JP2012047869A - Rear converter lens and imaging optical system having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rear converter lens capable of easily retaining a high optical performance even if being mounted on a main lens system.SOLUTION: This rear converter lens having a negative focal length is mounted on an image side of a main lens system to make a focal length of an entire system longer than a focal length of the main lens system alone. The rear converter lens comprises, in order from an object side to the image side, a front group and a rear group with a widest air interval formed therebetween as a border, and the rear group consists of an RL first lens group of positive refractive power, an RL second lens group of negative refractive power and an RL third lens group of positive refractive power. Assuming that the focal length and the Abbe number of a material of (i)th positive lens counted from the object side of the rear group are Rfpi and Rνdpi respectively and the focal length of the rear converter lens is f, the rear group satisfies the following conditional expression: 5<Σ(Rfpi×Rνdpi)/|f|<54.

Description

  The present invention relates to a rear converter lens that is detachably attached to the rear side (image side) of a main lens system and displaces the focal length of the entire lens system in a longer direction, and a photographing optical system having the rear converter lens. The present invention is particularly suitable for imaging devices such as digital cameras, video cameras, electronic still cameras, TV cameras, and silver halide photography cameras.

  Conventionally, in a single-lens reflex camera, a negative focal length (refractive power) lens (rear converter lens) is detachably mounted between the interchangeable lens (main lens system) (photographing lens) and the camera body. Displacement of the focal length in the longer direction is widely practiced. This method has advantages that the focal length of the entire lens system can be easily displaced, and that the entire lens system can be made compact compared to a method of mounting on the object side of the main lens system. However, this system increases the open F number of the entire system in proportion to the magnification of the focal length of the entire system, that is, the brightness of the entire lens system decreases.

  Further, when a rear converter lens is attached to the main lens system, the residual aberration of the main lens system is increased in proportion to the enlargement magnification. For this reason, in many cases, when the rear converter lens is attached to the main lens system, the image quality deteriorates. For example, when the enlargement magnification when the rear converter lens is attached to the main lens system is double, the lateral aberration is doubled and the image quality is deteriorated. Longitudinal aberrations are magnified to the square of the magnification, or 4 times. However, in the case of a rear converter lens, the F number of the main lens system is also doubled. Eventually, longitudinal aberrations are also magnified by a factor of 2 per unit depth of focus.

  For this reason, in order to satisfactorily maintain the various aberrations of the entire system when the rear converter lens is attached to the main lens system, it is necessary to correct the various aberrations of the rear converter lens itself. Conventionally, rear converter lenses are known that maintain good optical performance when mounted on a main lens system by optimizing power arrangement (refractive power arrangement), lens configuration, and the like (Patent Documents 1 and 2).

JP 63-147127 A JP 2009-80176 A

  When attached to the main lens system, the rear converter lens that shifts the focal length of the entire system in the longer direction has negative refractive power. For this reason, since it itself has a large negative Petzval sum, when it is attached to the main lens system, the curvature of field often deteriorates. In addition, when the rear converter lens is attached to the main lens system, the aperture stop is included in the main lens system, and therefore the aperture stop in the main lens system is often used. For this reason, in the rear converter lens, the principal ray of the off-axis light beam does not cross the optical axis and passes above and below the optical axis.

  For this reason, it is generally difficult for the rear converter lens to cancel various aberrations caused by passing off the optical axis. When the rear converter lens is attached to the main lens system, correction of spherical aberration, coma aberration, curvature of field, etc. is relatively easy, but correction of lateral chromatic aberration becomes difficult.

  An object of the present invention is to provide a rear converter lens that can easily maintain high optical performance even when attached to a main lens system, and a photographing optical system having the rear converter lens.

The rear converter lens of the present invention is a rear converter lens having a negative focal length that is attached to the image plane side of the main lens system to make the focal length of the entire system longer than the focal length of the main lens system alone. The rear converter lens is composed of a front group and a rear group in order from the object side to the image side, with the widest air gap as a boundary, and the rear group includes a first RL1 lens group having a positive refractive power and a first lens having a negative refractive power. The RL2 lens group and the RL3 lens group having a positive refractive power, the focal length of the i-th positive lens and the Abbe number of the material counted from the object side of the rear group are Rfpi, Rνdpi, and the rear converter lens, respectively. When the focal length is f,
5 <Σ (Rfpi × Rνdpi) / | f | <54
It satisfies the following conditional expression.

  According to the present invention, a rear converter lens capable of easily maintaining high optical performance even when attached to a main lens system, and a photographing optical system having the same are obtained.

Lens cross section when the rear converter lens of Numerical Example 1 is attached to the main lens system Lens sectional view of rear converter lens of Numerical Example 1 Aberration diagram when the rear converter lens of Numerical Example 1 is attached to the main lens system Lens cross section of rear converter lens of Numerical Example 2 Aberration diagram when the rear converter lens of Numerical Example 2 is attached to the main lens system Lens cross section of rear converter lens of Numerical Example 3 Aberration diagram when the rear converter lens of Numerical Example 3 is attached to the main lens system Schematic diagram of main parts of an imaging apparatus of the present invention

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The rear converter lens of the present invention is detachably mounted on the image plane side of the main lens system, and has a negative focal length that displaces the focal length of the entire system longer than the focal length of the main lens system alone. ing. The rear converter lens is composed of a front group and a rear group in order from the object side to the image side, with the widest air gap as a boundary. The front group consists of one or two lens groups. The rear group includes an RL1 lens group having a positive refractive power, an RL2 lens group having a negative refractive power, and an RL3 lens group having a positive refractive power.

  FIG. 1 is a lens cross-sectional view of the entire system when the rear converter lens of Example 1 of the present invention is detachably mounted on the image plane side of the main lens system. Although the telephoto lens is mentioned here as an example of the main lens system, the main lens system is not limited to the present embodiment, and any lens system such as a zoom lens or a standard lens may be used.

  FIG. 2 is a single lens cross-sectional view of the rear converter lens according to the first embodiment of the present invention. FIG. 3 is an aberration diagram for photographing the entire system at infinity when the rear converter lens of Example 1 is attached to the main lens system. FIG. 4 is a sectional view of a single lens of the rear converter lens according to the second embodiment of the present invention, and FIG. 5 shows aberrations at the time of photographing an infinitely distant object when the rear converter lens according to the second embodiment is attached to the main lens system. FIG. FIG. 6 is a sectional view of a single lens of the rear converter lens according to the third embodiment of the present invention, and FIG. 7 is an aberration at the time of photographing an infinitely distant object when the rear converter lens according to the third embodiment is mounted on the main lens system. FIG.

  A lens system in which a rear converter lens is attached to the main lens system of each embodiment is a photographic lens system used in an imaging apparatus such as a digital still camera or a silver halide film camera. In the lens cross-sectional view, the left side is the subject side (front), and the right side is the image side (rear).

  In FIG. 1, ML is a main lens system, RCL is a rear converter lens, and OL is a lens system (entire system) (imaging optical system) in which a rear converter lens RCL is attached to the main lens system ML. In the main lens system ML, SP is an aperture stop, and G is an optical block corresponding to protective glass or the like. In the rear converter lens RCL, FS is a flare cut stop. IP is an image plane. When used as a photographing optical system for a video camera or a digital still camera, an imaging plane of a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor, or a camera for a silver salt film It corresponds to a photosensitive surface such as a film surface.

  In the aberration diagrams, d and g are d-line and g-line, respectively, and S · C is a sine condition. M and S are the d-line meridional image surface and sagittal image surface. Lateral chromatic aberration is represented by the g-line. Fno is the F number, and ω is the shooting half angle of view. The rear converter lens RCL of each embodiment is composed of a front group FL and a rear group RL in order from the object side to the image side, with the widest air gap as a boundary. The front group FL includes a first FL1 lens group FL1 and a second FL2 lens group FL2. The rear group RL includes a first RL1 lens group RL1 having a positive refractive power, a RL2 lens group RL2 having a negative refractive power, and a RL3 lens group RL3 having a positive refractive power.

  Here, the lens group refers to lenses separated by a finite air interval, such as a single lens or a cemented lens obtained by cementing a plurality of lenses.

In the rear converter lens RCL of each embodiment, the focal length and the Abbe number of the i-th positive lens counted from the object side of the rear group RL are Rfpi and Rνdpi, respectively. Further, the focal length of the rear converter lens RCL is assumed to be f. At this time,
5 <Σ (Rfpi × Rνdpi) / | f | <54 (1)
The following conditional expression is satisfied.

  In the rear converter lens RCL of each embodiment, the rear group RL including the first RL1 lens group RL1, the RL2 lens group RL2, and the RL3 lens group RL3, which are relatively close to the image plane, has a higher incident height of off-axis rays. . Therefore, this lens group is suitable for correcting lateral chromatic aberration. Therefore, in each embodiment, the chromatic aberration of magnification is satisfactorily corrected while suppressing the occurrence of field curvature so as to satisfy the conditional expression (1) with respect to the focal length of the positive lens constituting the rear group RL and the Abbe number of the material. Yes.

  Conditional expression (1) relates to achromaticity in the rear group RL. When the lower limit of conditional expression (1) is exceeded, achromaticity due to the positive lens becomes too small, making it difficult to correct lateral chromatic aberration. If the upper limit of conditional expression (1) is exceeded, the achromaticity by the positive lens becomes too large, and the lateral chromatic aberration is overcorrected, which is not good. Conditional expression (1) is more preferably set as follows.

10 <Σ (Rfpi × Rνdpi) / | f | <52 (1a)
As described above, according to the rear converter lens of the present invention, particularly, the lateral chromatic aberration is corrected well, and high optical performance can be obtained even when mounted on the main lens system.

  In the rear converter lens of each embodiment, when it is mounted on the main lens system ML, it is preferable to satisfy one or more of the following conditions in order to maintain further excellent optical performance. The RL2 lens group RL2 includes a single negative lens, and the Abbe number and the partial dispersion ratio of the negative lens material are Rνd2n and RθgF2n, respectively. The RL3 lens group RL3 has a negative lens, and the Abbe number and the partial dispersion ratio of the negative lens material are Rνd3n and RθgF3n, respectively. Let Rr2r be the radius of curvature of the lens surface closest to the image side of the RL2 lens group RL2, and Rr3f be the radius of curvature of the lens surface closest to the object side of the RL3 lens group RL3.

  Let Rr1r be the radius of curvature of the lens surface closest to the image side of the RL1 lens group RL1, and let Rr2f be the radius of curvature of the lens surface closest to the object side of the RL2 lens group RL2. The front group FL has one or more lens groups, and the distance on the optical axis from the most image side lens surface of the front group FL to the object side lens surface of the RL1 lens group RL1 is dfr. Let td be the length of the rear converter lens RCL on the optical axis. The curvature radius of the lens surface closest to the image side of the first FL1 lens unit FL1 is Fr1r, and the curvature radius Fr2f of the lens surface closest to the object side of the second FL2 lens unit FL2.

At this time,
0 <RθgF2n−0.6438 + 0.001682 × Rνd2n <0.05
... (2)
0.00 <RθgF3n−0.6438 + 0.001682 × Rνd3n <0.05
... (3)
0.03 <(Rr2r−Rr3f) / (Rr2r + Rr3f) <0.25
... (4)
−0.30 <(Rr1r−Rr2f) / (Rr1r + Rr2f) <− 0.07
... (5)
0.08 <dfr / td <0.27 (6)
0.03 <(Fr1r−Fr2f) / (Fr1r + Fr2f) <0.25
... (7)
It is preferable to satisfy one or more of the following conditional expressions.

  Next, the technical meaning of each conditional expression described above will be described. Conditional expression (2) relates to partial dispersion of the material of the negative lens arranged in the second RL2 lens group RL2. If the partial dispersion of the material of the negative lens is too small beyond the lower limit of conditional expression (2), the effect of correcting the lateral chromatic aberration is not likely to be small. If the upper limit of conditional expression (2) is exceeded and the partial dispersion of the material of the negative lens becomes too large, the glass material that can be selected is likely to decrease. Conditional expression (2) is more preferably set as follows.

0.01 <RθgF2n−0.6438 + 0.001682 × Rνd2n <0.04
... (2a)
Conditional expression (3) relates to partial dispersion of the material of the negative lens arranged in the RL3 lens group RL3.

  If the partial dispersion of the material of the negative lens is too small beyond the lower limit of conditional expression (3), the effect of correcting the lateral chromatic aberration is not likely to be small. If the upper limit of conditional expression (3) is exceeded and the partial dispersion of the negative lens material becomes too large, the glass material that can be selected is likely to be small. Conditional expression (3) is more preferably set as follows.

0.01 <RθgF3n−0.6438 + 0.001682 × Rνd3n <0.04
... (3a)
Conditional expression (4) relates to the shape of the air lens between the RL2 lens group RL2 and the RL3 lens group RL3.

  If the lower limit of conditional expression (4) is exceeded, the field curvature will not be good. If the upper limit of conditional expression (4) is exceeded, the field curvature will not be good. Conditional expression (4) is more preferably set as follows.

0.05 <(Rr2r−Rr3f) / (Rr2r + Rr3f) <0.22
... (4a)
Conditional expression (5) relates to the shape of the air lens between the RL1 lens group RL1 and the RL2 group RL2. If the lower limit of conditional expression (5) is exceeded, the field curvature will not be good. If the upper limit of conditional expression (5) is exceeded, negative distortion is not likely to increase. Conditional expression (5) is more preferably set as follows.

−0.26 <(Rr1r−Rr2f) / (Rr1r + Rr2f) <− 0.09
... (5a)
Conditional expressions (4) and (5) should be satisfied at the same time, and an appropriate balance should be achieved. According to this, it becomes easy to correct field curvature and distortion more satisfactorily.

  Conditional expression (6) indicates that the length of the rear converter lens RCL on the optical axis, that is, the length on the optical axis from the most object side lens surface to the most image side lens surface, is the second FL2 lens group FL2 and the RL1 lens. It is related to the ratio between groups. If the lower limit of conditional expression (6) is exceeded and the distance dfr becomes too small, the rear group RL approaches the stop SP of the main lens ML, so that the separation between the on-axis light beam and the off-axis light beam becomes worse, and the lateral chromatic aberration is effectively reduced. It becomes difficult to correct. If the upper limit of conditional expression (6) is exceeded, the ratio of the distance dfr in the total length of the rear converter lens RCL will be too large, and it will be difficult to place a desired lens group, which is not good. Conditional expression (6) is more preferably set as follows.

0.085 <dfr / td <0.250 (6a)
Since the first FL1 lens group FL1 and the second FL2 lens group FL2 of the front group FL have a relatively high incident height of axial rays, it is easy to effectively correct spherical aberration and axial chromatic aberration.

  Conditional expression (7) relates to the shape of the air lens between the first FL1 lens group FL1 and the second FL2 lens group FL2. When the lower limit or upper limit of conditional expression (7) is exceeded, it is difficult to correct spherical aberration and axial chromatic aberration in a balanced manner. Conditional expression (7) is more preferably set as follows.

0.04 <(Fr1r−Fr2f) / (Fr1r + Fr2f) <0.22
... (7a)
Each of the RL1 lens group RL1 and the RL3 lens group RL3 is preferably composed of a cemented lens in which a positive lens and a negative lens are cemented.

  According to this, it becomes easy to correct lateral chromatic aberration satisfactorily. The first FL1 lens group FL1 is preferably composed of a positive lens and a negative lens, and the second FL2 lens group FL2 is preferably composed of a positive lens and a negative lens. According to this, it becomes easy to correct the longitudinal chromatic aberration remaining when correcting the lateral chromatic aberration in the rear group RL. The front group FL may be composed of only one lens group.

  Next, an embodiment of a single-lens reflex camera (imaging device) in which the rear converter lens of the present invention is mounted on the image side of the main lens system and used as a photographing optical system will be described with reference to FIG. In the figure, reference numeral 10 denotes a photographing lens (interchangeable lens) having the photographing optical system 1 including any one of the rear converter lens of Examples 1, 2, and 3 and a main lens system. The photographing optical system 1 is held by a lens barrel 2 that is a holding member.

  Reference numeral 20 denotes a camera body, which includes a quick return mirror 3 that reflects the light beam from the photographing lens 10 upward and a focusing screen 4 that is disposed at an image forming position of the photographing lens 10. Further, it has a penta roof prism 5 for converting an inverted image formed on the focusing screen 4 into an erect image, an eyepiece 6 for enlarging the erect image, and the like. Reference numeral 7 denotes a photosensitive surface, on which a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor, or a silver film is disposed as a light receiving means (recording means) for receiving an image. At the time of photographing, the quick return mirror 3 is retracted from the optical path, and an image is formed on the photosensitive surface 7 by the photographing lens 10.

  Although the preferred embodiments of the optical system of the present invention have been described above, it is needless to say that the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the gist thereof. For example, the present invention can be applied to an imaging device without a quick return mirror.

  Next, numerical examples 1 to 3 of the main lens system and rear converter lenses corresponding to the examples 1 to 3 are shown. In numerical examples 1 to 3, only the data of the rear converter lens is shown. Rs is the distance from the most image side surface of the main lens system to the most object side lens surface of the rear converter lens. The main lens system in Numerical Examples 1 to 3 uses a telephoto lens in which each aberration with a focal length of 390 mm is corrected sufficiently satisfactorily. Therefore, if each aberration is well corrected with the rear converter lens attached to this main lens system, good characteristics can be obtained with respect to the original optical performance even when attached to other lens systems. it can.

  In the numerical examples, i indicates the order counted from the object side. ri is the radius of curvature of the i-th optical surface, di is the axial distance between the i-th surface and the (i + 1) -th surface, and ndi and νdi are the i-th and (i + 1) -th surfaces with respect to the d-line, respectively. The refractive index and Abbe number of the medium in between are shown. β indicates the magnification when the rear converter lens is mounted on the main lens system ML. Table 1 shows the relationship between the above-described conditional expressions and numerical values in the numerical examples.

[Main lens]

Unit mm

Surface data surface number rd nd νd Effective diameter
1 270.270 14.50 1.48749 70.4 134.51
2 -835.232 31.14 133.96
3 138.907 20.35 1.43387 95.1 118.33
4 -523.327 0.19 116.36
5 -499.366 4.30 1.72047 34.7 116.33
6 310.896 27.04 111.57
7 96.687 14.92 1.43387 95.1 98.15
8 664.543 0.24 96.16
9 63.598 6.00 1.51633 64.1 84.19
10 51.493 24.99 76.37
11 461.177 5.35 1.80809 22.8 70.03
12 -178.181 3.20 1.83400 37.2 69.78
13 117.578 73.69 65.06
14 (Aperture) ∞ 10.03 44.75
15 285.718 7.19 1.72916 54.7 41.58
16 -69.727 2.18 1.84666 23.8 40.79
17 -247.741 0.84 40.02
18 81.104 5.31 1.84666 23.8 38.15
19 -151.144 2.00 1.69680 55.5 37.25
20 41.763 5.33 35.01
21 -163.217 1.70 1.88300 40.8 35.07
22 95.192 2.18 35.79
23 93.284 5.74 1.83400 37.2 37.48
24 -356.898 8.32 38.04
25 63.599 9.04 1.74950 35.3 40.70
26 -95.263 1.87 1.80809 22.8 40.10
27 101.811 11.05 39.24
28 ∞ 2.20 1.48749 70.2 39.72
29 ∞ 27.79 39.79
30 ∞ BF 40.77
Image plane ∞

Various data

Focal length 390.09
F number 2.90
Angle of View 3.17
Statue height 21.64
Total lens length 367.66
BF 39.00

[Numerical Example 1]

Unit mm

Surface data surface number rd nd νd Effective diameter
1 136.661 1.20 1.81600 46.6 22.08
2 19.932 5.97 1.57501 41.5 21.80
3 -46.059 2.78 22.04
4 -41.442 1.20 1.72916 54.7 21.81
5 25.523 5.38 1.61340 44.3 22.72
6 -48.774 10.56 23.21
7 -46.012 1.49 1.85026 32.3 24.07
8 65.320 12.02 1.61340 44.3 25.39
9 -24.563 0.20 27.97
10 -39.870 1.20 1.59282 68.6 27.66
11 61.918 0.20 28.68
12 42.253 9.47 1.48749 70.2 29.57
13 -36.822 2.50 1.59282 68.6 29.79
14 -331.730 30.86

Various data

Focal length -83.98

Front principal point 7.09
Rear principal point position -33.52

RS 17.79
β 1.996

Single lens Data lens Start surface Focal length
1 1 -28.73
2 2 25.02
3 4 -21.50
4 5 28.09
5 7 -31.56
6 8 30.66
7 10 -40.73
8 12 42.01
9 13 -70.09

[Numerical Example 2]

Unit mm

Surface data surface number rd nd νd Effective diameter
1 153.654 1.20 1.88300 40.8 24.00
2 19.178 6.00 1.57501 41.5 20.85
3 -59.856 3.98 21.03
4 -46.053 1.20 1.83481 42.7 21.07
5 36.347 5.08 1.73800 32.3 21.97
6 -37.255 9.83 22.42
7 -33.407 1.50 1.83481 42.7 22.53
8 127.368 7.00 1.65412 39.7 24.06
9 -22.677 0.20 25.22
10 -36.679 1.20 1.59282 68.6 25.04
11 67.265 0.20 25.99
12 44.571 6.00 1.61340 44.3 26.56
13 -39.447 1.50 1.80809 22.8 26.72
14 -567.554 27.33

Focal length -87.27


Front principal point position 5.62
Rear principal point position -26.88

RS 17.79
β 1.988

Single lens Data lens Start surface Focal length
1 1 -24.92
2 2 25.98
3 4 -24.17
4 5 25.68
5 7 -31.57
6 8 29.98
7 10 -39.87
8 12 35.07
9 13 -52.53

[Numerical Example 3]

Unit mm

Surface data surface number rd nd νd Effective diameter
1 43.119 1.20 1.88300 40.8 21.80
2 11.681 8.01 1.62588 35.7 19.20
3 -39.543 2.48 18.80
4 -26.576 1.20 1.88300 40.8 17.60
5 19.128 5.73 1.61340 44.3 17.20
6 -18.945 3.52 17.20
7 -35.911 1.20 1.88300 40.8 15.72
8 25.159 7.16 1.58144 40.8 16.25
9 -13.507 0.20 17.14
10 -16.814 1.20 1.59282 68.6 16.77
11 30.102 0.23 18.00
12 26.729 5.61 1.73800 32.3 18.48
13 -22.968 1.50 1.80809 22.8 18.77
14 229.102 19.43


Focal length -41.91


Front principal point position 12.17
Rear principal point position -13.81

RS 27.79
β 2.779

Single lens Data lens Start surface Focal length
1 1 -18.48
2 2 15.33
3 4 -12.44
4 5 16.46
5 7 -16.60
6 8 16.22
7 10 -18.03
8 12 17.58
9 13 -25.76

FL: front group RL: rear group FL1: first FL1 lens group FL2: second FL2 lens group RL1: first RL1 lens group RL2: RL2 lens group RL3: RL3 lens group SP: stop FP: flare-cut stop IP: image plane ML: main photographing lens RCL: rear converter lens S: sagittal image plane M: meridional image plane d: d line g: g line

Claims (11)

In the rear converter lens having a negative focal length, which is attached to the image plane side of the main lens system to make the focal length of the entire system longer than the focal length of the main lens system alone, the rear converter lens is In order from the image side to the image side, it is composed of a front group and a rear group with the widest air interval as a boundary, and the rear group includes a first RL1 lens group having a positive refractive power, a RL2 lens group having a negative refractive power, and a positive refraction. When the focal length of the ith positive lens and the Abbe number of the material counted from the object side of the rear group are Rfpi and Rνdpi, and the focal length of the rear converter lens is f, respectively. ,
5 <Σ (Rfpi × Rνdpi) / | f | <54
A rear converter lens that satisfies the following conditional expression: The RL2 lens group is composed of a single negative lens, and when the Abbe number and the partial dispersion ratio of the negative lens material are Rνd2n and RθgF2n, respectively.
0 <RθgF2n−0.6438 + 0.001682 × Rνd2n <0.05
The rear converter lens according to claim 1, wherein the following conditional expression is satisfied. The RL3 lens group includes a negative lens, and the Abbe number and partial dispersion ratio of the negative lens material are Rνd3n and RθgF3n, respectively.
0.00 <RθgF3n−0.6438 + 0.001682 × Rνd3n <0.05
The rear converter lens according to claim 1, wherein the following conditional expression is satisfied. When the curvature radius of the lens surface closest to the image side of the RL2 lens group is Rr2r, and the curvature radius of the lens surface closest to the object side of the RL3 lens group is Rr3f,
0.03 <(Rr2r−Rr3f) / (Rr2r + Rr3f) <0.25
The rear converter lens according to claim 1, wherein the following conditional expression is satisfied. When the radius of curvature of the lens surface closest to the image side of the RL1 lens group is Rr1r, and the radius of curvature of the lens surface closest to the object side of the RL2 lens group is Rr2f,
−0.30 <(Rr1r−Rr2f) / (Rr1r + Rr2f) <− 0.07
The rear converter lens according to claim 1, wherein the following conditional expression is satisfied.   6. The rear converter lens according to claim 1, wherein each of the first RL1 lens group and the RL3 lens group is a cemented lens in which a positive lens and a negative lens are cemented. The front group has one or more lens groups, and the distance on the optical axis from the most image side lens surface of the front group to the object side lens surface of the RL1 lens group is dfr, and the rear converter lens When the length on the optical axis is td,
0.08 <dfr / td <0.27
The rear converter lens according to claim 1, wherein the following conditional expression is satisfied.   The front group includes a first FL1 lens group and a second FL2 lens group in order from the object side to the image side. The first FL1 lens group includes a positive lens and a negative lens, and the second FL2 lens group includes a positive lens and a negative lens. The rear converter lens according to any one of claims 1 to 7. When the radius of curvature of the lens surface closest to the image side of the first FL1 lens unit is Fr1r, and the radius of curvature Fr2f of the lens surface closest to the object side of the second FL2 lens unit,
0.03 <(Fr1r−Fr2f) / (Fr1r + Fr2f) <0.25
The rear converter lens according to claim 8, wherein the following conditional expression is satisfied.   An imaging optical system comprising: a main lens system; and the rear converter lens according to claim 1 detachably attached to an image side of the main lens system.   An imaging apparatus comprising: the imaging optical system according to claim 10; and a light receiving unit that receives an image formed by the imaging optical system.