CN111624749A

CN111624749A - Converter lens, interchangeable lens, and image capturing apparatus

Info

Publication number: CN111624749A
Application number: CN202010122238.7A
Authority: CN
Inventors: 江部裕基; 斋藤慎一郎
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2019-02-27
Filing date: 2020-02-27
Publication date: 2020-09-04
Anticipated expiration: 2040-02-27
Also published as: JP7208130B2; JP2020140197A; CN111624749B

Abstract

The invention relates to a converter lens, an interchangeable lens, and an image pickup apparatus. The converter lens according to an exemplary embodiment of the present invention is a converter lens that has a negative refractive power and increases the focal length of the entire system. The converter lens includes a front group having a positive refractive power and a rear group having a negative refractive power, and the front group is a lens unit having a composite refractive power having a maximum positive refractive power in a case where the composite refractive power is obtained by sequentially synthesizing the refractive powers of the respective lenses from a lens closest to the object to the image side. At this time, the focal length of the front group, the focal length of the converter lens, and the lateral magnification of the converter lens when the converter lens is disposed on the image side of the main lens are appropriately determined.

Description

Converter lens, interchangeable lens, and image capturing apparatus

Technical Field

The invention relates to a converter lens, an interchangeable lens, and an image pickup apparatus.

Background

A rear converter lens (hereinafter, referred to as "converter lens") attached between an image pickup device and an interchangeable lens including a main lens so that the focal length of the entire system is increased is known.

WO17/134928 discusses a converter lens constituted by a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power in order from the object side to the image side. The converter lens is configured such that it can be used with a main lens having a relatively short back focal length.

In the case where the back focal length of the main lens is short, the converter lens is disposed at a position where the height of the principal ray of the off-axis light output from the main lens is relatively high. At this time, unless the refractive power of the lens group disposed on the object side in the converter lens and the refractive power of the entire converter lens are appropriately set, the diameter of the lens group disposed on the image side in the converter lens increases and/or it becomes difficult to correct curvature of field and chromatic aberration of magnification.

The present invention aims to provide a converter lens having a small size and high optical performance when the converter lens is disposed on the image side of a main lens.

Disclosure of Invention

According to an aspect of the present invention, a converter lens, which has a negative refractive power as a whole and is disposed on the image side of a main lens, such that the focal length of the entire system becomes longer than that of the main lens alone. The converter lens includes five or more lenses. The five or more lenses are composed of a front group having a positive refractive power and a rear group having a negative refractive power. The front group is a lens unit having a composite refractive power having a maximum positive refractive power in the case where the composite refractive power is obtained by synthesizing the refractive powers of the respective lenses in order from the lens closest to the object to the image side. The following conditional expressions are satisfied:

0.10<f1/(|EXT_f|×β)<0.36

here, f1 is the focal length of the front group, EXT _ f is the focal length of the converter lens, and β is the lateral magnification of the converter lens when the converter lens is disposed on the image side of the main lens.

Other features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Drawings

Fig. 1 is a sectional view illustrating a main lens according to an exemplary embodiment and a converter lens according to a first exemplary embodiment.

Fig. 2 is a sectional view showing a converter lens according to the first exemplary embodiment.

Fig. 3 is an aberration diagram at the time of wireless telephoto object focusing with the converter lens disposed on the image side of the main lens according to the first exemplary embodiment.

Fig. 4 is a sectional view illustrating a converter lens according to a second exemplary embodiment.

Fig. 5 is an aberration diagram at the time of wireless telephoto object focusing with the converter lens disposed on the image side of the main lens according to the second exemplary embodiment.

Fig. 6 is a sectional view illustrating a converter lens according to a third exemplary embodiment.

Fig. 7 is an aberration diagram at the time of wireless telephoto object focusing with the converter lens disposed on the image side of the main lens according to the third exemplary embodiment.

Fig. 8 is a sectional view illustrating a converter lens according to a fourth exemplary embodiment.

Fig. 9 is an aberration diagram at the time of wireless telephoto object focusing with the converter lens disposed on the image side of the main lens according to the fourth exemplary embodiment.

Fig. 10A and 10B each show a configuration of an image capturing system.

Detailed Description

A converter lens and an image photographing apparatus according to exemplary embodiments of the present invention will be described below with reference to the accompanying drawings.

As shown in fig. 1, the converter lens TCL according to the exemplary embodiment of the present invention is disposed on the image side of the main lens ML such as an interchangeable lens, so that the focal length of the image taking optical system OL (entire system) including the main lens ML and the converter lens TCL becomes longer than the focal length of the image taking optical system constituted only by the main lens ML.

The main lens ML is an image taking lens system used in an image taking apparatus such as a digital video camera, a digital camera, a silver-halide film camera, or a Television (TV) camera.

In the sectional views of the main lens ML shown in fig. 1 and the converter lens TCL shown in fig. 2, 4, 6, and 8, the left-hand side is the object side (front), and the right-hand side is the image side (rear). The aperture stop SP determines (limits) the luminous flux (Fno) of the full aperture f-number.

In the case where the image pickup device is a digital video camera or a digital camera, the image plane IP corresponds to an image pickup surface of an image sensor (photoelectric conversion element) such as a Charge Coupled Device (CCD) sensor or a Complementary Metal Oxide Semiconductor (CMOS) sensor. In the case where the image capturing device is a silver halide film camera, the image plane IP corresponds to the film surface.

Fig. 3, 5, 7, and 9 are aberration diagrams illustrating the converter lens TCL according to exemplary embodiments described below. In each spherical aberration diagram, the solid line represents the d-line, and the dashed two-dotted line represents the g-line. In each astigmatism diagram, a broken line M represents the amount of aberration on the meridional image plane, and a solid line S represents the amount of aberration on the sagittal image plane. A distortion (%) diagram (distortion aberration diagram) in each of the aberration diagrams shows a distortion aberration amount with respect to the d-line. A chromatic aberration diagram (chromatic aberration of magnification diagram) in each of the aberration diagrams shows a chromatic aberration amount with respect to the g-line. And, ω is a half angle of view (degree) and is an angle of view obtained by paraxial calculation. Fno denotes f-number.

In the case where the back focal length of the main lens ML is short, the converter lens TCL is disposed at a position where the height of the principal ray of the off-axis rays output from the main lens ML is high. At this time, if the positive refractive power of the object side of the converter lens TCL is weak, the diameter of the lens on the image side of the converter lens TCL increases.

Therefore, according to an exemplary embodiment of the present invention, the converter lens TCL includes a front group FL having a positive refractive power and a rear group RL disposed on the image side of the front group FL. The front group FL herein is a lens unit having a synthetic refractive power having a maximum positive refractive power in a case where the synthetic refractive power is obtained by combining the refractive powers of the lenses in order from the object side to the image side.

Also, the converter lens TCL is configured so that the following conditional expression (1) is satisfied:

0.10<f1/(|EXT_f|×β)<0.36(1)

in conditional expression (1), f1 is the focal length of the front group FL, EXT _ f is the focal length of the converter lens TCL, and β is the lateral magnification of the converter lens TCL when the converter lens TCL is disposed on the image side of the main lens ML.

Conditional expression (1) expresses a desired range of a value obtained by normalizing the ratio of the focal length f1 of the front group FL and the absolute value of the focal length EXT _ f of the converter lens TCL with the lateral magnification β of the converter lens TCL when the converter lens TCL is disposed on the image side of the main lens ML (hereinafter, the lateral magnification β of the converter lens TCL when the converter lens TCL is disposed on the image side of the main lens ML will be referred to as "the lateral magnification of the converter lens TCL"). A desired value of the ratio of the focal length f1 of the front group FL to the absolute value of the focal length EXT _ f of the converter lens TCL is changed according to the lateral magnification β of the converter lens TCL, so that in conditional expression (1), normalization is performed to eliminate the dependency on the lateral magnification β of the converter lens TCL. For similar reasons, the normalization of the lateral magnification β by the converter lens TCL is also performed in conditional expressions (2), (3), and (4) described below.

Conditional expression (1) indicates that the absolute value of the focal length EXT _ f of the converter lens TCL is relatively larger than the focal length f1 of the front group FL. This indicates that the refractive power of the front group FL is increased to bring the principal point closer to the object, so that the refractive power of the entire system of the converter lens TCL is reduced compared with the refractive power of the front group FL.

In this way, the positive refractive power is set stronger on the object side than on the image side of the converter lens TCL, so that the angle of the principal ray of the off-axis ray incident on the converter lens TCL is closer to the direction parallel to the optical axis (toward the telephoto direction). This controls the increase of the maximum diameter of the converter lens TCL. The angle of the principal ray of the off-axis rays is set in such a way as to allow the curvature of the positive lens contained in the converter lens TCL to be increased. This makes it easy to reduce the refractive index of the material of the positive lens at the d-line and to make the Petzval sum, which tends to have a large negative component, closer to zero by increasing the positive component of the Petzval sum (Petzval sum) of the converter lens TCL. In this way, the curvature of field is corrected.

The negative refractive power of the entire system of the converter lens TCL is prevented from becoming excessively strong, thereby preventing the refractive power of each lens disposed on the image side of the converter lens TCL from becoming excessively strong. In this way, field curvature, astigmatism, and chromatic aberration of magnification are successfully corrected.

If the focal length f1 of the front group FL is reduced to such an extent that the value of f1/(| EXT _ f | ×. β) is lower than the lower limit value of the conditional expression (1) and the refractive power of the front group FL increases, it becomes difficult to correct the on-axis chromatic aberration and coma. Therefore, it is not desirable that the value of f1/(| EXT _ f | ×) be lower than the lower limit value of conditional expression (1). If the refractive power of the front group FL decreases to the extent that the focal length f1 of the front group FL increases to the extent that the value of f1/(| EXT _ f | ×. β) exceeds the upper limit value of the conditional expression (1), it becomes difficult to decrease the absolute value of the petzval sum of the converter lens TCL having a negative focal length. Therefore, field curvature and chromatic aberration of magnification increase. Therefore, it is not desirable that the value of f1/(| EXT _ f | ×) exceeds the upper limit value of conditional expression (1).

As described above, according to the exemplary embodiments of the present invention, the converter lens TCL having a small size and having high optical performance is obtained. The converter lens TCL according to the exemplary embodiment of the present invention is particularly suitable for use in a converter device provided between a mirror-less camera and an interchangeable lens that is attachable to the mirror-less camera and has a relatively short back focal length.

It is desirable that the numerical range of conditional formula (1) is as follows:

0.11<f1/(|EXT_f|×β)<0.34(1a)。

more desirably, the numerical range of conditional formula (1) is as follows:

0.12<f1/(|EXT_f|×β)<0.32(1b)。

also, it is desirable that the converter lens TCL includes five or more lenses. For compatibility with a lens having a short back focal length, the converter lens TCL is disposed at a position where the height of the principal ray of the off-axis light ray output from the main lens is relatively high. Therefore, in order to successfully correct chromatic aberration of magnification and curvature of field, it is desirable that the number of lenses be five or more.

Also, it is desirable that the rear group RL of the converter lens TCL according to the exemplary embodiment of the present invention includes the first subgroup a having a positive refractive power and the second subgroup B having a positive refractive power. The second subgroup B here is composed of a single positive lens or cemented lens (first cemented lens) having a positive refractive power and disposed at a position closest to the image plane in the rear group RL. A cemented lens (or a single positive lens) having a positive refractive power is disposed at a position closest to the image plane in the rear group, i.e., a position at which the height of the light ray on the axis is low, so that field curvature is corrected without increasing other aberration components.

Also, it is desirable that the converter lens TCL satisfies one or more of the following conditional expressions (2) to (7):

0.03<fa/(|EXT_f|×β)<0.15(2)，

0.10<fb/(|EXT_f|×β)<0.35(3)，

1.00<L/(sk×β)<4.00(4)，

0.10<d/sk<0.60(5)，

-0.20< | EXT _ f | × [ 1/Rn × (1/N' -1/N) } <0.20(6), and

-3.0<(R2+R1)/(R2-R1)<-0.1(7)

in conditional expressions (2) to (7), fa is the focal length of the first sub-group a, and fb is the focal length of the second sub-group B. Further, L is a distance on the optical axis from the lens surface closest to the object to the lens surface closest to the image plane in the converter lens TCL, and sk is a distance on the optical axis from the lens surface closest to the image plane in the converter lens TCL to the image plane when the converter lens TCL is disposed on the image side of the main lens ML. Further, d is a distance on the optical axis from the lens surface closest to the image plane in the front group FL to the lens surface closest to the object in the rear group RL. Rn is a radius of curvature of the nth lens surface of the converter lens TCL from the object, N' is a refractive index of the medium on the light exit side of the nth lens surface, and N is a refractive index of the medium on the light incident side of the nth lens surface. Further, R1 is a radius of curvature of the object-side surface of the lens closest to the image plane in the converter lens TCL. Further, R2 is a radius of curvature of the image side surface of the lens closest to the image plane in the converter lens TCL.

Conditional expression (2) expresses a desired range of values obtained by normalizing the ratio of the focal length fa of the first sub-group a to the absolute value of the focal length EXT _ f of the converter lens TCL with the lateral magnification β of the converter lens TCL. If the focal length fa of the first sub-group a becomes shorter than the absolute value of the focal length EXT _ f of the converter lens TCL to such an extent that the value of fa/(| EXT _ f | ×. β) is lower than the lower limit value of the conditional expression (2), and the refractive power of the first sub-group a increases, the on-axis chromatic aberration, the spherical aberration, and the coma aberration increase. Therefore, it is not desirable that the value of fa/(| EXT _ f | ×. β) be lower than the lower limit value. On the other hand, if the absolute value of the focal length EXT _ f of the converter lens TCL is reduced to such an extent that the value of fa/(| EXT _ f | ×. β) exceeds the upper limit value of the conditional expression (2), the negative component of the petzval sum increases and the field curvature increases. Therefore, it is not desirable that the value of fa/(| EXT _ f | ×. β) exceeds the upper limit value of conditional expression (2).

Conditional expression (3) expresses a desired range of values obtained by normalizing the ratio of the focal length fb of the second sub-group B to the absolute value of the focal length EXT _ f of the converter lens TCL with the lateral magnification β of the converter lens TCL. The chromatic aberration of magnification particularly increases if the focal length fb of the second sub-group B becomes shorter than the absolute value of the focal length EXT _ f of the converter lens TCL to such an extent that the value of fb/(| EXT _ f | ×) is lower than the lower limit value of the conditional expression (3), and the refractive power of the second sub-group B increases. Therefore, it is not desirable that the value of fb/(| EXT _ f | ×. β) be lower than the lower limit value of conditional expression (3). On the other hand, if the focal length fb of the second sub-group B is increased to such an extent that the value of fb/(| EXT _ f | ×. β) exceeds the upper limit value of the conditional expression (3), and the refractive power of the second sub-group B is decreased, it becomes difficult to correct field curvature. Therefore, it is not desirable that the value of fb/(| EXT _ f | ×. β) exceeds the upper limit value of conditional expression (3).

The conditional expression (4) expresses a desired range of values obtained by normalizing the ratio of the distance L from the lens surface closest to the object to the lens surface closest to the image plane in the converter lens TCL and the distance sk on the optical axis from the lens surface closest to the image plane in the converter lens TCL with the lateral magnification β of the converter lens TCL. If the distance L is reduced to such an extent that the value of L/(sk × β) is lower than the lower limit value of conditional expression (4), it becomes difficult to increase the thickness of the positive lens due to space. Therefore, it also becomes difficult to increase the curvature of the positive lens. Therefore, field curvature and chromatic aberration of magnification increase. Therefore, it is not desirable that the value of L/(sk × β) be lower than the lower limit of the conditional expression. On the other hand, if the distance L increases to such an extent that the value of L/(sk × β) exceeds the upper limit value of the conditional expression (4), the length of the converter lens TCL increases and the size increases. Therefore, it is not desirable that the value of L/(sk × β) exceeds the upper limit value of conditional expression (4).

Conditional expression (5) defines a ratio of a distance d on the optical axis from the lens surface closest to the image plane in the front group FL to the lens surface closest to the object in the rear group RL to a distance sk on the optical axis from the lens surface closest to the image plane in the converter lens TCL to the image plane when the converter lens TCL is disposed on the image side of the main lens ML. In other words, the distance d represents the interval on the optical axis between the front group FL and the rear group RL. If the interval on the optical axis between the front group FL and the rear group RL is reduced to such an extent that the value of d/sk is lower than the lower limit value of conditional expression (5), the rear group RL needs to be disposed at a position where the height of the principal ray of the off-axis ray refracted on the front group FL is high, resulting in an increase in the diameter of the rear group RL. Therefore, it is not desirable that the value of d/sk is lower than the lower limit of conditional expression (5). On the other hand, if the interval on the optical axis between the front group FL and the rear group RL is increased to such an extent that the value of d/sk exceeds the upper limit value of the conditional expression (5), the negative lens of the rear group RL needs to be disposed at a position where the height of the marginal ray of the on-axis ray is low. At this time, the refractive power of the negative lens needs to be increased to form an image at a predetermined position, but if the refractive power of the negative lens is increased, the negative component of the petzval sum is increased and curvature of field is increased. Therefore, it is not desirable that the value of d/sk exceeds the lower limit of conditional expression (5).

The conditional expression (6) expresses a desired range of values obtained by multiplying the focal length EXT _ f of the converter lens TCL by the petzval sum. If the petzval sum increases in the negative direction to such an extent that the value of | EXT _ f | × {1/Rn × (1/N' -1/N) } is lower than the lower limit value of conditional expression (6), the refractive index of the material of the positive lens decreases and the Abbe number increases, while the refractive index of the material of the negative lens increases and the Abbe number decreases. Therefore, the on-axis chromatic aberration and the chromatic aberration of magnification increase. Therefore, it is not desirable that the value of | EXT _ f | × [ 1/Rn × (1/N' -1/N) } is lower than the lower limit value of conditional expression (6). On the other hand, if the value of | EXT _ f | × {1/Rn × (1/N' -1/N) } exceeds the upper limit value of conditional expression (6), the field curvature increases. Therefore, it is not desirable that the value of | EXT _ f | × [ 1/Rn × (1/N' -1/N) } exceeds the upper limit value of conditional expression (6).

Conditional expression (7) expresses a desired range of the shape factor of the lens closest to the image plane in the converter lens TCL. The astigmatism increases if the absolute value of the curvature of the lens of the image-side surface decreases to the extent that the value of (R2+ R1)/(R2-R1) is lower than the lower limit value of the conditional expression (7). Therefore, it is not desirable that the value of (R2+ R1)/(R2-R1) is lower than the lower limit of conditional formula (7). On the other hand, if the absolute value of the curvature of the lens of the image-side surface is increased to the extent that the value of (R2+ R1)/(R2-R1) exceeds the upper limit value of conditional expression (7), the lens size needs to be increased to maintain the edge thickness. Therefore, it is not desirable that the value of (R2+ R1)/(R2-R1) exceeds the upper limit of conditional formula (7).

The numerical ranges of conditional expressions (2) to (7) are desirably as follows:

0.04<fa/(|EXT_f|×β)<0.12(2a)，

0.11<fb/(|EXT_f|×β)<0.32(3a)，

1.20<L/(sk×β)<3.50(4a)，

0.11<d/sk<0.50(5a)，

-0.18< | EXT _ f | × [ 1/Rn × (1/N' -1/N) } <0.15(6a), and

-2.5<(R2+R1)/(R2-R1)<-0.15(7a)。

more preferably, the numerical ranges of conditional expressions (2) to (7) are as follows:

0.05<fa/(|EXT_f|×β)<0.10(2b)，

0.12<fb/(|EXT_f|×β)<0.30(3b)，

1.50<L/(sk×β)<3.10(4b)，

0.13<d/sk<0.47(5b)，

-0.16< | EXT _ f | × [ 1/Rn × (1/N' -1/N) } <0.10(6b), and

-2.1<(R2+R1)/(R2-R1)<-0.2(7b)。

satisfying at least one of the above conditional expressions makes it possible to reduce the size of the converter lens TCL and achieve high optical performance by appropriately correcting various aberrations such as field curvature and astigmatism.

It is desirable that the first sub-group a includes a cemented lens (second cemented lens). As an alternative to a large number of individual lenses, cemented lenses are provided in the first subgroup a to thereby reduce the boundary surface with air, so that unnecessary light generated due to reflection on the boundary surface is reduced.

Further, it is desirable that at least one negative lens of the cemented lenses included in the first sub-group a has a refractive index of 1.80 or more. It is desirable that the refractive index of the at least one negative lens is 1.85 or higher, more desirably 1.90 or higher. The use of the negative lens having a high refractive index in the first subgroup a controls the increase in curvature of the negative lens included in the first subgroup a. This makes it easy to correct curvature of field without increasing other aberration components.

It is desirable that the front group FL be composed of two or less lenses. The number of lenses of the front group FL including lenses having a high refractive power and tending to have a large curvature is reduced to thereby reduce the boundary surface between the lenses and the air, so that the generation of unnecessary light is controlled.

It is desirable that the number of lenses of the rear group RL is larger than that of the lenses of the front group FL. Reducing the refractive power of the lens disposed at a position where the height of the principal ray of the off-axis ray is high makes it easy to appropriately correct curvature of field and astigmatism.

The main lens ML according to an exemplary embodiment and the converter lens TCL according to an exemplary embodiment will be described below.

[ Main lens ]

In this specification, the composition of the main shot ML is common to the first to fourth exemplary embodiments of the converter shot TCL.

Fig. 1 is a sectional view showing the main lens ML when an infinite object is in focus. The main lens ML has an f-number of 2.90, a half angle of view of 3.16 degrees, and a back focus of 39 mm. The main lens ML described as an example is only an example, and may be any other optical system capable of forming an image on an image plane.

[ converter lens ]

The converter lens TCL according to the first to fourth exemplary embodiments will be described below.

A first exemplary embodiment of the present invention will be described below. Fig. 2 is a sectional view showing a converter lens TCL according to the first exemplary embodiment. Fig. 3 is an aberration diagram in focusing of an infinite object in a case where the converter lens TCL according to the first exemplary embodiment is disposed on the image side of the main lens ML. At this time, the lateral magnification was 1.400.

In the TCL according to the first exemplary embodiment, the front group FL is constituted by a positive lens disposed closest to the object in the converter lens TCL. Further, the rear group RL is constituted by six lenses disposed on the image side of the front group FL. In the rear group RL, the first sub-group a is constituted by a cemented lens (second cemented lens) formed by cementing three lenses, which are a negative lens, a positive lens, and a negative lens, together, and a cemented lens (second cemented lens) disposed on the image side of the cemented lens and formed by cementing the positive lens and the negative lens together. The second subgroup B is constituted by a single positive lens.

Satisfying the above conditional expressions (1) to (7) realizes a converter lens TCL having a small size and high optical performance. The cemented lens of the first subgroup a reduces unnecessary light due to surface reflection while correcting fluctuations in on-axis chromatic aberration and coma for each wavelength. The second subgroup B corrects for field curvature.

A second exemplary embodiment of the present invention will be described below. Fig. 4 is a sectional view illustrating the converter lens TCL according to the present exemplary embodiment. Fig. 5 is an aberration diagram in focusing of an infinite object in a case where the converter lens TCL according to the second exemplary embodiment is disposed on the image side of the main lens ML. At this time, the lateral magnification was 1.400.

In the converter lens TCL according to the second exemplary embodiment, the front group FL is constituted by the cemented lens disposed closest to the object in the converter lens TCL. The cemented lens is composed of a negative lens and a positive lens disposed on the image side of the negative lens and adjacent to the negative lens. Further, the rear group RL is constituted by five lenses disposed closer to the image than the front group FL.

In the rear group RL, the first sub-group a is constituted by a cemented lens (second cemented lens) disposed closest to the object in the rear group RL and a negative lens disposed on the image side of the cemented lens and adjacent to the cemented lens. The second sub-group B is constituted by a cemented lens (first cemented lens) formed by cementing together a negative lens and a positive lens.

Satisfying the above conditional expressions (1) to (7) realizes a converter lens TCL having a small size and high optical performance. The cemented lens in the first subgroup a reduces unnecessary light due to surface reflection while correcting fluctuations in axial chromatic aberration and coma for each wavelength. The positive lenses of the cemented lenses of the second subgroup B correct curvature of field, and the negative lenses cemented with the positive lenses correct chromatic aberration of magnification caused by the positive lenses.

A third exemplary embodiment of the present invention will be described below. Fig. 6 is a sectional view showing a converter lens TCL according to a third exemplary embodiment. Fig. 7 is an aberration diagram in focusing of an infinite object in a case where the converter lens TCL according to the present exemplary embodiment is disposed on the image side of the main lens ML. In this case, the lateral magnification was 2.000.

In the converter lens TCL according to the third exemplary embodiment, the front group FL is constituted by the positive lens disposed closest to the object in the converter lens TCL. The rear group RL is constituted by eight lenses disposed on the image side of the front group FL.

In the rear group RL, the first sub-group a is constituted by a cemented lens (second cemented lens) formed by cementing together three lenses that are a negative lens, a positive lens, and a negative lens, a single positive lens, and a cemented lens (second cemented lens) formed by cementing together three lenses that are a negative lens, a positive lens, and a negative lens. The second subgroup B is constituted by a single positive lens.

Satisfying the above conditional expressions (1) to (7) realizes a converter lens TCL having a small size and high optical performance.

The cemented lens on the object side of the first subgroup a reduces unnecessary light due to surface reflection while correcting fluctuations of axial chromatic aberration and coma for each wavelength. The positive lens between the two cemented lenses of the first subgroup a corrects particularly the on-axis chromatic aberration. The cemented lens on the image side in the first sub-group a reduces unnecessary light due to surface reflection while correcting specifically chromatic aberration of magnification. The positive lenses of the second subgroup B correct the curvature of field.

A fourth exemplary embodiment of the present invention will be described below. Fig. 8 is a sectional view illustrating the converter lens TCL according to the present exemplary embodiment. Fig. 9 is an aberration diagram in focusing of an infinite object in a case where the converter lens TCL according to the present exemplary embodiment is disposed on the image side of the main lens ML. In this case, the lateral magnification was 1.998.

In the converter lens TCL according to the fourth exemplary embodiment, the front group FL is constituted by the positive lens disposed closest to the object in the converter lens TCL. The rear group RL is constituted by eight lenses disposed on the image side of the front group FL.

In the rear group RL, the first sub-group a is constituted by a cemented lens (second cemented lens) formed by cementing three lenses as a negative lens, a positive lens, and a negative lens together, a cemented lens (second cemented lens) formed by cementing a positive lens and a negative lens together, and a single positive lens. The second sub-group B is constituted by a cemented lens (first cemented lens) formed by cementing together a negative lens and a positive lens.

Satisfying the above conditional expressions (1) to (7) realizes a converter lens TCL having a small size and high optical performance. The cemented lens on the object side of the first subgroup a reduces unnecessary light due to surface reflection while correcting fluctuations of axial chromatic aberration and coma for each wavelength. The cemented lens on the image side in the first sub-group a reduces unnecessary light due to surface reflection while correcting specifically chromatic aberration of magnification. The positive lens disposed on the image side of the cemented lens and adjacent to the cemented lens corrects field curvature.

The positive lenses of the cemented lenses in the second subgroup B correct curvature of field, while chromatic aberration of magnification caused by the positive lenses are corrected by the negative lenses cemented with the positive lenses.

[ numerical example embodiments ]

Numerical exemplary embodiments corresponding to the main lens ML and first to fourth numerical exemplary embodiments corresponding to the converter lens TCL according to the first to fourth exemplary embodiments will be described below.

In each numerical exemplary embodiment, the surface numbers indicate the order of the optical surfaces from the object side. And r represents a radius of curvature (mm) of each optical surface, d at a surface number i represents a space (mm) between the i-th optical surface and the (i +1) -th optical surface, nd is a refractive index of a material of the optical member at a d-line, and ν d is an Abbe number of the material of the optical member under the d-line as a reference. The Abbe number ν d of the material is expressed by the following formula:

νd＝(Nd-1)/(NF-NC)

here, Nd, NF, and NC are refractive indices of Frenshoff (Fraunhofer) d line (587.56nm), F line (486.13nm), and C line (656.27nm), respectively. The effective diameter is the diameter on the lens determined by the off-axis light passing range.

BF denotes the back focus. The back focal length of the main lens ML in the numerical exemplary embodiment is expressed by an air-reduced length of the distance on the optical axis from the surface closest to the image plane to the paraxial image plane. The back focal length of the converter lens TCL in the numerical exemplary embodiment is expressed by an air-converted length from a surface closest to the image plane in the converter lens TCL to a distance on the optical axis of the paraxial image plane when the converter lens TCL is disposed on the image side of the main lens ML.

The total lens length of the main lens ML in the numerical exemplary embodiment is the sum of the back focal length and the distance on the optical axis from the surface closest to the object (first lens surface) to the surface closest to the image plane (last lens surface) in the main lens ML.

The lens interval between the main lens ML and the converter lens TCL is a distance on the optical axis from a surface closest to the image plane in the main lens ML to a surface closest to the object in the converter lens TCL. The interval between the main lens ML and the converter lens TCL is specified by an air conversion length.

The front-side principal point position is a distance from a surface closest to the object to the front-side principal point, and the rear-side principal point position is a distance from a surface closest to the image plane to the rear-side principal point. Each numerical value of the front principal point position and the rear principal point position is a paraxial amount, and a direction from the object side to the image side is regarded as positive in sign.

Table 1 shows physical quantities used in the above-described conditional expressions in the first to fourth numerical example embodiments, and table 2 shows values corresponding to the conditional expressions.

[ Main lens ] -the unit common to the converter lenses according to the first to fourth exemplary embodiments is mm

Surface data

Various types of data

Single lens data

[ converter lens ]

[ first numerical example embodiment ]

Unit: mm is

Surface data

Various types of data

Single lens data

Interval between the main lens and the converter lens according to the first numerical exemplary embodiment: 6.00

Second numerical value exemplary embodiment

Unit: mm is

Surface data

Various types of data

Single lens data

Interval between the main lens and the converter lens according to the second numerical exemplary embodiment: 6.00

Third numerical example embodiment

Unit: mm

Surface data

Various types of data

Single lens data

Interval between main lens and converter lens according to the third numerical exemplary embodiment 6.00

[ fourth numerical example embodiment ]

Unit: mm

Surface data

Various types of data

Single lens data

Interval between main lens and converter lens according to the fourth numerical exemplary embodiment 6.00

[ Table 1]

[ Table 2]

[ image photographing device according to exemplary embodiment ]

Fig. 10A and 10B each show a configuration of the image capturing apparatus (digital camera) 10. Fig. 10A is a perspective view, and fig. 10B is a side view. The image capturing apparatus 10 includes a camera body 13, a main lens ML, a converter lens TCL according to any one of the first to fourth exemplary embodiments described above, and a light receiving element (image sensor) 12 configured to photoelectrically convert an image formed by the main lens ML and the converter lens TCL. An image sensor such as a CCD sensor and a CMOS sensor may be used as the light receiving element 12. The main lens ML and the converter lens TCL may be integrated with the camera body 13, or may be arranged to be attachable to and detachable from the camera body 13, respectively. In the case where the main lens ML and the converter lens TCL are integrated with the camera body 13, the converter lens TCL is arranged on the optical axis in an insertable and removable manner.

Interchangeable lens according to exemplary embodiment

The present invention is applicable to an interchangeable lens that includes a main lens ML and a converter lens TCL in the same lens barrel and that can be attached to and detached from an image capturing apparatus. The main lens ML may be a fixed focus lens or a zoom lens. In this case, the converter lens TCL is arranged on the optical axis in an insertable and removable manner. The converter lens TCL is disposed on the optical axis or off the optical axis through an operation part or a user interface based on a user instruction.

Although various exemplary embodiments of the present invention have been described, the present invention is not limited to the exemplary embodiments and examples, and various combinations, modifications, and changes are possible within the spirit of the present invention.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims

1. A converter lens having a negative refractive power as a whole and disposed on an image side of a main lens such that a focal length of an entire system becomes longer than a focal length of the main lens alone, the converter lens including five or more lenses,

wherein the five or more lenses are composed of a front group having a positive refractive power and a rear group having a negative refractive power,

wherein the front group is a lens unit having a composite refractive power having a maximum positive refractive power in a case where the composite refractive power is obtained by sequentially synthesizing the refractive powers of the respective lenses from a lens closest to the object to the image side, and

wherein the following conditional expressions are satisfied:

0.10<f1/(|EXT_f|×β)<0.36

2. The converter lens according to claim 1,

wherein the rear group comprises a first subgroup and a second subgroup, an

Wherein the second sub-group is constituted by a first cemented lens closest to the image plane in the rear group including a positive lens and a negative lens and having a positive refractive power as a whole, or a positive lens closest to the image plane in the rear group.

3. The converter lens according to claim 2, wherein the following conditional expression is satisfied:

0.03<fa/(|EXT_f|×β)<0.15

here, fa is the focal length of the first sub-group.

4. The converter lens of claim 2 wherein the first subgroup includes a second cemented lens comprising a positive lens and a negative lens.

5. The converter lens according to claim 4, wherein at least one negative lens of the second cemented lens has a refractive index of 1.80 or more with respect to d-line.

6. The converter lens according to claim 2, wherein the following conditional expression is satisfied:

0.10<fb/(|EXT_f|×β)<0.35

here fb is the focal length of the second subset.

7. The converter lens according to claim 1, wherein the following conditional expression is satisfied:

1.00<L/(sk×β)<4.00

here, L is a distance on the optical axis from a lens surface closest to the object in the converter lens to a lens surface closest to the image plane in the converter lens, and sk is a distance on the optical axis from the lens surface closest to the image plane in the converter lens to the image plane when the converter lens is disposed on the image side of the main lens.

8. The converter lens according to claim 1, wherein the following conditional expression is satisfied:

0.10<d/sk<0.60

here, d is a distance on the optical axis from the lens surface closest to the image plane in the front group to the lens surface closest to the object in the rear group, and sk is a distance on the optical axis from the lens surface closest to the image plane in the converter lens to the image plane when the converter lens is disposed on the image side of the main lens.

9. The converter lens according to claim 1, wherein the following conditional expression is satisfied:

-0.20<|EXT_f|×Σ{(1/N’-1/N)/Rn}<0.20

here, Rn is a radius of curvature of an N-th lens surface of the converter lens from the object, N' is a refractive index of a medium on a light emitting side of the N-th lens surface, and N is a refractive index of a medium on a light incident side of the N-th lens surface.

10. The converter lens according to claim 1, wherein the following conditional expression is satisfied:

-3.0<(R2+R1)/(R2-R1)<-0.1

here, R1 is a radius of curvature of an object-side surface of a lens closest to an image plane in the converter lens, and R2 is a radius of curvature of an image-side surface of a lens closest to the image plane in the converter lens.

11. The converter lens of claim 1 wherein the front group is comprised of two or fewer lenses.

12. An inverter lens according to any of claims 1 to 11 wherein the number of lenses of the rear group is greater than the number of lenses of the front group.

13. An interchangeable lens including a main lens and a converter lens which has a negative refractive power as a whole and is disposed in an optical path such that a focal length of the entire system becomes longer than that of the main lens alone,

wherein the converter lens includes five or more lenses,

wherein the following conditional expressions are satisfied:

0.10<f1/(|EXT_f|×β)<0.36

14. An image pickup apparatus includes a main lens, a converter lens and an image sensor, the converter lens having a negative refractive power as a whole and being in an optical path such that a focal length of the entire system becomes longer than that of the main lens alone,

wherein the converter lens includes five or more lenses,

wherein the front group is a lens unit having a composite refractive power having a maximum positive refractive power in a case where the composite refractive power is obtained by sequentially synthesizing the refractive powers of the respective lenses from a lens closest to the object to the image side, an

Wherein the following conditional expressions are satisfied:

0.10<f1/(|EXT_f|×β)<0.36