# Patent

## Title
Taking lens, and imaging device incorporating the same

## Abstract

A taking lens system includes, in order from an object side thereof, a first lens group (negative refracting), a second lens group (positive refracting), an aperture stop and a third lens group (positive refracting). Focusing from an infinite distance to a finite distance is performed by setting the second lens group and the third lens group such that the position of the second lens group upon focusing on a finite distance is located on an image side of the taking lens system with respect to the position of the second lens group upon focusing on an infinite distance and the position of the third lens group upon focusing on a finite distance is located on an object side of the taking lens system with respect to the position of the third lens group upon focusing on an infinite distance, while the first lens group remains fixed.

## Background
This application claims benefit of Japanese Application Nos. 2003-425107 and 2003-425108 filed in Japan on Dec. 22, 2003, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a taking lens and an imaging device that incorporates the same, and more particularly to a taking lens well fit for use on interchangeable lens cameras and an imaging device that incorporates the same.
So far, taking lens systems of the negative-positive-positive type with a leadoff negative lens group have been proposed in the art.
For some taking lenses of the negative-positive-positive type with a negative lens group out front wherein a front lens group is designed to move for focusing, however, it is required to rely on means for preventing entrance of dust into a lens barrel during focusing. With power-operated focusing, there is a problem that much power is consumed because of movement of a lens group susceptible of a large lens diameter.
A problem with taking lens systems of the type wherein the first lens group stays fixed is that aberrations are likely to fluctuate with focusing.
SUMMARY OF THE INVENTION
In view of such prior art problems as described above, the primary object of the invention is to provide a taking lens system of the type with a negative lens group out front, wherein entrance of dust into a lens barrel during focusing is substantially prevented and fluctuations of aberrations with zooming are substantially minimized, and a imaging device that incorporates the same.
According to the present invention, this object is achievable by the provision of a taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed.
The advantages of, and the requirements for, the taking lens system according to the invention are now explained.
When the taking lens system is constructed as being generally of the retrofocus type that comprises, in order from its object side, a negative lens group, a positive lens group and a positive lens group, it is easy to achieve a wide-angle arrangement with a long back focus.
The stop should be interposed between the second lens group of positive refracting power and the third lens group of positive refracting power for the purpose of minimizing negative distortion likely to occur throughout a lens system of the retrofocus type.
The first lens group should remain fixed upon focusing from an infinite distance to a finite distance, so that entrance of dust into a lens barrel during focusing can be prevented. It is also possible to keep power consumptions low at the time of power-operated focusing, again because the first lens group that tend to increase relatively in diameter stays fixed.
The positive second lens group and the positive third lens group with the aperture stop interposed between them should be set such that the position of the second lens group upon focusing on a finite distance is located on an image side of the taking lens system with respect to the position of the second lens group upon focusing on an infinite distance and the position of the third lens group upon focusing on a finite distance is located on an object side of the taking lens system with respect to the position of the third lens group upon focusing on an infinite distance, so that aberrations produced with focusing can be easily canceled out.
In this case, the taking lens system could be designed such that upon focusing from an infinite distance to a finite distance, the second lens group moves only toward the image side and the third lens group moves only toward the object side.
It is desired that the first lens group and the third lens group each comprise an aspheric surface, wherein the aspheric surface in the first lens group is located in at least the position of a refracting surface nearest to the image plane side of the first lens group.
Location of the aspheric lens in each of the first and third lens groups is favorable for correction of off-axis aberrations, and location of the aspheric surface in the position of the refracting surface nearest to the image plane side of the first lens group is particularly favorable for compactness while aberrations are corrected.
For the taking lens system, it is desired to satisfy the following condition (1):

fB/IM>1.5  (1)

where fB is a distance of the exit surface in the third lens group to an image plane as calculated on an air basis, and IM is an image circle (diameter) of the taking lens system.

Condition (1) is to define a back focus with respect to the image circle. Falling short of the lower limit of 1.5 to that condition is not preferable because the back focus becomes too small with respect to the image circle, leading to an unacceptable decrease in space areas for receiving optical members such as a quick return mirror, an optical path splitter prism and a filter.
For the taking lens system, it is desirable to satisfy the following condition (2):

f2/f>5.0  (2)

where f2 is a focal length of the second lens group, and f is a focal length of the taking lens system.

Condition (2) is to define the focal length of the second lens group. As the lower limit of 5.0 to that condition is not reached, the focal length of the second lens group becomes too short, rendering aberrations more likely to occur.
The taking lens system could be constructed such that a second lens located in the first lens group as counted from its object side is a positive lens.
The taking lens system could be constructed such that the second lens located in the first lens group as counted from its object side is a positive meniscus lens convex on its object side.
The taking lens system could be constructed such that a lens located nearest to the image plane side of the first lens group is a positive lens.
The taking lens system could be constructed such that the lens located nearest to the image plane side of the first lens group is a positive meniscus lens convex on its object side.
The taking lens system could be constructed such that the third lens group located in the image side thereof with respect to the aperture stop comprises, in order from an object side thereof, a subunit 3-1 having negative refracting power, a subunit 3-2 having positive refracting power, and a subunit 3-3 comprising a lens of negative refracting power and a lens of positive refracting power.
The taking lens system could be constructed such that the third lens group located in the image side thereof with respect to the aperture stop comprises, in order from an object side thereof, a subunit 3-1 having negative refracting power, and a subunit 3-2 having positive refracting power.
For the taking lens system, it is desirable to further comprise a holder frame for holding the first, the second and the third lens group in place, and a mount for making that holder frame attachable to, and detachable from, the body portion thereof.
The taking lens system of the invention can be easily assembled with a long back focus, and so lend itself to an interchangeable lens for lens interchangeable type single-lens reflex cameras as an example.
According to the invention, therefore, it is possible to achieve an imaging device comprising such a taking lens system as mentioned above and an image pickup device located on an image plane side thereof.
With the taking lens system of the negative-positive-position type wherein focusing from an infinite distance to a finite distance is performed by setting the second lens group and the third lens group such that the position of the second lens group upon focusing on a finite distance is located on an image side of the taking lens system with respect to the position of the second lens group upon focusing on an infinite distance and the position of the third lens group upon focusing on a finite distance is located on an object side of the taking lens system with respect to the position of the third lens group upon focusing on an infinite distance, while the first lens group remains fixed, it is possible to achieve a wide-angle arrangement that is generally of the retrofocus type with a long back focus. By interposing the stop between the second lens group of positive refracting power and the third lens group of positive refracting power, it is possible to minimize negative distortion. By allowing the first lens group to stay fixed upon focusing, it is possible to prevent entrance of dust into the lens barrel during focusing. When focusing is carried out in a power-operated fashion, power consumptions can be kept low because the first lens group susceptible of a large lens diameter is permitted to remain fixed.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a) and 1(b) are illustrative in axial section of the taking lens system of Example 1 according to the invention upon focusing on (a) an object point at infinity and (b) a nearby object point, respectively.
FIGS. 2(a) and 2(b) are similar to FIGS. 1(a) and 1(b), illustrating the taking lens system of Example 2 according to the invention.
FIGS. 3(a) and 3(b) are similar to FIGS. 1(a) and 1(b), illustrating the taking lens system of Example 3 according to the invention.
FIGS. 4(a) and 4(b) are similar to FIGS. 1(a) and 1(b), illustrating the taking lens system of Example 4 according to the invention.
FIGS. 5(a) and 5(b) are aberration diagrams for spherical aberration, field curvature, distortion and chromatic aberration of magnification of Example 1 upon focusing on (a) an object point at infinity and (b) a nearby object point.
FIGS. 6(a) and 6(b) are similar to FIGS. 5(a) and (b), showing aberrations for Example 2.
FIGS. 7(a) and 7(b) are similar to FIGS. 5(a) and (b), showing aberrations for Example 3.
FIGS. 8(a) and 8(b) are similar to FIGS. 5(a) and (b), showing aberrations for Example 4.
FIG. 9 is illustrative in section of a single-lens reflex camera in which the inventive taking lens system is incorporated.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Examples 1–4 of the inventive taking lens system are now explained with reference to the accompanying drawings.
FIGS. 1 to 4 are illustrative in axial section of Examples 1 to 4 upon focusing on (a) an object point at infinity and (b) a nearby object point. Throughout FIGS. 1–4, G1 stands for a first lens group; G2 a second lens group; S an aperture stop; G3 a third lens group; F a plane-parallel plate such as a low-pass filter; and I an image plane. The plane-parallel plate F is provided on its surface with an infrared cut coating. FIGS. 5–8 are aberration diagrams for spherical aberration, field curvature, distortion and chromatic aberration of magnification of Examples 1–4 upon focusing on (a) an object point at infinity and (b) a nearby object point. Throughout FIGS. 5–8, “FIY” is indicative of an image height.
EXAMPLE 1
This example is directed to a taking lens system made up of, in order from its object side, a first lens group G1 of negative refracting power, a second lens group G2 of positive refracting power, an aperture stop S and a third lens group G3 of positive refracting power, as shown in FIGS. 1(a) and 1(b). Upon focusing from an infinite distance to a finite distance, the first lens group G1 and the aperture stop S remain fixed, and the second lens group G2 and the third lens group G3 move toward the image side and the object side of the taking lens system, respectively.
Referring to the makeup of each lens group, the first lens group G1 consists of one negative meniscus lens convex on its object side, the second lens group G2 consists of four lenses, i.e., a negative meniscus lens convex on its object side, a doublet of a double-convex positive lens and a double-concave negative lens, and a double-convex positive lens. The third lens group G3 consists of a subunit S3-1 (3-1) of negative refracting power, a subunit S3-2 (3-2) of positive refracting power and a subunit S3-3 (3-3) composed of a lens of negative refracting power and a lens of positive refracting power, wherein the subunit S3-1 (3-1) consists of one double-concave negative lens, the subunit S3-2 (3-2) consists of three lenses, i.e., a positive meniscus lens convex on its image side, a double-convex positive lens and a positive meniscus lens convex on its image side, and the subunit S3-3 (3-3) consists of two lenses, i.e., a double-concave negative lens and a double-convex positive lens.
Two aspheric surfaces are used, one at the image-side surface of the negative meniscus lens in the first lens group G1 and another at the image-side surface of the positive meniscus lens on the object side of the subunit S3-2 (3-2) in the third lens group G3.
The taking lens system of the instant example is focusable from an infinite object distance up to a nearby distance of 150 mm.
EXAMPLE 2
This example is directed to a taking lens system made up of, in order from its object side, a first lens group G1 of negative refracting power, a second lens group G2 of positive refracting power, an aperture stop S and a third lens group G3 of positive refracting power, as shown in FIGS. 2(a) and 2(b). Upon focusing from an infinite distance to a finite distance, the first lens group G1 remains fixed, and the second lens group G2 and the aperture stop S move toward the image side of the taking lens system while the third lens group G3 moves toward the object side thereof.
Referring to the makeup of each lens group, the first lens group G1 consists of three lenses, i.e., a negative meniscus lens convex on its object side, a positive meniscus lens convex on its object side and a negative meniscus lens convex on its object side, and the second lens group G2 consists of three lenses, i.e., a doublet of a double-convex positive lens and a double-concave negative lens, and a double-convex positive lens. The third lens group G3 consists of a subunit S3-1 (3-1) of negative refracting power, a subunit S3-2 (3-2) of positive refracting power and a subunit S3-3 (3-3) composed of a lens of negative refracting power and a lens of positive refracting power, wherein the subunit S3-1 (3-1) consists of two lenses, i.e., a doublet of double-concave negative lens and a double-convex positive lens, the subunit S3-2 (3-2) consists of one double-convex positive lens, and the subunit S3-3 (3-3) consists of two lenses, i.e., a doublet of a negative meniscus lens convex on its image side and a positive meniscus lens convex on its image side.
Two aspheric surfaces are used, one at the image-side surface of the negative meniscus lens on the image side of the first lens group G1 and another at the image-side surface of the double-convex positive lens in the subunit S3-2 (3-2) in the third lens group G3.
In the instant example, the taking lens system is focusable from an infinite object distance up to a nearby distance of 150 mm.
EXAMPLE 3
This example is directed to a taking lens system made up of, in order from its object side, a first lens group G1 of negative refracting power, a second lens group G2 of positive refracting power, an aperture stop S and a third lens group G3 of positive refracting power, as shown in FIGS. 3(a) and 3(b). Upon focusing from an infinite distance to a finite distance, the first lens group G1 stays fixed, and both the second lens group G2 and the aperture stop S move toward the image side of the taking lens system while the third lens group G3 moves toward the object side thereof.
Referring to the makeup of each lens group, the first lens group G1 consists of three lenses, i.e., two negative meniscus lenses each convex on its object side and a positive meniscus lens convex on its object side, and the second lens group G2 consists of five lenses, i.e., a doublet of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, a doublet of a positive meniscus lens convex on its image side and a negative meniscus lens convex on its image side, and a double-convex positive lens. The third lens group G3 consists of a subunit S3-1 (3-1) of negative refracting power and a subunit S3-2 (3-2) of positive refracting power, wherein the subunit S3-1 (3-1) is composed of two lenses, i.e., a doublet of a double-concave negative lens and a double-convex positive lens, and the subunit S3-2 (3-2) is composed of two lenses, i.e., a double-convex positive lens and a positive meniscus lens convex on its image side.
Three aspheric surfaces are used, one at the image-side surface of the negative meniscus lens on the image side of the first lens group G1 and two at both surfaces of the double-convex positive lens in the subunit S3-2 (3-2) in the third lens group G3.
In the instant example, the taking lens system is focusable from an infinite object distance up to a nearby distance of 150 mm.
EXAMPLE 4
This example is directed to a taking lens system made up of, in order from its object side, a first lens group G1 of negative refracting power, a second lens group G2 of positive refracting power, an aperture stop S and a third lens group G3 of positive refracting power, as shown in FIGS. 4(a) and 4(b). Upon focusing from an infinite distance to a finite distance, the first lens group G1 stays fixed, the second lens group G2 moves toward the image side of the taking lens system, the aperture stop S moves toward the object side thereof, and the third lens group G3 moves toward the object side thereof.
Referring to the makeup of each lens group, the first lens group G1 consists of three negative meniscus lenses each convex on its object side, and the second lens group G2 consists of four lenses, i.e., a doublet of a negative meniscus lens convex on its object side and a double-convex positive lens and a doublet of a positive meniscus lens convex on its image side and a negative meniscus lens convex on its image side. The third lens group G3 consists of a subunit S3-1 (3-1) of negative refracting power and a subunit S3-2 (3-2) of positive refracting power, wherein the subunit S3-1 (3-1) is composed of two lenses, i.e., a doublet of a double-convex negative lens and a double-convex positive lens and the subunit S3-2 (3-2) is composed of two lenses, i.e., a double-convex positive lens and a positive meniscus lens convex on its image side.
Two aspheric surfaces are used, one at the image-side surface of the negative meniscus lens nearest to the image side of the first lens group G1 and another at the image-side surface of the double-convex positive lens in the subunit S3-2 (3-2) in the third lens group G3.
In the instant example, the taking lens system is focusable from an infinite object distance up to a nearby distance of 150 mm.
Numerical data on Examples 1–4 given above are set out just below. The symbols used hereinafter but not hereinbefore have the following meanings.
f: focal length of the taking lens system,
ω: half angle of view,
fB: back focus,
r1, r2, . . . : radius of curvature of each lens surface,
d1, d2, . . . : spacing between adjacent lens surfaces,
nd1, nd2, . . . d-line refractive index of each lens
vd1, vd2, . . . : Abbe number of each lens, and
d0: object distance.
Here let x represent an optical axis provided that the direction of propagation of light is positive, and y indicate a direction orthogonal to that optical axis. Then, aspheric configuration is given by

x=(y2/r)/[1+{1−(K+1)(y/r)2}1/2]+A4y4+A6y6+A8y8+A10y10

where r is a paraxial radius of curvature, K is a conical coefficient, and A4, A6, A8 and A10 are the 4th 6th, 8th and 10th aspherical coefficients, respectively.

EXAMPLE 1















f = 12.55



ω = 40.77°



fB = 35.277



IM = 21.64



f2 = 375.76



fB/IM = 1.63



f2/f = 29.94














r1 = 33.6504
d1 = 4.0000
nd1 = 1.77250
νd1 = 49.60


r2 = 14.0282(Aspheric)
d2 = (Variable)


r3 = 79.9189
d3 = 2.0000
nd2 = 1.80400
νd2 = 46.57


r4 = 20.1501
d4 = 1.5000


r5 = 41.1144
d5 = 7.2206
nd3 = 1.54814
νd3 = 45.79


r6 = −12.9519
d6 = 2.0000
nd4 = 1.77250
νd4 = 49.60


r7 = 312.8052
d7 = 0.1500


r8 = 55.6171
d8 = 7.8101
nd5 = 1.80518
νd5 = 25.42


r9 = −42.3338
d9 = (Variable)


r10 = ∞ (Stop)
d10 = (Variable)


r11 = −60.8936
d11 = 2.0000
nd6 = 1.80518
νd6 = 25.42


r12 = 210.3818
d12 = 2.0000


r13 = −103.2315
d13 = 3.4446
nd7 = 1.78800
νd7 = 47.37


r14 =
d14 = 0.3248


−43.3899(Aspheric)


r15 = 355.8635
d15 = 5.9879
nd8 = 1.49700
νd8 = 81.54


r16 = −38.2367
d16 = 0.1500


r17 = −447.6224
d17 = 5.4317
nd9 = 1.60311
νd9 = 60.64


r18 = −40.0638
d18 = 0.1500


r19 = −283.3681
d19 = 2.0000
nd10 = 1.80518
νd10 = 25.42


r20 = 35.6465
d20 = 9.0405
nd11 = 1.60311
νd11 = 60.64


r21 = −33.7007
d21 = (Variable)


r22 = ∞
d22 = 4.2000
nd12 = 1.51633
νd12 = 64.14


r23 = ∞
d23 = 2.5069


r24 = ∞(Image plane)










Aspherical Coefficients












2nd surface



K = −0.9181



A4 = 2.2158 × 10−5



A6 = 1.7270 × 10−8



A8 = 1.0768 × 10−10



A10 = 0.0000



14th surface



K = −2.8760



A4 = 9.1181 × 10−6



A6 = 1.1697 × 10−8



A8 = −3.2446 × 10−12



A10 = 2.1853 × 10−14











Variable Spaces














d0
∞
150.00000



d2
22.94977
24.49437



d9
7.34522
5.80062



d10
3.83282
3.00000



d21
30.00000
30.83282










EXAMPLE 2















f = 12.55



ω = 40.77°



fB = 35.277



IM = 21.64



f2 = 122.75



fB/IM = 1.63



f2/f = 9.78














r1 = 34.7869
d1 = 2.0000
nd1 = 1.77250
νd1 = 49.60


r2 = 16.2237
d2 = 7.1717


r3 = 41.7532
d3 = 3.5469
nd2 = 1.80518
νd2 = 25.42


r4 = 103.0721
d4 = 0.1500


r5 = 25.9065
d5 = 2.0000
nd3 = 1.48749
νd3 = 70.23


r6 = 10.6696(Aspheric)
d6 = (Variable)


r7 = 95.4643
d7 = 4.5445
nd4 = 1.57501
νd4 = 41.50


r8 = −10.1692
d8 = 2.0000
nd5 = 1.77250
νd5 = 49.60


r9 = 193.6844
d9 = 0.1000


r10 = 42.5027
d10 = 2.6873
nd6 = 1.80518
νd6 = 25.42


r11 = −128.1902
d11 = (Variable)


r12 = ∞ (Stop)
d12 = (Variable)


r13 = −35.0299
d13 = 2.0000
nd7 = 1.80518
νd7 = 25.42


r14 = 27.1767
d14 = 4.7099
nd8 = 1.58913
νd8 = 61.14


r15 = −38.0739
d15 = 0.1500


r16 = 47.8802
d16 = 4.1653
nd9 = 1.58913
νd9 = 61.14


r17 =
d17 = 0.1500


−36.6897(Aspheric)


r18 = −68.0376
d18 = 2.0000
nd10 = 1.80518
νd10 = 25.42


r19 = −172.1490
d19 = 4.2606
nd11 = 1.77250
νd11 = 49.60


r20 = −24.2548
d20 = (Variable)


r21 = ∞
d21 = 4.2000
nd12 = 1.51633
νd12 = 64.14


r22 = ∞
d22 = 2.5069


r23 = ∞ (Image plane)










Aspherical Coefficients












6th surface



K = −0.4515



A4 = −4.6778 × 10−5



A6 = −3.4610 × 10−7



A8 = −2.6741 × 10−9



A10 = 0.0000



17th surface



K = 0



A4 = 2.9864 × 10−5



A6 = 8.1338 × 10−9



A8 = 1.9789 × 10−12



A10 = 0.0000











Variable Spaces














d0
∞
150.00000



d6
17.05685
17.28525



d11
4.52680
4.70882



d12
6.11814
4.69773



d20
30.00000
31.00999










EXAMPLE 3















f = 12.20



ω = 41.56°



fB = 35.274



IM = 21.64



f2 = 249.68



fB/IM = 1.63



f2/f = 20.46














r1 = 25.7587
d1 = 2.0000
nd1 = 1.72916
νd1 = 54.68


r2 = 14.5160
d2 = 8.0114


r3 = 21.4779
d3 = 2.0509
nd2 = 1.51633
νd2 = 64.14


r4 = 10.7046(Aspheric)
d4 = 5.9987


r5 = 34.2468
d5 = 4.1207
nd3 = 1.69895
νd3 = 30.13


r6 = 60.1762
d6 = (Variable)


r7 = 41.0046
d7 = 2.0000
nd4 = 1.72916
νd4 = 54.68


r8 = 8.8714
d8 = 5.8748
nd5 = 1.64769
νd5 = 33.79


r9 = 90.3027
d9 = 1.0000


r10 = −22.7154
d10 = 5.0119
nd6 = 1.69895
νd6 = 30.13


r11 = −8.5801
d11 = 2.5582
nd7 = 1.80100
νd7 = 34.97


r12 = −36.0826
d12 = 0.8188


r13 = 47.3492
d13 = 3.7261
nd8 = 1.77250
νd8 = 49.60


r14 = −577.3278
d14 = (Variable)


r15 = ∞ (Stop)
d15 = (Variable)


r16 = −51.1868
d16 = 1.5000
nd9 = 1.80518
νd9 = 25.42


r17 = 30.5800
d17 = 4.9360
nd10 = 1.48749
νd10 = 70.23


r18 = −81.0257
d18 = 0.1500


r19 =
d19 = 6.6319
nd11 = 1.43875
νd11 = 94.93


34.6262(Aspheric)


r20 =
d20 = 0.1500


−21.7282(Aspheric)


r21 = −134.0324
d21 = 5.7167
nd12 = 1.51633
νd12 = 64.14


r22 = −21.3461
d22 = (Variable)


r23 = ∞
d23 = 4.2000
nd13 = 1.51633
νd13 = 64.14


r24 = ∞
d24 = 2.5037


r25 = ∞(Image plane)










Aspherical Coefficients












4th surface



K = −0.9388



A4 = −6.7494 × 10−6



A6 = −1.2010 × 10−7



A8 = −1.1977 × 10−9



A10 = 0.0000



19th surface



K = −6.0521



A4 = −1.4568 × 10−5



A6 = 7.8521 × 10−8



A8 = −3.7664 × 10−10



A10 = 0.0000



20th surface



K = 0



A4 = 2.6936 × 10−5



A6 = 6.8110 × 10−8



A8 = −1.5114 × 10−10



A10 = 0.0000











Variable Spaces














d0
∞
150.00000



d6
5.53502
5.95355



d14
1.00000
1.02741



d15
4.50900
3.12971



d22
30.00000
30.93335










EXAMPLE 4















f = 12.20



ω = 41.57°



fB = 35.270



IM = 21.64



f2 = 51.73



fB/IM = 1.63



f2/f = 4.24














r1 = 27.1466
d1 = 2.0000
nd1 = 1.72916
νd1 = 54.68


r2 = 14.9781
d2 = 6.7472


r3 = 19.4345
d3 = 2.0000
nd2 = 1.72916
νd2 = 54.68


r4 = 13.9731
d4 = 2.7937


r5 = 16.9952
d5 = 2.0000
nd3 = 1.49700
νd3 = 81.54


r6 = 10.6814(Aspheric)
d6 = (Variable)


r7 = 35.0430
d7 = 2.0000
nd4 = 1.72916
νd4 = 54.68


r8 = 10.2257
d8 = 6.7784
nd5 = 1.72825
νd5 = 28.46


r9 = −141.4052
d9 = 4.6624


r10 = −42.6858
d10 = 4.0090
nd6 = 1.60311
νd6 = 60.64


r11 = −10.1899
d11 = 1.4000
nd7 = 1.80518
νd7 = 25.42


r12 = −30.3384
d12 = (Variable)


r13 = ∞ (Stop)
d13 = (Variable)


r14 = −31.5466
d14 = 1.5000
nd8 = 1.80100
νd8 = 34.97


r15 = 25.0509
d15 = 4.5966
nd9 = 1.48749
νd9 = 70.23


r16 = −31.4552
d16 = 0.1500


r17 = 49.5784
d17 = 3.6959
nd10 = 1.49700
νd10 = 81.54


r18 =
d18 = 0.1500


−42.7447(Aspheric)


r19 = −415.3378
d19 = 5.5499
nd11 = 1.72916
νd11 = 54.68


r20 = −22.6252
d20 = (Variable)


r21 = ∞
d21 = 4.2000
nd12 = 1.51633
νd12 = 64.14


r22 = ∞
d22 = 2.4997


r23 = ∞(Image plane)










Aspherical Coefficients












6th surface



K = −0.8190



A4 = −4.5482 × 10−5



A6 = −3.0420 × 10−7



A8 = −1.0248 × 10−9



A10 = 0.0000



18th surface



K = 0



A4 = 2.8776 × 10−5



A6 = 1.2964 × 10−8



A8 = −1.8297 × 10−11



A10 = 0.0000











Variable Spaces














d0
∞
150.00000



d6
11.90487
13.59517



d12
2.87862
1.00000



d13
8.48326
8.49462



d20
30.00000
30.17692










FIG. 9 is a sectional schematic of a single-lens reflex camera that incorporates the inventive taking lens system and uses a small-format CCD or C-MOS as an image pickup device. In FIG. 9, reference numeral 1 stands for a single-lens reflex camera, 2 a taking lens system located within a lens barrel rigged with a focusing mechanism, and 3 a lens barrel's mount that makes the taking lens system 2 attachable to, or detachable from, the single-lens reflex camera 1 and for which, for instance, a screw or bayonet type mount could be used. In this embodiment, the bayonet type mount is used.
Reference numeral 4 represents an image pickup device's plane, 5 a quick return mirror interposed between the taking lens system 2 and the plane 4 of the image pickup device on the optical path of that lens system, 7 a finder screen located on a path of light reflected at the quick return mirror 5, 8 a penta prism, 9 a finder and E the eye of a viewer (eye point).
For the taking lens system 2 in the thus assembled single-lens reflex camera 1, for instance, the inventive taking lens system according to each of Examples 1–4 is used.

## Claims

1. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
wherein upon focusing from an infinite distance to a finite distance, said second lens group moves only toward the image side and said third lens group moves only toward the object side.

2. An imaging device, comprising the taking lens system as recited in claim 1, and an image pickup device located on an image plane side of said taking lens system.

3. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
wherein each of said first lens group and said third lens group comprises an aspheric surface, wherein the aspheric surface in said first lens group is located in at least a position of a refracting surface nearest to an image plane side of the first lens group.

4. An imaging device, comprising the taking lens system as recited in claim 3, and an image pickup device located on an image plane side of said taking lens system.

5. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
the taking lens system satisfies condition (1):


fB/IM>1.5  (1)



where fB is a distance of an exit surface in said third lens group to an image plane as calculated on an air basis, and JIM is an image circle of the taking lens system.

6. An imaging device, comprising the taking lens system as recited in claim 5, and an image pickup device located on an image plane side of said taking lens system.

7. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
the taking lens system satisfies condition (2):


f2/f>5.0  (2)



where f2 is a focal length of said second lens group, and f is a focal length of the entire taking lens system.

8. An imaging device, comprising the taking lens system as recited in claim 7, and an image pickup device located on an image plane side of said taking lens system.

9. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
wherein a second lens element located in said first lens group as counted from an object side thereof is a positive meniscus lens element convex on an object side thereof.

10. An imaging device, comprising the taking lens system as recited in claim 9, and an image pickup device located on an image plane side of said taking lens system.

11. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
wherein said third lens group located in the image side thereof with respect to said aperture stop comprises, in order from an object side thereof a subunit 3-1 having negative refracting power, a subunit 3-2 having positive refracting power, and a subunit 3-3 comprising a lens element of negative refracting power and a lens element of positive refracting power.

12. An imaging device, comprising the taking lens system as recited in claim 11, and an image pickup device located on an image plane side of said taking lens system.

13. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
wherein said third lens group located in the image side thereof with respect to said aperture stop comprises, in order from an object side thereof, a subunit 3-1 having negative refracting power, and a subunit 3-2 having positive refracting power.

14. An imaging device, comprising the taking lens system as recited in claim 13, and an image pickup device located on an image plane side of said taking lens system.

15. A taking lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, an aperture stop and a third lens group having positive refracting power, wherein:
focusing from an infinite distance to a finite distance is performed by setting said second lens group and said third lens group such that a position of said second lens group upon focusing on a finite distance is located on an image side of said taking lens system with respect to a position of said second lens group upon focusing on an infinite distance and a position of said third lens group upon focusing on a finite distance is located on an object side of said taking lens system with respect to a position of said third lens group upon focusing on an infinite distance, while said first lens group remains fixed, and
which further comprises a holder frame for holding said first lens group, said second lens group and said third lens group in place, and a mount for making said holder frame attachable to, and detachable from, a body portion of said taking lens system.

16. An imaging device, comprising the taking lens system as recited in claim 15, and an image pickup device located on an image plane side of said taking lens system.