REAR CONVERSION LENS
Technical Field
[0001] The present technology relates to a rear conversion lens, and especially relates to a rear conversion- lens with which an image-pickup lens for a three-chip camera is used with a single-chip color camera,
Background Art
[0002] In the past, a rear conversion lens is used to magnify the focal length of an image-pickup lens {for example, see Patent Document 1 and Patent Document 2).
[0003] Further, as another usage of a rear conversion lens, it is desirable to provide a rear conversion lens, which is to be arranged between an image-pickup lens and a camera having different standards and to be used, in order to use the image-pickup lens and the camera in combination. [0004] For example, there is a need to use a 2/3-inch-format zoom lens with a single-chip color camera including an APS-C size image sensor, and there is a need to use the same 2/3-inch-format image-pickup lens with a super-35-mm-format single-chip cinema camera. [0005] Patent Document 1: Japanese Patent

Application Laid-open No. 2005-107261
Patent Document 2: Japanese Patent Application Laid-open No. 2006-349904 Summary of Invention
Problem to be solved by the Invention [0006] However, for example, some 2/3-inch-format image-pickup lenses are designed based on the assumption that they are used with three-chip cameras with color-separation prisms. If such an image-pickup lens is used with a single-chip color camera, since there is no color-separation prism, which is the assumption of the designing, a spherical aberration occurs in large amounts and it is therefore difficult to use them in combination.
[0007] In other words, it is not possible to use such a 2/3-inch-format image-pickup lens and a single-chip color camera, vvrhich is designed based on the assumption that it is used without a color-separation prism, in combination only by converting a mount system of the camera, by converting the focal length of the image-pickup lens, or by adjusting the flange focal length or the backfocus.
[0008] In view of the circumstances as described above, it is an object of the present technology to provide a rear conversion lens, with which it is possible to use an image-pickup lens for a three-chip

camera, which is designed based on the assumption that it is used with a color-separation prism in combination, and a single-chip color camera in combination.
Means for solving the Problem
[0009] In order to achieve the above-mentioned object, according to an embodiment of the present technology, a rear conversion lens includes: a chassis including two opposite ends, the chassis including light-transmitting holes on the ends, respectively; a first coupler configured to detachably couple one of the ends of the chassis with an image field side of an image-pickup lens of a three-chip camera including three first image sensors, the image-pickup lens being designed based on an assumption that the image-pickup lens is to be used with a color-separation prism in combination; a second coupler configured to detachably couple the other end of the chassis with a single-chip color camera including a second image sensor, the second image sensor including an image field larger than an image field of the first image sensors of the three-chip camera; and an optical system including a first lens group, a second lens group, and a third lens group arranged in the chassis in this order from an object side, the first lens group having a negative compound focal length, the second lens group being

configured to correct a spherical aberration, the third lens group being configured to form an image. [0010] According to the present technology, because the compound focal length of the first lens group is negative, the rear conversion lens may be downsized while the rear conversion*lens is used with an image-pickup lens for a three-chip camera, which is telecentric at the image side, in combination. [0011] In order to achieve the above-mentioned object, according to an embodiment of the present technology, in the rear conversion lens, the first lens group may include first three-bonded lenses, the second lens group may include a concave lens, a convex lens, and second three-bonded lenses in this order from the object side, and the third lens group may include a convex lens.
[0012] According to the present technology, because an aberration is corrected precisely by using many lenses, it is possible to correct an aberration more appropriately than another rear conversion lens, which includes fewer lenses and may thus be downsized. [0013] In order to achieve the above-mentioned object, according to an embodiment of the present technology, in the rear conversion lens, a compound focal length of the second three-bonded lenses and the third lens group may be positive.

[0014] According to the present technology, because the compound focal length of the second three-bonded lenses and the third lens group is positive, the rear conversion lens may be downsized while the rear conversion lens is used with an image-pickup lens for a three-chip camera, which is telecentric at the image side, in combination.
[00153 In order to achieve the above-mentioned object, according to an embodiment of the present technology, in the rear conversion lens, the first three-bonded lenses and the second three-bonded lenses satisfy a conditional expression (1), j ~3.0<{f2G)/{flG)<-1.2 ... (1)
where fIG is a compound focal length of the first three-bonded lenses, and f2G is a compound focal length of the second three-bonded lenses.
[0016] According to the present technology, because the compound focal length of the first three-bonded lenses and the compound focal length of the second three-bonded lenses are determined such that the conditional expression (1) is satisfied, astigmatism on both a sagittal plane and a meridional plane is within a practical range. Further, it is possible to prevent the image quality of a periphery of an image sensor from being degraded. [0017] In order to achieve the above-mentioned

object, according to an embodiment of the present technology, in the rear conversion lens, the first three-bonded lenses may satisfy a conditional expression (2) ,
nlGl>nlG3>nlG2 ... (2)
where nlGl is a refractive index of a first lens
of the first three-bonded lenses from the object side,
nlG2 is a refractive index of a second lens of the
first three-bonded lenses from the object side, and
nlG3 is a refractive index of a third lens of the first
three-bonded lenses from the object side.
[0018] According to the present technology, because
the refractive indexes of the- respective lenses of the
i first three-bonded lenses are determined such that the
conditional expression (2) is satisfied, it is possible
to generate a high-order spherical aberration, to
control spherical aberration of the image-pickup lens
for a three-chip camera, and to increase the image
quality.
[0019] In order to achieve the above-mentioned
object, according to an embodiment of the present
technology, in the rear conversion lens, the second
three-bonded lenses may satisfy conditional expressions
(3) and (4),
n2G2>n2Gl ... (3)
n2G2>n2G3 ... (4)

where n2Gl is a refractive index of a first lens of the second three-bonded lenses from the object side, n2G2 is a refractive index of a second lens of the second three-bonded lenses from the object side, and n2G3 is a refractive index of a third lens of the second three-bonded lenses from the object side. [0020] According to the present technology, because the refractive indexes of the respective lenses of the second three-bonded lenses are determined such that the conditional expressions (3) and (4) are satisfied, it is possible to generate a high-order spherical aberration, to control spherical aberration of the image-pickup lens for a th:rfee-chip camera, and to increase the image quality. Effect of the Invention
[0021] As described above, according to the present technology, it is possible to use an image-pickup lens for a three-chip camera, which is designed based on the assumption that it is used v^ith a color-separation prism in combination, and a single-chip color camera in combination.
Brief Description of Drawings
[Fig. 1] A cross-sectional view taken along the optical axis of the optical system of a rear conversion lens of the present technology. [Fig. 2] An optical path diagram of the optical system

of the rear conversion lens of the present technology. [Fig. 3] Coma aberration diagrams of the optical system of the rear conversion lens of the present technology.
[Fig. 4] A spherical aberration diagram of the optical system of the rear conversion lens of the present technology.
[Fig. 5] Diagrams showing, starting from the left, astigmatism,, distortion, and chromatic aberration of magnification of the optical system of the rear conversion lens of the present technology, [Fig. 6] Spot diagrams of the optical system of the rear conversion lens of the present technology. [Fig. 7] White MTF {Modulation Transfer Function) characteristic diagrams of the optical system of the rear conversion lens of the present technology. [Fig. 8] Defocused MTF characteristic diagrams of the optical system of the rear conversion lens of the present technology.
[Fig. 9] A spherical aberration diagram of an image-pickup lens for a three-chip camera with a color-separation prism.
[Fig. 10] Coma aberration diagrams of the image-pickup lens for a three-chip camera with a color-separation prism. [Fig. 11] Diagrams showing, starting from the left,

astigmatism, distortion, and chromatic aberration of magnification of the image-pickup lens for a three-chip camera v/ith a color-separation prism. [Fig. 12] White MTF characteristic diagrams of the image-pickup lens for a three-chip camera with a color-separation prism.
[Fig. 13] A spherical aberration diagram of an image-pickup lens for a three-chip camera without a color-separation prism.
[Fig. 14] Diagrams showing, starting from the left,
astigmatism, distortion, and chromatic aberration of
magnification of the image-pickup lens for a three-chip
camera without a color-separation prism. i
[Fig. 15] Coma aberration diagrams of the image-pickup lens for a three-chip camera without a color-separation prism.
[Fig. 16] White MTF characteristic diagrams of the image-pickup lens for a three-chip camera without a color-separation prism.
[Fig. 17] An outside view showing the rear conversion lens of the present technology. Modes for Carrying Out the Invention [0023] Hereinafter, an embodiment of the present technology will be described with reference to the drawings. [Features of the present technology]

[0024] (Correction of spherical aberration and longitudinal chromatic aberration)
As described above, a rear conversion lens (hereinafter referred to as the optical system) of the present technology is to be arranged behind an image-pickup lens for a three-chip camera and used. An image-pickup lens for a three-chip camera is designed based on the assumption that it is used with a color-separation prism. For that reason, if an image-pickup lens for a three-chip camera is used with a single-chip color camera without a color-separation prism in combination, a spherical aberration largely displaced to an object side will occur because there is no color- i separation prism, and a longitudinal chromatic aberration will occur resulting from a difference of a refractive index affected by the wavelength of a prism glass. Since the optical system corrects such aberrations, the aberration shape when an image-pickup lens for a three-chip camera is used v^ith a single-chip color camera in combination is substantially equivalent to the aberration shape when an image-pickup lens for a three-chip camera is used together with a three-chip camera. [0025] (Magnification of image circle)
A typical rear conversion lens is only configured to convert the focal length of an image-pickup lens.

and magnifies the image in the vicinity of the center of an image-pickup lens without changing the size of an image circle. In contrast, the optical system is configured to magnify about 2,5 times the diameter of an image circle of the 2/3~inch format to thereby obtain an image circle of the APS-C format. The reason is as follows. While the area of the image field of an image sensor for a three-chip camera is small, there is a need to use an image sensor, whose image field has a larger area, for a single-chip color camera to be used in combination.
[0026] The optical system not only simply magnifies an image circle, but also magnifies the above-mentioned aberration shape by a magnification factor. As a result, the entire resolution capability and the entire peripheral brightness of the magnified image field are approximately the same as the resolution capability and the peripheral brightness of the original image field before magnification. [0027] (Telecentricity)
It is assumed that the optical system is used for an image-pickup lens for a three-chip camera, which is telocentric at an image side to solve a color-cast problem of a color-separation prism. Further, because the optical system itself is telocentric at an image side and causes light beams to enter photosensitive

elements of an image field perpendicularly, light beams perpendicularly enter color filters of photosensitive elements even of a periphery of an image field. Therefore a captured image is brilliantly colored without murkiness even in its periphery. [0028] [Image-pickup lens assumed to be used in the present technology]
The optical system of the present technology is designed based on the assumption that it is arranged behind an image-pickup lens for a three-chip camera. Originally, an image-pickup lens for a three-chip camera is designed based on the assumption that it is used with a three-chip camera in combination. [0029] Under such a supposition, it is assumed that a color-separation prism and filters, e.g., a quartz filter 3.15 mm, an N-BAF 52 filter 33.02 mm, S-BSL 7 filter 10.05 mm, and the like, i.e., glass members of 40 mm or more in total, exist between an image-pickup lens for a three-chip camera and an image sensor. It should be noted that the flange focal length of an image-pickup lens is, for example, 48 mm (in Air). [0030] For that reason, if the image-pickup lens for a three-chip camera is used with a camera without such glass members, an extremely large aberration (mainly spherical aberration) may occur. In view of the above, it is most important to correct such an aberration when

the image-pickup lens for a three-chip camera is used with a single-chip color camera in combination. [0031] [Problem in using typical designing technique]
Typically, a rear conversion lens (tele conversion lens) is designed based on the assumption that the principal ray is emitted from a negative exit pupil position in a front image-picjcup lens. In other words, it is assumed that light beams emitted from an image-pickup lens, whose exit pupil distance is finite negative, enters a rear conversion lens. In addition, the rear conversion lens is designed such that light beams from the rear conversion lens have a finite-negative exit pupil distance, too.
[0032] In designing such a rear conversion lens, according to a general basic designing, a group of convex lenses is arranged as lenses closest to the object side, divergent of a principal ray is prevented once, a group of concave lenses are arranged at the image field side of the lens group, light beams are narrowed, and the focal length is thereby made larger. [0033] This basic designing is employed in order to downsize the rear conversion lens. Instead of achievement of downsizing, in general, the exit pupil distance at the image side of the rear conversion lens is smaller than the exit pupil distance of an image-

pickup lens.
[0034] In a film camera, a rear conversion lens, which makes the exit pupil distance smaller, makes the angle of light beams entering a film surface larger, which is not a problem. However, in a digital camera using an image sensor instead of a film, it is desirable that light beams enter perpendicular to photosensitive elements of the image sensor. However, in a rear conversion lens designed based on a typical technique,- the exit pupil distance is shortened, the angle of light beams entering photosensitive elements in a periphery of the image sensor is not perpendicular, and as a result the light amount of a periphery of an image may be insufficient or murkiness may occur,
[0035] Particularly, unlike a typical rear conversion lens, it is another object of the present technology to magnify an image circle. If the above-mentioned basic designing is employed, the incident angle of light beams entering photosensitive elements in a periphery of the image sensor is largely different from a perpendicular, which is a problem. [0036] In other words, according to a design constraint for a digital camera, i.e., a digital camera {including cinema camera) using an image sensor to capture an image, the exit pupil position should be at

infinity such that light beams perpendicularly enter
photosensitive elements even in a periphery of an image
sensor.
[0037] [Overview of lens structure]
Fig. 1 is a cross-sectional view taken along the optical axis of the optical system. A flange plane FP of an image-pickup lens for a three-chip camera is at the left side of Fig. 1, and an image field IMG of the image sensor is at the right side of Fig. 1. Sn is the n-th surface from the object side, Ln is the n-th lens from the object side, Fn is the n-th filter from the object side, and Dn is the n-th distance between ^surfaces from the object side. [0038] It should be noted that a filter FLl functions as a cover glass or an ND (Neutral Density) glass, and each of filters FL2 to FL4 is glass such as a quartz filter or another filter built in a camera. Further, for example, as shov-zn in Fig. 17, the optical system is combined with a chassis 10, which holds the optical system. The chassis 10 includes a first mount (first coupler) 20 at the object side, which is configured to connect the chassis 10 and an image-pickup lens for a three-chip camera, and a second mount {second coupler) 30 at the image field side of the chassis 10, which is configured to connect the chassis 10 and a single-chip color camera.

[0039] The optical system is an optical system including three groups of lenses {nine lenses), is designed based on the assumption that light beams entering from an image-pickup lens for a three-chip camera are telecentric, and is optically designed such that light beams are telecentric at the image field side.
[0040] The lens structure of the optical system includes, in the stated order from the object side,
(1) concave meniscus lens Ll including concave surface facing an image side
(2) biconvex lens L2
(3) biconcave lens L3
(4) biconcave lens L4
(5) biconvex lens L5
(6) biconvex lens L6
(7) biconcave lens L7
(8) biconvex lens L8
(9) biconvex lens L9.
[0041] Further, the lenses Ll, L2, and L3 are bonded together, and a concave lens group of three-bonded lenses {first lens group, first three-bonded lenses) is thereby formed.
[0042] Further, the lenses L6, L7, and L8 are bonded together, an afocal lens group of three-bonded lenses (second three-bonded lenses) is thereby formed, and the

SP351212XX00
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afocal lens group and the single lenses L4 and L5 form an aberration correction group {second lens group). [0043] A single lens L9 forms a convex lens group {third lens group) configured to form an image. 5 [0044] The optical system employs, as a method of correcting a spherical aberration, which occurs in large amounts when an image-pickup lens for a three-chip camera is used without a color-separation prism, a method of bonding pieces of glass, each of which has a 10 large curvature and different refractive index, thereby generating a high-order spherical aberration, and correcting the spherical aberration of the image-pickup
lens for a three-chip carr/era.
i [0045] [Requirements]
15 The requirements of the optical system are as
follows.
[0046] (First requirement)
As the first requirement, the compound-focal
length of the first lens group, i.e., the first three-20 bonded lenses including the bonded lenses Ll to L3,
should be negative.
[0047] This requirement is effective to downsize the
optical system, i.e., a magnifying optical system to be
combined with an image-pickup lens for a three-chip 25 camera, which is telecentric at the image side.
Further, this requirement is effective to, when the

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image-pickup lens for a three-chip camera, which is
telecentric at the image side, is used, make the
optical system to be telecentric at the image side.
Also from a viewpoint of magnification of an image,
5 which is an object of the optical system, the focal
length of the first lens group should be negative.
[0048] Because the exit pupil distance of a typical
image-pickup lens is negative, if the focal length of
the first lens group at the rear conversion lens side
10 is negative, the rear lenses of the second lens group
and the latter are too large to use. The optical
system solves this problem.
[0049] If the compound focal length is positive,
i then a.sufficient magnification factor cannot be
15 obtained, an image field curvature aberration is
increased, and a high-performance optical system cannot thus be obtained. Unlike an existing rear conversion lens, whose image cxrcle is constant in size even if an image is magnified, the optical system magnifies an
20 image circle to occupy a larger area and is largely
affected by an image field curvature aberration. For that reason, this requirement is important to reduce an image field curvature aberration. [0050] {Second requirement)
25 As the second requirement, the compound focal
length of the second three-bonded lenses and the third

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lens group, i.e., the lenses L6 to L9, should be positive.
[0051] Similar to the above-mentioned requirement, this requirement is effective to downsize the optical 5 system, i.e., a magnifying optical system to be
combined with an image-pickup lens for a three-chip camera, which is telecentric at the image side. The negative focal length of the first lens group and the positive focal length of the second lens group are
10 essential requirements to downsize the optical system. [0052] If the compound focal length is negative, then the optical system cannot be telecentric at the image field side, an image field curvature aberration is increased, and a high-performance optical system
15 cannot thus be obtained.
[0053] (Third requirement)
As the third requirement, the following conditional expression (1) should be satisfied. '-3.G<{f2G)/(flG)<-1.2 ... (1)
20 [0054] Where flG is the compound focal length of the first three-bonded lenses, and f2G is the compound focal length of the second three-bonded lenses. [0055] If the value (f2G)/(flG) is equal to or larger than -1.2, then astigmatism on both a sagittal
25 plane and a meridional plane is largely displaced to
the object side, the image quality of a periphery of an

SP351212XX00
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image sensor is degraded, and the optical system is thus not practical.
[0056] In addition, if the value (f2G}/(flG) is equal to or smaller than -3,0, then astigmatism on a 5 meridional plane is displaced to the object side,
astigmatism on a sagittal plane is displaced to the image field side, the image quality of the periphery is degraded, and the optical system is thus not practical. [0057] (Fourth requirement)
10 As the fourth requirement, the following
conditional expression (2) should be satisfied.
nlGl>nlG3>nlG2 ... (2) [0058] Where: nlGl is the refractive index of the lens Ll of the first three-bonded lenses, which is the
15 first lens from the object side; nlG2 is the refractive index of the lens L2 of the first three-bonded lenses, which is the second lens from the object side; and nlG3 is the refractive index of the lens L3 of the first three-bonded lenses, which is the third lens from the
20 object side.
[0059] As described above, this requirement is necessary to generate a high-order spherical aberration of the first lens group, to control spherical aberration of the image-pickup lens for a three-chip
25 camera, and to increase the image quality. The
smallest refractive index (nlG2) of the central lens L2

SP351212XX00 21
out of the refractive indexes of the three lenses is an absolute requirement. The requirement, in which the refractive index (nlGl) of the lens Ll closest to the object side out of the three lenses is larger than the 5 refractive index (nlG3) of the lens L3 closest to the image field, is defined based on the design condition of the currently-assumed image-pickup lens for a three-chip camera. [0060] If this requirement is not satisfied, then it
10 is not possible to correct a high-order aberration and a low-order aberration in a balanced manner and to attain a sufficient resolution. [0061] (Fifth requirement)
As the fifth requirement, the following
15 conditional expressions (3) and (4) should be satisfied.
n2G2>n2Gl ... (3) n2G2>n2G3 ... (4) [00 62] Where: n2Gl is the refractive index of the
20 lens L6 of the second three-bonded lenses, which is the first lens from the object side; n2G2 is the refractive index of the lens L7 of the second three-bonded lenses, v-zhich is the second lens from the object side; and n2G3 is the refractive index of the lens L8 of the second
25 three-bonded lenses, which is the third lens from the object side.

i/

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[0063] Similar to the above-mentioned fourth requirement, this requirement is necessary to generate a high-order spherical aberration of the second three-bonded lenses, to control spherical aberration of the 5 -image-pickup lens for a three-chip camera, and to
increase the image quality. The highest refractive index (n2G2) of the central lens L7 out of the refractive indexes of the three lenses is a typical requirement to control a spherical aberration.
10 [0064] If this requirement is not satisfied, then it is not possible to correct a high-order aberration and a low-order aberration in a balanced manner and to attain a sufficient resolution, [0065] [Lens surface data]
15 Specific numerical examples of lens surface data
are as follows, where R is a curvature radius of each lens surface (optical surface), D is a distance on the optical axis between lens surfaces, Nd is a refractive index with respect to the d-line (587.6 nm) of each
20 lens or filter (optical medium), and Vd is an Abbe number of the d-line of each lens (optical medium). Note that the curvature radius R, the distance between surfaces D, and an effective radius are measured in millimeters (mm).
25 [0066] {Lens surface data)
Surface No. R D Nd Vd Effective radius

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11 52.20 11.00 1.593 35.45 13.9 6th lens L6
12 -22.35 6.30 1.834 37.34 13.9 7th lens L7
13 27.80 10.50 ■ 1.541 47.00 15.3 8th lens L8
14 -168.00 0.51 16.6
10
15
20
25

Notes
0 10.00
flange plane FP
1 - 3.00 1.517 etc. FLl
2 ~ 10.60
3 156.00 1.70 1.883 LI
4 18.40 8.20 1.640 L2
5 -200.90 1.70 1.729 L3
6 35.85 11.17
7 -42.50 2.00 1.729 L4
8 100.30 • 1.00
9 64.40 4.50 1.620 L5
10 -125.00 0.71

image-pickup lens
64.20 17.9 filter,
40.80 13.6 1st lens
34.57 12.4 2nd lens
54.67 12.0 3rd lens
11.5 54.67 11.9 4th lens
12.5 36.30 13.1 5th lens
13.4

24

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10
15
20
25

18.3 17.7 filter,
17.7 filter,
17.7 filter,
16.5
16.5 sensor
16.5
28.0 image
15 69.50 11.50 1.518 58.96 17.9 9th lens L9
16 -31.74 1.00
17 - 0.25 1.523 59.44 etc. FL2
18 - 2.50 1.549 69.92 etc. FL3
19 - 0.30 1.517 64.20 etc, FL4
20 <- 11.56
21 - 0.70 1.517 64.20 cover SC
22 - 1.00
23 - 0.00 field IMG
[0067] [Optical path diagram, aberration diagrams, and the like]
Here, description will be made with reference to an optical path diagram, aberration diagrams, and the like. Fig. 2 is an optical path diagram of the optical system. As to aberration diagrams, aberration diagrams of the optical system will be provided first, and aberration diagrams of an image-pickup lens for a three-chip camera, in which the optical system is to be used, with a color-separation prism will be provided next. Finally, aberration diagrams of the image-pickup

SP351212XX0O
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10
15
20
25

lens for a three-chip camera without a color-separation prism will be provided.
[0068] For example, the scale difference between coma aberration diagrams of an image-pickup lens for a three-chip camera with a color-separation prism and coma aberration diagrams of an image-pickup lens for a three-chip camera without a color-separation prism is one of the important points. Since the scale of the former is ±0.01 mm and the scale of the latter is ±0.5 mm, it can be seen that the order of a coma aberration with a color-separation prism is fifty times as large as the order of a coma aberration without a color-i separation prism,
i
[0069] Fig. 3 shows coma aberration diagrams of the optical system, in which the drawing scale is ±0,02 mm. The left diagrams show meridional planes, and the right diagrams show sagittal planes. In descending order, aberrations are 14 mm, 11 mm, 7 mm, and 0 mm in image height.
[0070] Fig. 4 is a spherical aberration diagram of the optical system, in v/hich the perpendicular axis represents a light beam height and the horizontal axis represents a distance in the optical axis direction. The drawing scale is ±0.1 mm.
[0071] Fig. 5 shows, starting from the left, astigmatism, distortion, and chromatic aberration of

^

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magnification of the optical system. In the astigmatism dxagram, the perpendicular axis represents an image height, and the horizontal axis represents a defocusing amount. In the distortion diagram, the 5 perpendicular axis represents an image height, and the horizontal axis represents an amount of a distortion of an image. The drawing scales are ±0.1 mm, ±2%, and ±0.02 mm, respectively. In the astigmatism diagram, the solid line represents the meridional direction, and
10 the dashed line represents the sagittal direction. [0072] Fig. 6 shows spot diagrams of the optical system, in which the drawing scale between the center of each ciross and ,each end thereof is 0.05 mm. The lower-left diagram represents the image height of 0 mm,
15 the upper-left diagram represents the image height of 7 mm, the lower-right diagram represents the image height of 11 mm, and the upper-right diagram represents the image height of 14 mm. [0073] Fig. 7 shows v/hite MTF (Modulation Transfer
20 Function) characteristic diagrams of the optical
system, in which the perpendicular axis represents a contrast (modulation), and the horizontal axis represents a spatial frequency. The lower-left diagram represents the image height of 0 mm, the upper-left
25 diagram represents the image height of 7 mm, the lower-right diagram represents the image height of 11 mm, and

//

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the upper-right diagram represents the image height of 14 mm.
[0074] Fig. 8 shows defocused MTF characteristic diagrams of the optical system, in which the 5 perpendicular axis represents a contrast (modulation), and the horizontal axis represents a focus shift position. A defocused range is +0.1 mm. The lower-left diagram represents the image height of 0 mm, the upper-left diagram represents the image height of 7 mm,
10 the lower-right diagram represents the image height of 11 mm, and the upper-right diagram represents the image height of 14 mm.
[0075] Fig. 9 is a^spherical aberration diagram of an image-pickup lens for a three-chip camera with a
15 color-separation prism, in v-zhich the perpendicular axis represents a light beam height and the horizontal axis represents a distance in the optical axis direction, The drawing scale is ±0.01 mm. [0076] Fig. 10 shov/s coma aberration diagrams of the
20 image-pickup lens for a three-chip camera v;ith a color-separation prism, in which the drawing scale is +0.01 mm. The left diagrams show meridional planes, and the right diagrams show sagittal planes. In descending order, aberrations are 5.8 mm, 5 mm, 4 mm, and 0 mm in
25 image height.
[0077] Fig. 11 shows, starting from the left,

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astigmatism, distortion, and chromatic aberration of
magnification of the image-pickup lens for a three-chip
camera with a color-separation prism. In the
astigmatism diagram, the perpendicular axis represents
5 an image height, and the horizontal axis represents a
defocusing amount. In the distortion diagram, the
perpendicular axis represents an image height, and the
horizontal axis represents an amount of a distortion of
an image. The drawing scales are ±0.01 mm, ±0.3%, and
10 ±0.002 mm, respectively. In the astigmatism diagram,
the solid line represents the meridional direction, and
the dashed line represents the sagittal direction.
[0078] Fig. 12 shows white MTFicharacteristic
J diagrams of the image-pickup lens for a three-chip
15 camera with a color-separation prism, in which the
perpendicular axis represents a contrast (modulation), and the horizontal axis represents a spatial frequency. The lower-left diagram represents the image height of 0 mm, the upper-left diagram represents the image height
20 of 4 mm, the lower-right diagram represents the image height of 5 mm, and the upper-right diagram represents the image height of 5.8 mm.
[0079] Fig. 13 is a spherical aberration diagram of an image-pickup lens for a three-chip camera without a
25 color-separation prism, in which the perpendicular axis represents a light beam height and the horizontal axis

represents a distance in the optical axis direction. The drawing scale is ±0.5 mm.
[0080] Fig. 14 shows, starting from the left, astigmatism, distortion, and chromatic aberration of magnification of the image-pickup lens for a three-chip camera without a color-separation prism. In the astigmatism diagram, the perpendicular axis represents an image height, and the horizontal axis represents a defocusing amount. In the distortion diagram, the perpendicular axis represents an image height, and the horizontal axis represents an amount of a distortion of an image. The drawing scales are ±0.01 mm, ±0.3%, and ±0,002 mm, respectively. In the astigmatism tiiagram, the solid line represents the meridional direction, and the dashed line represents the sagittal direction. [0081] Fig. 15 shows coma aberration diagrams of the image-pickup lens for a three-chip camera without a color-separation prism, in v/hich the drav;ing scale is ±0.5 mm. The left diagrams shovj meridional planes, and the right diagrams show sagittal planes. In descending order, aberrations are 5.8 mm, 5 mm, 4 mm, and 0 mm in image height.
[0082] Fig. 16 shows white MTF characteristic diagrams of the image-pickup lens for a three-chip camera without a color-separation prism, in which the perpendicular axis represents a contrast (modulation) ,

and the horizontal axis represents a spatial frequency. The lower-left diagram represents the image height of 0 mm, the upper-left diagram represents the image height of 4 mm, the lower-right diagram represents the image height of 5 mm, and the upper-right diagram represents the image height of 5.8 mm.
[0083] [Supplementary note]
The present technology is not limited to the above-mentioned embodiment, but may be variously modified within the gist of the present technology, as a matter of course.
[0084] [Other structures of the present technology]
It should be noted that the present technology may ^
i employ the following structures.
(1) A rear conversion lens, including:
a chassis including two opposite ends, the chassis including light-transmitting holes on the ends, respectively;
a first coupler configured to detachably couple one of the ends of the chassis v/ith an image field side of an image-pickup lens of a three-chip camera including three first image sensors, the image-pickup lens being designed based on an assumption that the image-pickup lens is to be used with a color-separation prism in combination;
a second coupler configured to detachably couple

the other end of the chassis with a single-chip color camera including a second image sensor, the second image sensor including an image field larger than an image field of the first image sensors of the three-chip camera; and
an optical system including a first lens group, a second lens group, and a third lens group arranged in the chassis in this order from an object side, the first lens group having a negative compound focal length, the second lens group being configured to correct a spherical aberration, the third lens group being configured to form an image.
(2) The rear conversion lens according to (1), in
v/hich
the first lens group includes first three-bonded lenses,
the second lens group includes a concave lens, a convex lens, and second three-bonded lenses in this order from the object side, and
the third lens group includes a convex lens.
(3) The rear conversion lens according to (2) , in
which
a compound focal length of the second three-bonded lenses and the third lens group is positive.
(4) The rear conversion lens according to (2) or (3),
in which

the first three-bonded lenses and the second three-bonded lenses satisfy a conditional expression
-3.0<(f2G)/{flG)<~1.2 ... (1)
where
flG is a compound focal length of the first three-bonded lenses, and
f2G is a compound focal length of the second three-bonded lenses.
(5) The rear conversion lens according to any one of
(2) to (4), in which
the first three-bonded lenses satisfy a conditional expression (2),
nlGl>nlG3>nlG2 ... (2)
where
nlGl is a refractive index of a first lens of the first three-bonded lenses from the object side,
nlG2 is a refractive index of a second lens of the first three-bonded lenses from the object side, and
nlG3 is a refractive index of a third lens of the first three-bonded lenses from the object side.
(6) The rear conversion lens according to any one of
{2} to (5), in which
the second three-bonded lenses satisfy conditional expressions (3) and (4), n2G2>n2Gl ... (3)

n2G2>n2G3 ... (4)
where
n2Gl is a refractive index of a first lens of the second three-bonded lenses from the object side,
n2G2 is a refractive index of a second lens of the second three-bonded lenses from the object side, and
n2G3 is a refractive index of a third lens of the second three-bonded lenses from the object side. Description of Symbols
[0085] Dn ... n-th distance between surfaces from object side
FLn ... n-th filter, etc. from object side FP ... flange plane on image-pickup lens for three-chip camera
IMG ... image field of image sensor Ln .,. n-th lens from object side SO ... sensor cover
Sn ... n-th surface of lens or filter from object side 10 ... chassis 20 ... first mount 30 ... second mount

Claims [1]
A rear conversion lens, comprising:
a chassis including two opposite ends, the chassis including light-transmitting holes on the ends, respectively;
a first coupler configured to detachably couple one of the ends of the chassis with an image field side of an image-pickup lens of a three-chip camera including three first image sensors, the image-pickup lens being designed based on an assumption that the image-pickup lens is to be used with a color-separation prism in combination;
a second coupler configured to detachably couple the other end of the chassis with a single-chip color camera including a second image sensor, the second image sensor including an image field larger than an image field of the first image sensors of the three-chip camera; and
an optical system including a first lens group, a second lens group, and a third lens group arranged in the chassis in this order from an object side, the first lens group having a negative compound focal length, the second lens group being configured to correct a spherical aberration, the third lens group being configured to form an image. [2] The rear conversion lens according to claim 1,

wherein
the first lens group includes first three-bonded lenses,
the second lens group includes a concave lens, a convex lens, and second three-bonded lenses in this order from the object side, and
the third lens group includes a convex lens. [3] The rear conversion lens according to claim 2, wherein
a compound focal length of the second three-bonded lenses and the third lens group is positive. [4] The rear conversion lens according to claim 3, wherein
the first three-bonded lenses and the second three-bonded lenses satisfy a conditional expression
-3.0<(f2G)/(flG)<-1.2 ... (1)
where
fIG is a compound focal length of the first three-bonded lenses, and
f2G is a compound focal length of the second three-bonded lenses.
[5] The rear conversion lens according to claim 4, wherein
the first three-bonded lenses satisfy a conditional expression (2),

nlGl>nlG3>nlG2 .*.. (2)
whef'e
nlGl is a refractive index of a first lens of the first three-bonded lenses frojn the object side,
nlG2 is a refractive index of a second lens of the first three-bonded lenses frqin the object side, and
nlGS is a refractive indax of a third lens of the firs-t three-bonded lenses froiti the object side. [6] The rear conversion lens according to claim 5, v^herein
the second three-bonded lenses satisfy conditional expressions (3) and (4),
neG2>n2Gl ... (3)
n2G2>n2G3 ... (4)
where ' •
n2Gl is a refractive index of a first lens of the second three-bonded lenses from the object side, ,
n2G2 is a refractive index of a second lens of the second three-bonded lenses from the object side, and
n2G3 is a refractive index of a third lens of the second three-bonded lenses from the object side.