JP5093657B2 - Retrofocus lens, image pickup apparatus, and focusing method of retrofocus lens - Google Patents

Description

  The present invention relates to a retrofocus lens suitable for a single-lens reflex camera, a digital camera, or the like.

In general, focusing of the photographing lens is performed by extending the entire lens system toward the object side. However, even with a retrofocus type wide-angle lens, when focusing is performed with this method, since there are large variations in spherical aberration, astigmatism, etc. when focusing on a short distance object, good imaging performance is obtained. There was a problem that I could not. In view of this, a retrofocus lens that employs a floating system has been proposed in order to satisfactorily suppress fluctuations in aberrations when focusing on a short-distance object (see, for example, Patent Document 1). In recent years, for such retrofocus lenses, not only aberration performance but also ghost and flare, which are one of the factors that impair optical performance, are becoming more severe, and are therefore applied to the lens surface. Higher performance is also required for the antireflection film, and multilayer film design technology and multilayer film formation technology continue to advance to meet the demand (see, for example, Patent Document 2).
JP 2005-181852 A JP 2000-356704 A

  For example, a lens disclosed in Japanese Patent Application Laid-Open No. 2005-181852 includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a positive refraction arranged in order from the object side. The first lens unit is fixed and the distance between the first lens unit and the second lens unit increases, and the distance between the second lens unit and the third lens unit increases. The interval is configured to be reduced. However, in such a retrofocus lens, the second lens group has a positive refractive power, so that there is a problem in that a large amount of distortion occurs during focusing.

  In addition, since the first lens group is fixed at the time of focusing as described above, there is a problem in that it is not possible to sufficiently perform aberration correction at the time of focusing on a short-distance object. In addition, there is a problem in that reflected light that becomes ghost or flare is likely to be generated from the optical surfaces of the front group and rear group in such a retrofocus lens.

  The present invention has been made in view of such problems, and a retrofocus lens and an imaging apparatus in which aberration fluctuation is small even when focusing on a short-distance object with a high imaging magnification, and ghost and flare are further reduced. And a method of focusing a retrofocus lens.

In order to achieve such an object, retrofocus lenses are arranged in order from the object side along the optical axis, a first lens group having a negative refractive power, a second lens group having a negative refractive power, and a positive lens group. A third lens group having a refractive power of 2 mm, and when focusing from an object at infinity to an object at a short distance, the distance between the first lens group and the second lens group increases and the second lens group The first lens group, the second lens group, and the third lens group are configured to move toward the object side along the optical axis so that the distance between the first lens group and the third lens group decreases. An antireflection film is provided on at least one of the optical surfaces in the first lens group or the third lens group , and the antireflection film is configured to include at least one layer having a refractive index of 1.30 or less. The

In the retrofocus lens according to the first invention, when the shooting magnification when focusing on the closest object is βm,
(−βm)> 0.25
Satisfy the conditions.

In the retrofocus lens according to the second aspect of the present invention, the imaging magnification when focusing on a short distance object is β, and the first lens group and the second lens group when focusing on an object at infinity When the air distance between the first lens group and the second lens group when the air distance between the first lens group and the second lens group is in focus when the distance from the object at infinity is in focus is set to ΔD1,
−30 <ΔD1 / (β × D1) <− 7
Satisfy the conditions of
The photographing magnification when the object is focused on the short distance object is the photographing magnification when the object is focused on the closest object.

In the retrofocus lens according to a third aspect of the invention, the first lens group is composed of a negative meniscus lens arranged in order from the object side along the optical axis and having a convex surface facing the object side, and a positive lens. The

In the first or third aspect of the invention, the imaging magnification when focusing on a short distance object is β, and the air space between the first lens group and the second lens group when focusing on an object at infinity Is D1, and when the amount of change in the air gap between the first lens group and the second lens group when focusing from an infinite object to a close object is ΔD1,
−30 <ΔD1 / (β × D1) <− 7
Satisfy the conditions of
It is preferable that the photographing magnification when focusing on the short distance object is the photographing magnification when focusing on the closest object .

In the second or third invention, when the shooting magnification when focusing on the closest object is βm,
(−βm)> 0.25
It is preferable to satisfy the following conditions .

In the first or second invention, it is preferable that the first lens group includes a negative meniscus lens having a convex surface facing the object side, which is arranged in order from the object side along the optical axis, and a positive lens. .

In the third invention, the radius of curvature of the object side lens surface of the positive lens in the first lens group is r1, and the radius of curvature of the image side lens surface of the positive lens in the first lens group is r2. When
0.5 <(r2 + r1) / (r2-r1) <3.0
It is preferable to satisfy the following conditions .

In each of the above-described inventions, the antireflection film is a multilayer film, and the layer having a refractive index of 1.30 or less is preferably the most surface layer among the layers constituting the multilayer film. .

In each of the above inventions, it is preferable that an aperture stop is disposed in the third lens group .

Moreover, in the above-mentioned invention, it is preferable that the optical surface is a concave surface with respect to the aperture stop .

In each of the above-described inventions, the amount of change in the air gap between the first lens group and the second lens group when focusing from an infinite object to a close object is ΔD1, When the amount of change in the air gap between the second lens group and the third lens group when focusing on a distance object is ΔD2,
ΔD1 = −ΔD2
It is preferable to satisfy the following conditions .

In each of the above-described inventions, when the focal length of the first lens group is f1, and the focal length of the second lens group is f2,
0.1 <f1 / f2 <10
It is preferable to satisfy the following conditions.

  In each of the above-described inventions, it is preferable that each lens surface of the retrofocus lens is formed as a spherical surface or a flat surface.

  Further, an imaging apparatus according to the present invention is an imaging apparatus including a retrofocus lens that forms an image of an object on a predetermined surface, wherein the retrofocus lens is a retrofocus lens according to each invention. To do.

The focusing method according to the present invention includes a first lens group having a negative refractive power, a second lens group having a negative refractive power, and a positive refraction arranged in order from the object side along the optical axis. In a focusing method of a retrofocus lens configured with a third lens group having power, when focusing from an object at infinity to an object at a short distance, the first lens group and the second lens group The first lens group, the second lens group, and the third lens group are moved along the optical axis so that the distance between the second lens group and the third lens group decreases as the distance increases. And an antireflection film is provided on at least one of the optical surfaces of the first lens group or the third lens group , and the antireflection film has a refractive index of 1.30 or less. It was constructed to include at least one layer, closest range object When was βm shooting magnification upon focusing, the following equation
(−βm)> 0.25
To meet the requirements of

  According to the present invention, it is possible to reduce fluctuations in aberrations even when focusing on a short-distance object with a high photographing magnification, and it is possible to further reduce ghosts and flares.

  Hereinafter, preferred embodiments of the present application will be described with reference to the drawings. A single-lens reflex camera CAM provided with a retrofocus lens according to the present application is shown in FIG. This single-lens reflex camera CAM includes a retrofocus lens RL, a quick return mirror M, an imaging device CCD for photographing, a focusing screen F, a pentaprism P, and an eyepiece lens EL. The quick return mirror M, the image sensor CCD, the focusing screen F, the pentaprism P, and the eyepiece lens EL are built in the camera body B, and the retrofocus lens RL is detachably attached to the camera body B.

  The retrofocus lens RL forms an image of a subject (object) (not shown) on the image sensor CCD or the focusing screen F. The quick return mirror M is inserted at an angle of 45 degrees with respect to the optical axis that passes through the retrofocus lens RL, and reflects light from the subject that has passed through the retrofocus lens RL during normal times (in a shooting standby state). Then, an image is formed on the focusing screen F, and when the shutter is released, the mirror is raised and jumps up so that light from the subject passing through the retrofocus lens RL forms an image on the image sensor CCD. That is, the image sensor CCD and the focusing screen F are disposed at optically conjugate positions.

  The pentaprism P is an upright image obtained by inverting the subject image (inverted image) on the focusing screen F formed by the retrofocus lens RL vertically and horizontally, and the eyepiece EL is an object that is an upright image by the pentaprism P. An image is formed on an eye point (not shown). Thereby, the object image formed on the focusing screen F by the retrofocus lens RL can be observed by the eyepiece lens EL.

  In such a single-lens reflex camera CAM, when a release button (not shown) is pressed by the photographer, the quick return mirror M is in a mirror-up state and retracts out of the optical path, and light from the subject passes through the retrofocus lens RL. To the image sensor CCD. Thereby, the light from the subject is picked up by the image pickup device CCD and recorded as a subject image in a memory (not shown). In this way, the photographer can shoot a subject with the single-lens reflex camera CAM.

  By the way, as shown in FIG. 1, for example, the retrofocus lens RL includes a first lens group G1 having a negative refractive power and a second lens having a negative refractive power, which are arranged in order from the object side along the optical axis. It is composed of a group G2 and a third lens group G3 having a positive refractive power. When focusing from an object at infinity to a near object, the distance between the first lens group G1 and the second lens group G2 increases and the distance between the second lens group G2 and the third lens group G3 decreases. As described above, the first lens group G1, the second lens group G2, and the third lens group G3 move to the object side along the optical axis. This makes it possible to simultaneously correct coma, astigmatism and distortion that occur during focusing.

  Furthermore, an antireflection film is provided on at least one of the optical surfaces in the first lens group G1 or the third lens group G3, and the antireflection film is a layer formed using a wet process (details will be described later). It is configured to include at least one layer. Thereby, it becomes possible to further reduce ghost and flare. In this way, it is possible to provide a retrofocus lens and a focus method of a retrofocus lens that have less aberration fluctuations when focusing on a short-distance object with a high shooting magnification and reduce ghosts and flares. become.

  When the antireflection film is a multilayer film, the layer formed by using the wet process is preferably the most surface layer among the layers constituting the multilayer film. In this way, since the difference in refractive index with air can be reduced, the reflection of light can be further reduced, and ghosts and flares can be further reduced.

  Moreover, when the refractive index of the layer formed using the wet process is nd, the refractive index nd is preferably 1.30 or less. In this way, since the difference in refractive index with air can be reduced, the reflection of light can be further reduced, and ghosts and flares can be further reduced.

  In the retrofocus lens RL of the present embodiment, it is preferable that an aperture stop S is disposed in the third lens group G3. Thereby, the field curvature aberration can be corrected satisfactorily.

  At this time, the optical surface on which the antireflection film is provided is preferably concave with respect to the aperture stop S. In this way, since a ghost is likely to occur on the concave surface with respect to the aperture stop S, ghosts and flares can be effectively reduced.

  Note that the antireflection film is not limited to a wet process, and may include at least one layer having a refractive index of 1.30 or less (by a dry process or the like). Even if it does in this way, the effect similar to the case where a wet process is used can be acquired. At this time, the layer having a refractive index of 1.30 or less is preferably the most surface layer among the layers constituting the multilayer film. The third lens group G3 is preferably provided with an aperture stop S, and the optical surface on which the antireflection film is provided is preferably concave with respect to the aperture stop S.

  In addition, the retrofocus lens RL of the present embodiment has a shooting magnification β when focused on a short distance object, and is between the first lens group G1 and the second lens group G2 when focused on an object at infinity. Is set to D1, and when the amount of change in the air distance between the first lens group G1 and the second lens group G2 when focusing from an object at infinity to a close object is ΔD1, the following conditional expression It is preferable to satisfy the condition represented by (1).

  −30 <ΔD1 / (β × D1) <− 7 (1)

  Conditional expression (1) is a relationship between the first lens group G1 and the second lens group G2 at the time of focusing in order to correct astigmatism that occurs during focusing in the retrofocus lens RL of the present embodiment. It is a conditional expression which prescribes | regulates the ratio of the variation | change_quantity of an air space | interval, and imaging | photography magnification. The retrofocus lens RL of the present embodiment can satisfactorily correct astigmatism and coma aberration by changing the air spacing of each lens group while satisfying conditional expression (1).

  When the condition exceeds the upper limit value of conditional expression (1), astigmatism is excessively corrected, and accordingly, the lower coma aberration is also excessively corrected. In addition, if the upper limit of conditional expression (1) is set to -10, the effect of this application can be exhibited more. On the other hand, when the condition is lower than the lower limit value of the conditional expression (1), astigmatism cannot be sufficiently corrected, and accordingly, the lower coma aberration cannot be sufficiently corrected. In addition, if the lower limit of conditional expression (1) is set to -15, the effect of this application can be exhibited more.

  At this time, it is desirable that the photographing magnification when the object is focused on the short distance object is the photographing magnification when the object is focused on the closest object. This is preferable because the above-described conditional expression (1) can be obtained even when focusing on the closest object.

  Moreover, it is preferable that the condition represented by the following conditional expression (2) is satisfied, where βm is an imaging magnification when focusing on the closest object.

  (−βm)> 0.25 (2)

  Thereby, a high imaging magnification can be achieved at the time of short-distance shooting. On the other hand, when the condition is lower than the lower limit value of the conditional expression (2), it is not preferable because a sufficient photographing magnification cannot be obtained at the time of short-distance photographing. In addition, if the lower limit of conditional expression (2) is set to 0.3, the effect of the present application can be exhibited more. Moreover, if the lower limit of conditional expression (2) is set to 0.4, the effect of the present application can be further exhibited.

  Further, the amount of change in the air gap between the first lens group G1 and the second lens group G2 when focusing from an infinite object to a close object is ΔD1, and the infinite object is in focus. When the amount of change in the air gap between the second lens group G2 and the third lens group G3 is ΔD2, it is preferable that the condition represented by the following conditional expression (3) is satisfied.

  ΔD1 = −ΔD2 (3)

  Conditional expression (3) is a conditional expression that prescribes the ratio of the amount of movement when the first lens group G1 and the third lens group G3 are in focus. Here, it goes without saying that if focusing is performed by moving all three lens groups at different movement ratios, the variation in aberration can be corrected well when focusing on a short-distance object. On the other hand, when the conditional expression (3) is satisfied, the movement amounts at the time of focusing of the first lens group G1 and the third lens group G3 become the same, and all the three lens groups are moved at different movement ratios. Therefore, the structure of the lens barrel can be simplified as compared with the method of focusing. For this reason, spherical aberration and curvature of field aberration due to mechanical accuracy such as decentration can be reduced, which is preferable.

  Further, when the focal length of the first lens group G1 is f1, and the focal length of the second lens group G2 is f2, it is preferable that the condition expressed by the following conditional expression (4) is satisfied.

  0.1 <f1 / f2 <10 (4)

  Conditional expression (4) is a conditional expression for defining an appropriate range between the focal length of the first lens group G1 and the focal length of the second lens group G2. When the condition exceeds the upper limit value of conditional expression (4), the refractive power of the second lens group G2 increases, and astigmatism and coma change due to focusing increase, which is not preferable. In addition, if the upper limit of conditional expression (4) is set to 6.0, the effect of this application can be exhibited more. On the other hand, when the condition is lower than the lower limit value of the conditional expression (4), the refractive power of the first lens group G1 increases, and astigmatism and coma change due to focusing increase, which is not preferable.

  The first lens group G1 is preferably composed of a negative meniscus lens arranged in order from the object side along the optical axis and having a convex surface directed toward the object side, and a positive lens. With this configuration, it is possible to suppress variations in spherical aberration and distortion during focusing.

  At this time, when the radius of curvature of the object-side lens surface of the positive lens in the first lens group G1 is r1, and the radius of curvature of the image-side lens surface of the positive lens in the first lens group G1 is r2, the following It is preferable that the condition represented by conditional expression (5) is satisfied.

  0.5 <(r2 + r1) / (r2-r1) <3.0 (5)

  Conditional expression (5) is a conditional expression for defining the shape of the second lens from the object side (positive lens in the first lens group G1) in the first lens group G1. When the condition exceeds the upper limit value of the conditional expression (5), the variation in spherical aberration that occurs with focusing becomes large, and the spherical aberration is excessively corrected when focusing on a short-distance object. In addition, if the upper limit of conditional expression (5) is set to 1.5, the effect of this application can be exhibited more. On the other hand, when the condition is lower than the lower limit value of the conditional expression (5), the declination of the off-axis ray is increased, and the sagittal image surface is particularly curved, which is not preferable. In addition, if the lower limit of conditional expression (5) is set to 0.55, the effect of the present application can be exhibited more.

  In the third lens group G3, when the Abbe number of the material used for the second lens from the image side with respect to the d-line is ν32, it is preferable that the condition represented by the following conditional expression (6) is satisfied.

  60 <ν32 <83 (6)

  Conditional expression (6) is a conditional expression that defines an appropriate range of the Abbe number of the second lens counted from the image side in the third lens group G3. The retrofocus lens of this embodiment can correct lateral chromatic aberration by selecting a glass that satisfies the conditional expression (6) as the glass material of the second lens. Therefore, if the upper limit value or lower limit value of conditional expression (6) is exceeded, the lateral chromatic aberration cannot be corrected sufficiently.

  Moreover, it is preferable that each lens surface in the retrofocus lens is formed as a spherical surface or a flat surface. In this way, since the manufacturing error of the lens is small, it is possible to easily construct a lens having high optical performance with few various aberrations such as spherical aberration.

  Further, the single-lens reflex camera (imaging device) CAM of the present embodiment includes the retrofocus lens RL having the above-described configuration. Thus, a single-lens reflex camera (imaging device) can be realized in which aberration variation is small even when focusing on a short-distance object with a high photographing magnification, and ghost and flare are further reduced.

  Embodiments of the present application will be described below with reference to the accompanying drawings. In addition, although the 1st-3rd Example described below is an Example of the retrofocus lens which concerns on this application, the detail of the anti-reflective film provided in these retrofocus lenses is demonstrated separately after each Example. .

(First embodiment)
The first embodiment of the present application will be described below. FIG. 1 is a lens configuration diagram of a retrofocus lens according to the first example. The retrofocus lens RL according to the first example includes a first lens group G1 having a negative refractive power, a second lens group G2 having a negative refractive power, arranged in order from the object side along the optical axis, The third lens group G3 has a positive refractive power.

  The first lens group G1 includes a negative meniscus lens L11 having a convex surface directed toward the object side and a biconvex positive lens L12 arranged in order from the object side along the optical axis, and an image in the negative meniscus lens L11. An antireflection film is provided on the lens surface on the side and the lens surface on the object side of the positive lens L12. The second lens group G2 includes a negative meniscus lens L21 having a convex surface facing the object side, which is arranged in order from the object side along the optical axis, and a cemented lens including a biconvex positive lens L22 and a biconcave negative lens L23. It consists of a positive lens. The third lens group G3 includes a biconvex positive lens L31, an aperture stop S, a biconcave negative lens L32, and a positive lens with a convex surface facing the image side, which are arranged in order from the object side along the optical axis. It comprises a meniscus lens L33 and a biconvex positive lens L34.

  With such a lens configuration, the retrofocus lens RL according to the present embodiment increases the distance between the first lens group G1 and the second lens group G2 when focusing from an object at infinity to an object at a short distance. At the same time, the first lens group G1, the second lens group G2, and the third lens group G3 move toward the object side along the optical axis so that the distance between the second lens group G2 and the third lens group G3 decreases. It is like that. More specifically, at the time of focusing, the movement ratios of the first lens group G1 and the third lens group G3 are the same, that is, the first lens group G1 and the third lens group G3 are moved integrally to the object side. It has become.

  Tables 1 to 3 shown below are tables listing values of specifications in the first to third examples. In [Overall specifications] in each table, f indicates a focal length, FNO indicates an F number, and 2ω indicates an angle of view. In [Lens Data], the surface number is the order of the lens surfaces counted from the object side, r is the radius of curvature of the lens surfaces, d is the distance between the lens surfaces, and nd is the d-line (wavelength λ = 587.6 nm). ), And νd represents the Abbe number for the d-line (wavelength λ = 587.6 nm), respectively. Note that r = ∞ indicates a plane, and the refractive index nd = 1 of air is omitted. Bf represents back focus.

  [Variable interval data] indicates the photographing magnification β, the distance d0 from the object to the first lens surface, and the variable interval between the lens groups. [Condition corresponding value] indicates the value of each conditional expression.

  In all the following specification values, “mm” is generally used as the focal length f, the radius of curvature r, and other length units unless otherwise specified. Even if proportional reduction is performed, the same optical performance can be obtained. The description of these symbols is the same in the other examples below.

  Table 1 below shows specifications in the first embodiment. The surface numbers 1 to 18 in Table 1 correspond to the surfaces 1 to 18 in FIG.

(Table 1)
[Overall specifications]
f = 41.2
FNO = 2.89
2ω = 72 °
[Lens data]
Surface number r d nd νd
Object ∞ ∞
1 49.752 3.0 1.83481 42.72
2 32.115 5.7
3 130.000 4.1 1.75692 31.59
4-492.062 (d4)
5 138.964 2.4 1.49782 82.56
6 19.308 16.0
7 42.141 4.6 1.78800 47.38
8-310.529 2.4 1.56732 42.72
9 73.800 (d9)
10 80.318 6.0 1.61800 63.38
11 -37.349 1.0
12 Aperture stop S 4.5
13 -28.492 9.2 1.71736 29.52
14 70.877 1.9
15 -95.520 3.2 1.48749 70.45
16 -25.721 0.1
17 270.998 3.4 1.80400 46.58
18 -59.752 Bf
Image plane ∞
[Variable interval data]
Infinite focus state Closest focus state β 0.00 −0.50
d0 ∞ 72.3
d4 1.00 5.28
d9 6.50 2.21
[Conditional value]
Conditional expression (1) ΔD1 / (β × D1) = − 8.56
Conditional expression (2) (−βm) = 0.50
Conditional expression (3) ΔD1 = 4.28
-ΔD2 = 4.28
Conditional expression (4) f1 / f2 = 6.30
Conditional expression (5) (r2 + r1) / (r2-r1) = 0.58
Conditional expression (6) ν32 = 70.45

  Thus, in this embodiment, it can be seen that all the conditional expressions (1) to (6) are satisfied.

  FIGS. 2A and 2B are graphs showing various aberrations of the retrofocus lens according to Example 1 when focused at infinity and when focused at a close distance (shooting magnification β = −0.5 times), respectively. . In each aberration diagram, FNO represents an F number, NA represents a numerical aperture, Y represents an image height, H represents an object height, and A represents a half angle of view (unit: “°”). The spherical aberration diagram shows the F-number or numerical aperture value corresponding to the maximum aperture, the astigmatism diagram and the distortion diagram show the maximum image height, and the coma diagram shows each half field angle or each object. Indicates a high value.

  In each aberration diagram, d represents an aberration curve of the d-line (wavelength λ = 587.6 nm), and g represents an aberration curve of the g-line (wavelength λ = 435.8 nm). Further, in the astigmatism diagram, the solid line indicates the sagittal image plane, and the broken line indicates the meridional image plane. It should be noted that the same reference numerals as in this example are used in the various aberration diagrams of each example described below. From the various aberration diagrams, it can be seen that the retrofocus lens according to the present example has excellent aberrations and excellent imaging performance.

  As shown in FIG. 3, when a light beam BM from the object side enters the retrofocus lens RL as shown in the drawing, the object-side lens surface (the first ghost generation surface and its surface number) in the positive lens L12. Is reflected again by the lens surface on the image plane I side of the negative meniscus lens L11 (the second ghost generation surface, whose surface number is 2) and reaches the image plane I. , Will cause ghosts. The first ghost generating surface 3 and the second ghost generating surface 2 are both concave with respect to the aperture stop S. A ghost can be effectively reduced by forming an antireflection film corresponding to a wide incident angle in a wider wavelength range on such a surface.

(Second embodiment)
The second embodiment of the present application will be described below. FIG. 4 is a lens configuration diagram of a retrofocus lens according to Example 2. The retrofocus lens RL according to the second example includes a first lens group G1 having negative refractive power, a second lens group G2 having negative refractive power, arranged in order from the object side along the optical axis, The third lens group G3 has a positive refractive power.

  The first lens group G1 includes a negative meniscus lens L11 having a convex surface directed toward the object side and a biconvex positive lens L12 arranged in order from the object side along the optical axis, and an image in the negative meniscus lens L11. An antireflection film is provided on the lens surface on the side and the lens surface on the object side of the positive lens L12. The second lens group G2 is arranged in order from the object side along the optical axis, the negative meniscus lens L21 having a convex surface on the object side, the positive meniscus lens L22 having a convex surface on the object side, and the convex surface on the object side. And a cemented positive lens composed of a negative meniscus lens L23. The third lens group G3 includes a biconvex positive lens L31, an aperture stop S, a biconcave negative lens L32, and a positive lens with a convex surface facing the image side, which are arranged in order from the object side along the optical axis. It is composed of a meniscus lens L33 and a biconvex positive lens L34, and antireflection films are provided on the object-side and image-side lens surfaces of the positive meniscus lens L33.

  With such a lens configuration, the retrofocus lens RL according to the present embodiment increases the distance between the first lens group G1 and the second lens group G2 when focusing from an object at infinity to an object at a short distance. At the same time, the first lens group G1, the second lens group G2, and the third lens group G3 move toward the object side along the optical axis so that the distance between the second lens group G2 and the third lens group G3 decreases. It is like that. More specifically, at the time of focusing, the movement ratios of the first lens group G1 and the third lens group G3 are the same, that is, the first lens group G1 and the third lens group G3 are moved integrally to the object side. It has become.

  Table 2 below shows specifications in the second embodiment. The surface numbers 1 to 18 in Table 2 correspond to the surfaces 1 to 18 in FIG.

(Table 2)
[Overall specifications]
f = 41.2
FNO = 2.89
2ω = 72 °
[Lens data]
Surface number r d nd νd
Object ∞ ∞
1 71.182 2.5 1.77250 49.61
2 33.485 5.8
3 102.661 4.0 1.75692 31.59
4-913.364 (d4)
5 88.128 2.0 1.49782 82.56
6 21.965 17.0
7 43.206 4.5 1.81600 46.63
8 628.340 2.0 1.57501 41.49
9 59.241 (d9)
10 59.135 5.0 1.60300 65.47
11 -43.390 3.0
12 Aperture stop S 4.0
13 -28.586 9.0 1.71736 29.52
14 72.203 2.0
15 -101.563 3.5 1.49782 82.56
16 -26.164 0.1
17 686.431 3.5 1.83481 42.72
18 -55.148 Bf
Image plane ∞
[Variable interval data]
Infinite focus state Closest focus state β 0.00 −0.50
d0 ∞ 74.0
d4 1.01 7.58
d9 7.01 0.44
[Conditional value]
Conditional expression (1) ΔD1 / (β × D1) = − 13.01
Conditional expression (2) (−βm) = 0.50
Conditional expression (3) ΔD1 = 6.58
-ΔD2 = 6.58
Conditional expression (4) f1 / f2 = 1.66
Conditional expression (5) (r2 + r1) / (r2-r1) = 0.80
Conditional expression (6) ν32 = 82.56

  Thus, in this embodiment, it can be seen that all the conditional expressions (1) to (6) are satisfied.

  FIGS. 5A and 5B are graphs showing various aberrations when the retrofocus lens according to Example 2 is focused at infinity and when focused at a close distance (shooting magnification β = −0.5 times), respectively. . From the various aberration diagrams, it can be seen that the retrofocus lens according to the present example has excellent aberrations and excellent imaging performance.

(Third embodiment)
The third embodiment of the present application will be described below. FIG. 6 is a lens configuration diagram of a retrofocus lens according to Example 3. The retrofocus lens RL according to the third example includes a first lens group G1 having negative refractive power, a second lens group G2 having negative refractive power, arranged in order from the object side along the optical axis, The third lens group G3 has a positive refractive power.

  The first lens group G1 includes a negative meniscus lens L11 having a convex surface facing the object side and a positive meniscus lens L12 having a convex surface facing the object side, which are arranged in order from the object side along the optical axis. The second lens group G2 includes a negative meniscus lens L21 having a convex surface facing the object side and a positive meniscus lens L22 having a convex surface facing the object side, which are arranged in order from the object side along the optical axis. The third lens group G3 includes a biconvex positive lens L31, an aperture stop S, a biconcave negative lens L32, and a positive lens with a convex surface facing the image side, which are arranged in order from the object side along the optical axis. A meniscus lens L33 and a positive meniscus lens L34 having a convex surface facing the image side are provided, and antireflection films are provided on the object-side and image-side lens surfaces of the positive meniscus lens L33.

  With such a lens configuration, the retrofocus lens RL according to the present embodiment increases the distance between the first lens group G1 and the second lens group G2 when focusing from an object at infinity to an object at a short distance. At the same time, the first lens group G1, the second lens group G2, and the third lens group G3 move toward the object side along the optical axis so that the distance between the second lens group G2 and the third lens group G3 decreases. It is like that. More specifically, at the time of focusing, the movement ratios of the first lens group G1 and the third lens group G3 are the same, that is, the first lens group G1 and the third lens group G3 are moved integrally to the object side. It has become.

  Table 3 below shows specifications in the third embodiment. The surface numbers 1 to 17 in Table 3 correspond to the surfaces 1 to 17 in FIG.

(Table 3)
[Overall specifications]
f = 41.2
FNO = 2.89
2ω = 72 °
[Lens data]
Surface number r d nd νd
Object ∞ ∞
1 45.878 2.5 1.77250 49.61
2 26.109 6.5
3 137.175 3.8 1.75692 31.59
4 1000.000 (d4)
5 85.239 2.0 1.49782 82.56
6 24.833 17.0
7 51.763 5.0 1.81600 46.63
8 149.355 (d8)
9 74.600 5.0 1.72916 54.66
10 -50.076 3.0
11 Aperture stop S 4.0
12-30.599 9.3 1.72825 28.46
13 63.406 2.0
14 -143.413 3.5 1.60300 65.47
15 -28.542 0.1
16 -465.372 3.5 1.77250 49.61
17 -53.935 Bf
Image plane ∞
[Variable interval data]
Infinite focus state Closest focus state β 0.00 −0.50
d0 ∞ 72.2
d4 1.01 6.66
d8 7.01 1.36
[Conditional value]
Conditional expression (1) ΔD1 / (β × D1) = − 11.19
Conditional expression (2) (−βm) = 0.50
Conditional expression (3) ΔD1 = 5.65
-ΔD2 = 5.65
Conditional expression (4) f1 / f2 = 0.18
Conditional expression (5) (r2 + r1) / (r2-r1) = 1.32
Conditional expression (6) ν32 = 65.47

  Thus, in this embodiment, it can be seen that all the conditional expressions (1) to (6) are satisfied.

  FIGS. 7A and 7B are graphs showing various aberrations of the retrofocus lens according to Example 3 when focused at infinity and when focused at a short distance (shooting magnification β = −0.5 times), respectively. . From the various aberration diagrams, it can be seen that the retrofocus lens according to the present example has excellent aberrations and excellent imaging performance.

  Here, the antireflection film used for the retrofocus lenses of the first to third embodiments will be described. FIG. 8 is a diagram showing a film configuration of the antireflection film. The antireflection film 101 is composed of seven layers and is formed on the optical surface of the optical member 102 such as a lens. The first layer 101a is formed of aluminum oxide deposited by a vacuum deposition method. Further, a second layer 101b made of a mixture of titanium oxide and zirconium oxide deposited by a vacuum deposition method is further formed on the first layer 101a. Further, a third layer 101c made of aluminum oxide deposited by a vacuum deposition method is formed on the second layer 101b, and titanium oxide and zirconium oxide deposited by a vacuum deposition method are formed on the third layer 101c. A fourth layer 101d made of the mixture is formed. Furthermore, a fifth layer 101e made of aluminum oxide deposited by vacuum deposition is formed on the fourth layer 101d, and titanium oxide and zirconium oxide deposited by vacuum deposition on the fifth layer 101e. A sixth layer 101f made of the mixture is formed.

  Then, the seventh layer 101g made of a mixture of silica and magnesium fluoride is formed on the sixth layer 101f formed in this way by a wet process, and the antireflection film 101 of this embodiment is formed. For the formation of the seventh layer 101g, a sol-gel method which is a kind of wet process is used. In the sol-gel method, an optical thin film material sol is applied on the optical surface of an optical member, a gel film is deposited, and then immersed in a liquid. This is a method for producing a film by vaporizing and drying. The wet process is not limited to the sol-gel method, and a method of obtaining a solid film without going through a gel state may be used.

  Thus, the first layer 101a to the sixth layer 101f of the antireflection film 101 are formed by electron beam evaporation which is a dry process, and the seventh layer 101g which is the uppermost layer is prepared by a hydrofluoric acid / magnesium acetate method. It is formed by the following procedure by a wet process using the prepared sol solution. First, an aluminum oxide layer to be the first layer 101a, a titanium oxide-zirconium oxide mixed layer to be the second layer 101b, a third layer on the lens film formation surface (the optical surface of the optical member 102 described above) in advance using a vacuum deposition apparatus, An aluminum oxide layer to be the layer 101c, a titanium oxide-zirconium oxide mixed layer to be the fourth layer 101d, an aluminum oxide layer to be the fifth layer 101e, and a titanium oxide-zirconium oxide mixed layer to be the sixth layer 101f are formed in this order. And after taking out the optical member 102 from a vapor deposition apparatus, the sol liquid prepared by the hydrofluoric acid / magnesium acetate method was apply | coated by the spin coat method, and the layer which consists of a mixture of the silica and magnesium fluoride used as the 7th layer 101g was formed. Form. The reaction formula when prepared by the hydrofluoric acid / magnesium acetate method is shown in the following formula (7).

    2HF + Mg (CH3COO) 2 → MgF2 + 2 + CH3COOH (7)

  The sol solution used for the film formation is used for film formation after mixing raw materials and subjecting to an autoclave at 140 ° C. for 24 hours at a high temperature and pressure. The optical member 102 is completed by heat treatment at 160 ° C. for 1 hour in the air after the seventh layer 101g is formed. By using such a sol-gel method, several to several tens of atoms or molecules are gathered to form particles with a size of several nanometers to several tens of nanometers. Secondary particles are formed and the secondary particles are deposited to form the seventh layer 101g.

The optical performance of the antireflection film 101 formed in this way will be described using the spectral characteristics shown in FIG. FIG. 9 shows the spectral characteristics when a light ray is vertically incident when the antireflection film 101 is designed under the conditions shown in Table 4 below when the reference wavelength λ is 550 nm. Table 4 shows aluminum oxide as Al ₂ O ₃ , titanium oxide-zirconium oxide mixture as ZrO ₂ + TiO ₂ , silica and magnesium fluoride as SiO ₂ + MgF ₂ , and a reference wavelength λ of 550 nm. Table 4 shows design values when the substrate has four kinds of refractive indexes of 1.46, 1.62, 1.74, and 1.85.

(Table 4)
Substance Refractive index Optical film thickness Optical film thickness Optical film thickness Optical film thickness Medium Air 1
7th layer SiO ₂ + MgF ₂ 1.26 0.275λ 0.268λ 0.271λ 0.269λ
6th layer ZrO ₂ + TiO ₂ 2.12 0.045λ 0.057λ 0.054λ 0.059λ
5th layer Al ₂ O ₃ 1.65 0.212λ 0.171λ 0.178λ 0.162λ
4th layer ZrO ₂ + TiO ₂ 2.12 0.077λ 0.127λ 0.13λ 0.158λ
3rd layer Al ₂ O ₃ 1.65 0.288λ 0.122λ 0.107λ 0.08λ
Second layer ZrO ₂ + TiO ₂ 2.12 0 0.059λ 0.075λ 0.105λ
1st layer Al ₂ O ₃ 1.65 0 0.257λ 0.03λ 0.03λ
Substrate refractive index 1.46 1.62 1.74 1.85

  As can be seen from FIG. 9, the reflectance is suppressed to 0.2% or less over the entire wavelength range of 420 nm to 720 nm.

  In the retrofocus lens of the first example, since the refractive index of the negative meniscus lens L11 is 1.83481, and the refractive index of the positive lens L12 is 1.75692, the negative meniscus lens L11 has an image plane I side. It is possible to use an antireflection film corresponding to the refractive index of the substrate of 1.85 on the lens surface, and an antireflection film corresponding to the refractive index of the substrate corresponding to 1.74 on the object side lens surface of the positive lens L12. A membrane can be used.

  In the retrofocus lens of the second example, the refractive index of the negative meniscus lens L11 is 1.77250, the refractive index of the positive lens L12 is 1.75692, and the refractive index of the positive meniscus lens L33 is 1.49782. Therefore, an antireflection film having a refractive index of 1.74 corresponding to the refractive index of the substrate can be used for the lens surface on the image plane I side in the negative meniscus lens L11, and the lens surface on the object side in the positive lens L12. It is possible to use an antireflection film corresponding to the refractive index of the substrate of 1.74, and to use antireflection films corresponding to the refractive index of the substrate of 1.46 on the lens surfaces on both sides of the positive meniscus lens L33. Is possible.

  Further, in the retrofocus lens of the third example, the refractive index of the positive meniscus lens L33 is 1.60300, and thus the reflection corresponding to the refractive index of the substrate corresponding to 1.62 is formed on both lens surfaces of the positive meniscus lens L33. It is possible to use a prevention film.

  Thus, by applying the antireflection film of the present embodiment to the retrofocus lenses of the first to third examples, it is suitable for a single-lens reflex camera or a digital camera, has an angle of view of 70 ° or more, and an imaging magnification. Even when focusing on an object at a short distance of about -0.5 times, there is little variation in aberrations, and a retrofocus lens with reduced ghost and flare can be realized. In addition, by installing the retrofocus lens of each example in the single-lens reflex camera of this embodiment, there is little aberration variation even when focusing on a short-distance object with a shooting magnification of about -0.5 times, and ghost and A single-lens reflex camera (imaging device) with further reduced flare can be realized.

  The antireflection film 101 can be used as an optical element provided on the optical surface of a plane-parallel plate, or can be used provided on the optical surface of a lens formed in a curved surface. The retrofocus lens of the second embodiment is not limited to four surfaces, and an antireflection film is provided on two surfaces of the negative meniscus lens L11, the image side lens surface and the positive meniscus lens L33, the object side lens surface. The antireflection film may be provided only on the object side lens surface of the positive meniscus lens L33. Furthermore, such a configuration can be applied not only to the retrofocus lens of the second embodiment but also to the retrofocus lenses of the first and third embodiments.

  Next, a modified example of the antireflection film will be described. This antireflection film consists of five layers and is configured under the conditions shown in Table 5 below. Note that the sol-gel method described above is used to form the fifth layer. Table 5 shows design values when the refractive index of the substrate is 1.52 when the reference wavelength λ is 550 nm.

(Table 5)
Material Refractive index Optical film thickness Medium Air 1
Fifth layer Mixture of silica and magnesium fluoride 1.26 0.269λ
Fourth layer Titanium oxide-zirconium oxide mixture 2.12 0.043λ
Third layer Aluminum oxide 1.65 0.217λ
Second layer Titanium oxide-zirconium oxide mixture 2.12 0.066λ
First layer Aluminum oxide 1.65 0.290λ
Substrate BK7 1.52

  FIG. 10 shows spectral characteristics when light is vertically incident on the antireflection film of the modification. As can be seen from FIG. 10, the reflectance is suppressed to 0.2% or less over the entire wavelength range of 420 nm to 720 nm. FIG. 11 shows spectral characteristics when the incident angles are 30 degrees, 45 degrees, and 60 degrees.

  For comparison, FIG. 12 shows spectral characteristics at the time of vertical incidence of a multilayer broadband antireflection film formed by only a dry process such as a conventional vacuum deposition method and configured under the conditions shown in Table 6 below. FIG. 13 shows spectral characteristics when the incident angles are 30, 45, and 60 degrees.

(Table 6)
Material Refractive index Optical film thickness Medium Air 1
Seventh layer Magnesium fluoride 1.39 0.243λ
Sixth layer Titanium oxide-zirconium oxide mixture 2.12 0.119λ
5th layer Aluminum oxide 1.65 0.057λ
Fourth layer Titanium oxide-zirconium oxide mixture 2.12 0.220λ
Third layer Aluminum oxide 1.65 0.064λ
Second layer Titanium oxide-zirconium oxide mixture 2.12 0.057λ
First layer Aluminum oxide 1.65 0.193λ
Substrate BK7 1.52

  Comparing the spectral characteristics of the modified example shown in FIGS. 10 and 11 with the spectral characteristics of the conventional example shown in FIGS. 12 and 13, the low reflectance of the antireflection film according to the modified example can be clearly seen.

  In the above-described embodiment, the example in which the antireflection film formed by the wet process is formed on at least one surface of the first lens group G1 or the third lens group G3 is shown, but it may be formed on the second lens group. I do not care. Further, in order to correct image blur caused by camera shake, a part of the lens group or one lens group may be moved as a vibration-proof lens group in a direction perpendicular to the optical axis. In the retrofocus lens RL of the present embodiment, it is particularly preferable that the positive lens L31 of the third lens group G3 is an anti-vibration lens group.

  Further, the lens surface of the lens constituting the retrofocus lens RL of the present embodiment may be an aspherical surface. This aspherical surface may be any one of an aspherical surface by grinding, a glass mold aspherical surface in which glass is molded into an aspherical shape, or a composite aspherical surface in which a resin provided on the glass surface is formed in an aspherical shape.

  In addition, each above-mentioned Example has shown one specific example of this application, and this application is not limited to these. For example, a retrofocus lens having a three-group configuration is shown as an example of the present application. Needless to say, a lens having a four-group configuration including the three groups and a lens having a group configuration of three or more groups are equivalent lenses having the effects of the present application. . In addition, in the configuration in each lens group, it goes without saying that a lens group in which an additional lens is added to the configuration in the embodiment is an equivalent lens group in which the effect of the present application is inherent.

1 is a lens configuration diagram of a retrofocus lens according to Example 1. FIG. FIGS. 9A and 9B are graphs showing various aberrations when the retrofocus lens according to Example 1 is focused at infinity and focused at a close distance (imaging magnification: −0.5 times), respectively. FIG. 3 is a configuration diagram of a retrofocus lens according to the first example, in which incident light rays are reflected by a first ghost generation surface and a second ghost generation surface. It is a lens block diagram of the retrofocus lens which concerns on 2nd Example. FIGS. 9A and 9B are graphs showing various aberrations when the retrofocus lens according to Example 2 is focused at infinity and focused at close distance (imaging magnification: −0.5 times), respectively. It is a lens block diagram of the retrofocus lens which concerns on 3rd Example. FIGS. 9A and 9B are graphs showing various aberrations when the retrofocus lens according to Example 3 is focused at infinity and focused at close distance (imaging magnification: −0.5 times), respectively. It is explanatory drawing which shows the structure of an antireflection film. It is a graph which shows the spectral characteristic of an antireflection film. It is a graph which shows the spectral characteristics of the antireflection film concerning a modification. It is a graph which shows the spectral characteristics of the antireflection film concerning a modification. It is a graph which shows the spectral characteristic of the anti-reflective film produced with the prior art. It is a graph which shows the spectral characteristic of the anti-reflective film produced with the prior art. It is a schematic block diagram of a single-lens reflex camera.

Explanation of symbols

CAM single-lens reflex camera (imaging device)
RL Retrofocus lens G1 First lens group G2 Second lens group G3 Third lens group I Image surface S Aperture stop 101 Antireflection film 101a First layer 101b Second layer 101c Third layer 101d Fourth layer 101e Fifth layer 101f Sixth layer 101g Seventh layer 102 Optical member

Claims (15)

A first lens group having negative refractive power, arranged in order from the object side along the optical axis, a second lens group having negative refractive power, and a third lens group having positive refractive power,
When focusing from an object at infinity to an object at a short distance, the distance between the first lens group and the second lens group increases and the distance between the second lens group and the third lens group decreases. Further, the first lens group, the second lens group, and the third lens group are configured to move toward the object side along the optical axis,
An antireflection film is provided on at least one of the optical surfaces of the first lens group or the third lens group, and the antireflection film includes at least one layer having a refractive index of 1.30 or less. And
When the shooting magnification when focusing on the closest object is βm,
(−βm)> 0.25
A retro-focus lens that satisfies the above conditions . A first lens group having negative refractive power, arranged in order from the object side along the optical axis, a second lens group having negative refractive power, and a third lens group having positive refractive power,
When focusing from an object at infinity to an object at a short distance, the distance between the first lens group and the second lens group increases and the distance between the second lens group and the third lens group decreases. Further, the first lens group, the second lens group, and the third lens group are configured to move toward the object side along the optical axis,
An antireflection film is provided on at least one of the optical surfaces of the first lens group or the third lens group, and the antireflection film includes at least one layer having a refractive index of 1.30 or less. And
The imaging magnification when focusing on a short distance object is β, and the air distance between the first lens group and the second lens group when focusing on an infinite object is D1, and the distance from the infinite object is near. When the amount of change in the air gap between the first lens group and the second lens group when focusing on a distance object is ΔD1,
−30 <ΔD1 / (β × D1) <− 7
Satisfy the conditions of
The shooting magnification when focusing on an object at a short distance is the shooting magnification when focusing on an object at the closest distance.
A retro focus lens that is characterized by A first lens group having negative refractive power, arranged in order from the object side along the optical axis, a second lens group having negative refractive power, and a third lens group having positive refractive power,
When focusing from an object at infinity to an object at a short distance, the distance between the first lens group and the second lens group increases and the distance between the second lens group and the third lens group decreases. Further, the first lens group, the second lens group, and the third lens group are configured to move toward the object side along the optical axis,
An antireflection film is provided on at least one of the optical surfaces of the first lens group or the third lens group, and the antireflection film includes at least one layer having a refractive index of 1.30 or less. And
The first lens group includes a negative meniscus lens arranged in order from the object side along the optical axis and having a convex surface directed toward the object side, and a positive lens. The imaging magnification when focusing on a short distance object is β, and the air distance between the first lens group and the second lens group when focusing on an infinite object is D1, and the distance from the infinite object is near. When the amount of change in the air gap between the first lens group and the second lens group when focusing on a distance object is ΔD1,
−30 <ΔD1 / (β × D1) <− 7
Satisfy the conditions of
4. The retrofocus lens according to claim 1, wherein an imaging magnification when focusing on the object at a short distance is an imaging magnification when focusing on an object at the closest distance. 5. When the shooting magnification when focusing on the closest object is βm,
(−βm)> 0.25
The retrofocus lens according to claim 2, wherein the following condition is satisfied. The first lens group includes a negative meniscus lens arranged in order from the object side along the optical axis and having a convex surface facing the object side, and a positive lens. The retrofocus lens described in 1. When the radius of curvature of the lens surface on the object side of the positive lens in the first lens group is r1, and the radius of curvature of the lens surface on the image side of the positive lens in the first lens group is r2, the following equation:
0.5 <(r2 + r1) / (r2-r1) <3.0
The retrofocus lens according to claim 3 or 6, wherein the following condition is satisfied. The antireflection film is a multilayer film,
8. The layer according to claim 1 , wherein the layer having a refractive index of 1.30 or less is a layer on the most surface side among layers constituting the multilayer film. Retro focus lens. The retrofocus lens according to any one of claims 1 to 8, wherein an aperture stop is disposed in the third lens group. The retrofocus lens according to claim 9 , wherein the optical surface is a concave surface with respect to the aperture stop. The amount of change in the air gap between the first lens group and the second lens group when focusing from an infinite object to a close object is ΔD1, and when the infinite object is focused to a close object When the amount of change in the air gap between the second lens group and the third lens group is ΔD2, the following expression ΔD1 = −ΔD2
The retrofocus lens according to any one of claims 1 to 10, wherein the following condition is satisfied. When the focal length of the first lens group is f1, and the focal length of the second lens group is f2, the following expression 0.1 <f1 / f2 <10
The retrofocus lens according to claim 1, wherein the following condition is satisfied. The retrofocus lens according to any one of claims 1 to 12 , wherein lens surfaces of the retrofocus lens are each formed of a spherical surface or a flat surface. In an imaging device including a retrofocus lens that forms an image of an object on a predetermined surface,
An imaging apparatus, wherein the retrofocus lens is the retrofocus lens according to any one of claims 1 to 13 . A first lens group having a negative refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power, arranged in order from the object side along the optical axis. In the focusing method of the configured retrofocus lens,
When focusing from an object at infinity to an object at a short distance, the distance between the first lens group and the second lens group increases and the distance between the second lens group and the third lens group decreases. And moving the first lens group, the second lens group, and the third lens group to the object side along the optical axis,
At least one surface the antireflection film in the optical surface is provided in the first lens group or the third lens group, the anti-reflecting layer constitutes a layer in which the refractive index is 1.30 or less so as to include at least one layer ,
When the shooting magnification when focusing on the closest object is βm,
(−βm)> 0.25
A retrofocus lens focusing method characterized by satisfying the above conditions .

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