens toward the periphery of 58.5% in the imaging lens of
the Embodiment 1, 59.5% in the imaging lens of the Embodiment 2,
28.3% in the imaging lens of the Embodiment 3, 20.4% in the imaging
lens of the Embodiment 4, 29.8% in the imaging lens of the
Embodiment 5, and 55.3% in the imaging lens of the Embodiment 6. As
a result, even in intermediate positions between the intersecting
point of the optical axis and the imaging surface and the periphery
of the lens, the angle of incidence onto the imaging device does
not deviate greatly from a right angle. Accordingly, since the
angle of incidence of the light does not deviate greatly from a
right angle even in intermediate positions from the periphery of
the lens, which serves as an important part of the image, the
problem of this part of the image becoming dark does not arise.
[0274] According to the imaging lens of the present invention as
described above, various aberrations can be favorably corrected,
and although the optical length is short, a sufficient back focus
can be secured.
[0275] As described above, the imaging lens of the present
invention may be used as a camera lens for installation in portable
telephones, personal computers, or digital cameras, and may also be
favorably applied as a camera lens for installation in PDAs
(personal digital assistants), a camera lens for installation in
toys comprising an image recognition function, and a camera lens
for installation in monitoring, surveying, and crime-prevention
devices and so on.
* * * * *