JPH01319009A - retro focus lens - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a retrofocus type wide-angle lens incorporating a short-range correction mechanism.

[Conventional technology]

-IG, the focusing method of retrofocus type photographic lenses is often carried out by extending the entire lens. However, since the movement of the entire lens system involves moving a large object, the total weight of the moving group including the lens barrel becomes heavy. In particular, with ultra-wide-angle lenses or lenses with a very long back focus compared to the focal length, the negative refractive power of the front group becomes large.
Since it becomes difficult to correct aberrations, the number of lenses increases, the diameter of the front lens generally increases, and the overall length and weight also increase significantly.

In particular, in the case of a camera lens or the like incorporating an automatic focusing mechanism, it is necessary to have good responsiveness, have small movable parts, be light, and have a small amount of movement. On the other hand, retrofocus lenses, which have a two-group configuration consisting of a front group with negative refractive power and a rear group with positive refractive power, are inherently highly asymmetric, so the object distance changes when focusing, resulting in the shadow magnification. When the aberrations change, various aberrations of the lens, especially astigmatism, curvature of field, and coma, change significantly compared to other aberrations. Therefore, efforts have been made to improve compactness and operability while correcting aberrations as described in (a) to (d) below.

First, in order to correct aberration changes during close-up photography, various retrofocus photographic lenses have been proposed in which a mechanism is added to move some lens groups while extending the entire lens. For example, (a) Tokuko Sho 45-39
It is disclosed in Japanese Patent No. 875.

The second is a method of focusing by fixing two lenses from the object side and moving all the lens groups on the image side.
It is disclosed in Publication No. 10.

The third method is to move only the lens group closest to the image side to focus, and (c) JP-A-55-1
This method is disclosed in Japanese Patent Application Laid-Open No. 43517 and Japanese Patent Application Laid-Open No. 58-202414.

[Problem that the invention seeks to solve]

However, each of the above (a) to (d) has the following drawbacks.

The method of adding a mechanism to move some of the lens groups while performing the first whole movement has the disadvantage that the weight of the moving group is the same as that of the whole movement, and the overall length is also thick. For example, ( b) In the whole extension second focusing method of Japanese Patent Publication No. 45-39875, when the bank focus is very long compared to the ultra-wide-angle lens or the focal length,
The negative refractive power of the front group becomes strong, making it difficult to correct aberrations.
The number of lenses increases, the diameter of the front lens generally increases, and the overall length and weight also increase. Therefore, the method of performing short-distance correction while simultaneously extending the entire shaft is inconvenient in terms of operation.

In a method of focusing by fixing two lenses from the second object side and moving all the lens groups on the image side, (b) JP-A-61-1409
In the No. 10 publication, the focusing group, which is also a movable group, is large and heavy (as mentioned above, this is also inconvenient for camera lenses that incorporate an automatic focusing mechanism). be.

Furthermore, this method of correcting various aberrations is insufficient, particularly in ultra-wide-angle lenses with a large angle of view, changes in coma aberration and curvature of field during close-range imaging are insufficiently corrected.

In the third rear group extension method in which only the lens group closest to the image side is moved and focused, (c) Japanese Patent Application Laid-Open No. 55-14351
The one in Publication No. 7 has a small movable lens group, which is convenient for camera lenses that incorporate an automatic focusing mechanism, but
The angle of view was relatively small at 90@ or less, and it had the drawback that changes in various aberrations, especially coma aberration, were significant during close-range shooting.

Therefore, if the angle of view is made even wider, it becomes extremely difficult to correct aberrations. In addition, (d) the method disclosed in Japanese Patent Application Laid-open No. 5!3-202414 allows the weight of the focusing group to be reduced and the number of lenses to be reduced, which is convenient for operation, but (c)
Similarly, the angle of view is as small as 84'' or less, and changes in various aberrations, especially coma aberration, during close-range photography are significant, making it difficult to achieve a wider angle.

Therefore, the patent application filed in Sho 6 proposed by the same applicant as the present invention
1-313854, an aspherical lens whose negative surface refractive power becomes stronger toward the periphery than the apex is introduced into the focusing group that moves out the rear group, while effectively correcting aberration fluctuations during close-up shooting. Moreover, the size and weight of the focusing group have also been improved to reduce weight.

However, with the introduction of this aspherical lens, it is necessary to provide back focus by the thickness of the aspherical lens.
Especially for retrofocus lenses with a wide angle of view,
Ensuring a sufficient bank focus is not preferable because it imposes many restrictions on aberration correction.

Therefore, the present invention solves the above-mentioned drawbacks, makes it possible to secure sufficient back focus without imposing restrictions on aberration correction, and further improves operability by reducing the weight, size, and amount of movement of the focusing group. It is an object of the present invention to provide a retrofocus type lens that is compact and has a wide angle of view, and has good imaging performance during close-range 1-lit photography.

[Means for solving problems]

In order to achieve the above object, the present invention provides a retrofocus wide-angle lens having, in order from the object side, a first lens group G1 having a negative refractive power and a second lens group G2 having a positive refractive power. Therefore, this second lens group is the front group G2F.
and a rear group G□ with positive refractive power, and this rear group G is composed of a positive lens and a negative lens, and has a shape in which the negative surface refractive power becomes stronger as it approaches the periphery from the apex. Or, it has at least one aspherical surface whose positive surface refractive power becomes weaker toward the periphery of the apex, and the diaphragm is placed closer to the object side than the aspherical lens in the rear group G□. The group G4 is extended toward the object along the optical axis to focus on a close object.

In order to achieve sufficient aberration correction in such a basic configuration of the present invention, it is desirable that the configuration satisfies each of the following conditions (1) to (3).

As-31 0<<0.2・-・・-(1)-
1,0≦α≦1.0・−・・−・(2)0.7≦□≦
6.0...(3) However, f: Focal length of the entire system AS-S: Difference in the optical axis direction between the aspheric surface at the outermost periphery of the effective diameter and a reference spherical surface having a predetermined apex radius of curvature.

α: The angle between the optical axis and the paraxial ray from the object point at infinity on the axis that enters the lens surface of the rear group GzR of the second lens group, which is the initial value α= 0, value obtained as h=f.

α1: Angle between the paraxial ray incident on the front lens surface and the optical axis.

hI: Height of incidence of paraxial rays incident on the frontmost lens surface.

f4: Focal length of the rear group 0 of the second lens group.

Note that the surface refractive power referred to here is the difference between the incident angle and the exit angle of a given ray that is incident on a given point on a certain refractive surface, that is, the deviation angle, which is the surface refraction of a minute surface near the refractive point. When a parallel beam of light incident near the refraction point converges after refraction, the surface refractive power of that surface is positive, and when it diverges after refraction, the surface refractive power of that surface is negative. Defined as surface refractive power.

In addition, the aspherical surface in the present invention can be expressed as an aspherical shape in which the negative surface stress rupture force becomes stronger as it approaches the periphery from the apex, and the positive surface stress rupture force becomes weaker as it approaches the periphery from the apex. It can also be expressed as an aspherical shape, but in any case, the effect of the surface is substantially equivalent.

[For production]

The aspherical surface of the present invention provided in the rear group G It has a function of extremely well correcting aberration fluctuations due to focus, and is disclosed in Japanese Patent Application No. 61-31385 proposed by the same person as the present invention.
It is possible to obtain the same effect as that of the aspheric lens for correcting aberration fluctuations due to short-distance focusing described in No. 4.

Therefore, in the present invention, there is no need to secure a space for arranging an aspherical lens for correcting aberration fluctuations due to short-distance focusing, and therefore it is possible to eliminate constraints on aberration correction due to securing back focus.

Therefore, the present invention has a structure that ensures sufficient back focus and is advantageous in terms of lens design.
By providing an aspheric surface in the focusing group to correct aberration fluctuations due to short-distance focusing, it is possible to bring out significantly superior imaging performance.

In addition, in the present invention, a retrofocus lens system is capable of practically correcting aberrations for light rays from an object point at infinity without introducing an aspheric surface, but cannot fully correct short-distance aberration fluctuations. In this case, if an aspherical surface such as the one of the present invention, which has a property of mainly preventing deterioration of short-range performance, is introduced into the rear group of the second lens group having a focusing function, it is possible to correct short-range aberrations. becomes possible.

As described above, the present invention provides a method of mainly correcting distortion aberration and field curvature by using an aspherical lens near the frontmost or rearmost part of an optical system, and a method of mainly correcting distortion and field curvature by using an aspherical lens near the frontmost or rearmost position of an optical system, Unlike conventional aspheric lenses, which correct spherical aberration and sagittal coma flare, the concept of the correction method is clearly different.

The conditions +1) to (3) of the present invention will be explained in detail below.

If the upper limit of condition (1) is exceeded, it becomes difficult to form an aspherical surface, causing problems such as processing tolerances and performance deterioration with respect to eccentricity, and as a result, productivity of the aspherical lens deteriorates. In addition, spherical aberration increases due to the influence of higher orders (fifth order or higher terms) due to Rand rays having a high incident height, and optical performance deteriorates. Therefore, it is realistically desirable for the amount of subtraction of the aspherical lens to fall within this range. Then, within the range of condition (1), an aspherical surface is set in the rear group of the second lens group such that at least one surface has a negative surface pressure refracting power for oblique rays that increases as the position moves closer to the periphery than the apex. By combining appropriate positive and negative surface pressure powers, fluctuations in comatic aberration can be particularly reduced even at close range.

Regarding spherical aberration, by determining the refractive power arrangement of the front group and the rear group so that the above α value becomes an appropriate value as in condition (2), the rear group G which is the focusing group! , l can be made nearly parallel to the optical axis. When the Rand rays become nearly parallel, even if the rear group G0 is extended to focus on a close-range object point, the height of incidence of the Rand rays on each surface of the rear group remains almost unchanged. Therefore, changes in the surface pressure of each surface with respect to the Rand ray are suppressed to a small extent, and fluctuations in spherical aberration due to extension for focusing are significantly reduced. Therefore, when the α value in the above condition (2) exceeds 1.0, the rear group G! has the above positive refractive power in order to focus from an object point at infinity to a near object point! , 1 toward the object, Ran incident on the surface of the rear group G□ closest to the object
Since the d ray is significantly converged, by extending it, the incident height of the Rand ray increases significantly, the positive refractive power for the Rand ray increases significantly, and the spherical aberration changes significantly in the direction of undercorrection. Moreover, when the α value is less than -1.0, the reason is exactly opposite to the above, and the incident height of the Rand ray that is incident on the surface closest to the object side of the rear group Gtll becomes low, resulting in positive surface refraction for the Rand ray. becomes weaker, and the spherical aberration changes significantly in the direction of overcorrection.

Furthermore, in the present invention, the curvature of field may vary in the negative direction during short-distance focusing, and in this case, the above α
It is desirable to set the value in a direction that slightly converges the light rays without setting it to completely 0. In that case, by setting the α value as described above, it is possible to improve the balance with the characteristics of the image plane. Therefore, the image quality tends to improve substantially, and it is possible to better balance the α value for aberration correction and the image plane characteristics. On the other hand, if the field curvature changes in the positive direction during short-distance focusing, contrary to the above, it is possible to set the α value in a direction where the light rays diverge slightly to improve the balance with the field characteristics. It is.

In addition, in the present invention, since the negative surface refractive power of the aspherical surface introduced into the rear group G2R increases toward the periphery, the Rand ray with a high incident height enters the aspherical surface, and its incident angle When α becomes large, due to the influence of higher order aberrations, even α
Even if the value is convergent (positive), the spherical aberration may tend to be overcorrected.゛This is because the surface refractive power tends to be negative toward the periphery of the aspherical lens (therefore, the surface refractive power of the entire focusing group Ga1l also becomes closer to negative toward the periphery, and the higher the incident light In this case, the lens surface with negative surface refractive power protrudes at a certain position, so it tends to be over-corrected.This tendency of higher-order spherical aberrations can be incorporated into coma aberration correction, which is the main purpose of the present invention. It becomes possible to satisfactorily balance aberrations and spherical aberrations.

The angle α between the paraxial ray from the axial eyeless far point that enters the lens surface closest to the object side of the rear group G21 and the optical axis is:
In the paraxial ray tracing formula, it is well known as the value α multiplied by the refractive index of the object-side medium immediately in front of the surface, and is determined by the following ray tracing formula. That is, the initial value α1,h of the ray incident on the first surface closest to the object is α1=0.
, h, = f (synthetic focal length of the lens system) is a value determined by the following formula.

α to '-α to +h, φX α to ◆1 ``α to hl*1-hK ej dK' However, αx=NxUx αX'miN X ' U K' a N z + 1
U K * 1φK = (NK' NK)
/rk6, 'wd'/NK' rl: Radius of curvature of the surface hk: Incident height φ of the surface,: Surface refractive power U at the vertex of the surface,: Paraxial incident ray on the surface Angle dK with respect to the optical axis of
*-+: Regarding the refractive index direction for the d-line and the above paraxial tracking formula, see, for example, "Lens Design Method" by Yoshiya Matsui (
Kyoritsu Shuppan), pages 19-20.

In addition, with an appropriate refractive power arrangement that complies with condition (3), the rear group G
By setting the focal length □, coma aberration and field curvature can be well corrected. If the upper limit of condition (3) is exceeded, that is, if the focal length of the rear group czm is excessively large, it is possible to reduce the residual aberration of the rear group G□, but the This is extremely difficult to achieve as a lens for a single-lens reflex camera, which has an increased amount of movement and has limited bank focus. When the focal length of the rear group G2 is large, that is, when the focal length of the rear group G2. When the refractive power of the rear group G is small,
The surface refractive power of each surface in the second lens is weakened, and it becomes easy to reduce short-range fluctuations in coma aberration, especially in the upper coma aberration. However, since the amount of movement is large, the incident height of the oblique rays incident on the diverging first lens group G) changes greatly when focusing on a close object point. Therefore, the change in coma aberration on the lower side also becomes larger.

Originally, the fluctuation of the lower coma aberration is due to the divergent first lens group G,
It must be kept sufficiently small.

This is because the aspheric surface in the rear group G□ has a limit in correcting fluctuations in lower coma aberration. On the other hand, if the lower limit of condition (3) is exceeded, there is an advantage that the amount of movement of the rear group G2R is reduced, but it becomes difficult to correct aberrations, and the rear group G2R moves less.
It becomes difficult to configure the lens in the aluminum with two lenses, and the weight also increases. Since the focal length of the rear group G□ becomes smaller, the refractive power of each surface becomes thicker, which increases aberration fluctuations due to extension for focusing.Also, it is possible to suppress the aberration fluctuations. Even if this were the case, the deviation of the aspherical surface from the spherical surface would become large, and the upper limit of condition (1) would be exceeded.

Therefore, in the present invention, comatic aberration, spherical aberration, and field curvature can be suppressed to a small value by setting appropriate values such as conditions (11 to (3)).

Furthermore, rear group G! Since the L can be constructed with the minimum number of two lenses that can correct chromatic aberration, the weight can be reduced and the rear group G! ll
The amount of movement can be reduced. Particularly in the case of a camera lens incorporating an automatic focusing mechanism, the movable part for focusing needs to be small and light, and the amount of movement thereof needs to be small, so the present invention is very effective.

By the way, especially in a retrofocus type lens with a wide angle of view, maintaining a sufficient back focus imposes many restrictions on aberration correction. Moreover, in the present invention, a rear focus method is used in a retrofocus type lens having a large angle of view exceeding 94 degrees to focus from infinity to close range. You have to secure a space for movement (which results in further restrictions).

Therefore, the present invention increases the number of lens components,
In addition, without increasing the diameter of the front lens, the problem can be solved by configuring the lens in the first lens group G and the rear group G2R of the second lens group to have an appropriate shape so that the principal point of each lens is more visible to the image side. There is.

However, in a retrofocus lens employing a rear focus system like the present invention, it is necessary to particularly consider the shape of the rear group. In other words, as mentioned above, aberration fluctuations due to the rear focus method are largely related to the movement and shape of the lens for focusing, so the rear group G of the second lens due to focusing.

Due to this movement, a change in the incident height of the oblique rays incident on the rear group G11l becomes a problem. Therefore, in a retrofocus lens with a wide angle of view, consideration must be given to correcting aberrations of oblique rays in particular.

Therefore, the rear group G of the second lens of the present invention needs to have an appropriate shape with a concave surface facing the aperture so that oblique rays are refracted at the minimum angle of deviation.

Moreover, the axial ray is the rear group G of the second lens! , l, the spherical aberration changes significantly during focusing. Therefore, it is necessary to have an appropriate shape to alleviate this.

Therefore, it is desirable that the present invention be configured to satisfy the following condition (4).

r, +r.

16□≦-1, 2 (4) r, -r.

However, r: When the positive lens and negative lens that constitute the rear group Gtm are cemented, it is the radius of curvature of the rear group Oil closest to the object side, and When the positive lens and negative lens are separated, the radius of curvature on the object side of the positive lens.

r: When the positive lens and negative lens that make up the rear group G4 are cemented, it is the radius of curvature of the rear group G□ closest to the image side, and also makes up the rear group G. When the positive lens and negative lens are separated, the radius of curvature on the image side of the positive lens.

In this condition (4), especially the rear group G! , l defines the optimal shape of the positive lens component or positive lens, that is, the optimal shape factor, and moves the principal point toward the image side to ensure bank focus while eliminating spherical aberration caused by axial rays. In addition, coma aberration and field curvature due to off-axis rays are corrected extremely well.

However, if the upper limit of this conditional expression is exceeded, it is undesirable to attempt to perform good aberration correction while ensuring back focus, as this will result in an increase in the number of lens components and an increase in the size of the entire lens system. In addition, the positive lens component or positive lens in the rear group G□ of the second lens approaches a plano-convex shape, and oblique rays passing through this lens are refracted at a large deviation angle, resulting in comatic aberration and image plane during focusing. It becomes difficult to satisfactorily correct the curvature with the aspherical lens of the present invention. As a result, spherical aberration and external C and M aberrations occur significantly,
In addition, the field curvature changes in the positive direction, and the fluctuation of coma aberration due to the difference in the angle of view also increases.On the other hand, if the lower conditional expression is exceeded, bank focus can be secured, but spherical aberration will be insufficiently corrected. In addition, coma aberration and field curvature vary greatly, and variations in coma aberration due to differences in field angles also increase.

Furthermore, the edge thickness of the lens becomes extremely thin, making it difficult to manufacture.

In addition, in the present invention, the following conditions (5), (
It is desirable that the configuration satisfies 6).

r, +ra −1,6≦□≦0 −・−(5) r, -r.

ν, −R, ≧25 ·−・−(6J However, r4: The radius of curvature on the object side of the positive lens located closest to the image side of the front group G2F of the second lens group.

r,: The radius of curvature on the image side of the positive lens located closest to the image side of the front group G z F of the second lens group.

ν2: Atsube number of the positive lens in the rear group G□.

Sill: Atsube number of the negative lens in the rear group G.

If the configuration satisfies condition (5), the burden of aberration correction on the rear group of the second lens group, especially the burden on spherical aberration, can be reduced, and good aberration correction can be performed. The lower limit of this conditional expression is -1,25
It is preferable that it is above.

If the range of condition (6) is exceeded, especially the rear group G due to focusing
As the chromatic aberration of magnification fluctuates with the movement of t*, the curvature of the chromatic aberration of magnification also increases, making it difficult to perform good correction. Furthermore, if the configuration is such that −ν1≧28 is satisfied, better correction of chromatic aberration can be expected.

In order to minimize aberration fluctuations caused by oblique rays during focusing, all lens surfaces in the rear group of the present invention have a meniscus shape with a concave surface facing the aperture. desirable.

In addition, in the present invention, a negative lens is used for one song from the object side.
Although it is possible to configure the rear group to consist of a positive lens, in order to achieve good aberration correction, it is recommended to configure this rear group G□ to consist of a positive lens and a negative lens in order from the object side. desirable.

〔Example〕

Each embodiment according to the present invention will be described below. 1st
The example is a retrofocus lens with a focal length of 18 mm and an angle of view reaching 100° while maintaining a brightness of about 3.5, and the second and third examples have a focal length of 20 m + i and an F number of about 3.5. It is a retrofocus lens that has an angle of view of 946 degrees while maintaining a brightness of about 2.8 degrees. 1 to 3 show lens configurations of first to third embodiments of the present invention, respectively.

As shown in FIG. 1, in the first embodiment, the first lens group G1 having a negative refractive power is composed of four negative, positive, negative, and negative meniscus lenses with a convex surface facing the object side, and has a positive refractive power. The front group Gzr, which has a positive refractive power, consists of five lenses: a negative lens, a thick positive lens cemented to this, a positive meniscus lens, a negative lens cemented to this, and a positive lens. The rear group G2 consists of two positive and negative meniscus lenses cemented with their convex surfaces facing the image side.

In addition, in the second embodiment shown in FIG. 2, the first lens group G having negative refractive power includes three positive and negative meniscus lenses with convex surfaces facing the object side, a negative lens, and a negative lens. The front group G2. consists of five lenses including a cemented thick positive lens and has positive refractive power. consists of five lenses: a positive meniscus lens with a convex surface facing the object side, a positive meniscus lens with a convex surface facing the image side, a positive meniscus lens, a negative lens cemented to this lens, and a positive lens. The rear group G□, which has a refractive power of It consists of two meniscus lenses, positive and negative.

In this way, the rear group G! , l is as follows:

Here, h is the vertical height from the optical axis, k is the conic constant,
Ax, ka, A1, As, A+o are aspheric coefficients, and r is a radius of curvature at the paraxial axis. This aspheric surface in each example is the rear group G! It is provided on the concave surface on the object side of the positive meniscus lens located closest to the object side of *, and has a shape in which the negative surface refractive power becomes stronger as it gets closer to the periphery than the apex.

As shown in FIGS. 4 and 5, the cutout portion of the thick positive lens located closest to the object side of the front group G□ of the second lens group in each example is for determining the light beam. It is.

Tables 1 and 2 below show the first embodiment and the second embodiment according to the present invention.
In the table 0 showing the specifications of the examples, the leftmost number represents the order from the object, and the refractive index and Ambe number are d! l#I(λ-58
7.6). In addition, F8° is the F number, R is the distance from the image plane to the object, and β is the imaging magnification.
En in the aspherical shape is expressed as 10-''.

-11.

Fso”3.52 r d Abbe8 1
2.302 2.5019 -14.529
(Bf) B f 37.958 39.5
05] Yo! , il January 1L A+o=-0,1569B-10 2・F. -2,85 r d Abbe nB
f 37.896 40.095A1゜, 0
．． 1233B-11 3,3 FNDmo2.86 d Abbe n B f 37.886 40.058A+ol
IO, 1742E-11 Condition corresponding values in the first to third embodiments of the present invention described above are shown.

However, in As-5l □, the effective diameter of the lens having an aspherical surface in the first to third embodiments is 16.
70, 18.75.18.93.

The aperture S in the first embodiment is the second stop from the object side in the front group.
0.0 from the rear of the image side (11th surface) of the thick positive lens.
The line S in Examples 2 and 3 is located at the rear of the image side (13th surface) of a positive meniscus lens with a convex surface facing the second image side from the object side in the front group. 0.650 and 0.600, respectively.

Since each lens making up the rear group G has a meniscus shape with a concave surface facing the aperture S, it is particularly effective in reducing off-axis aberrations and fluctuations caused by short-range focusing. It has a very advantageous shape.

Various aberration diagrams for the above-mentioned first to third embodiments are shown in FIGS. 4 to 6, respectively.

In the aberration diagrams shown in Figs. 4 to 6, (a) shows the infinity focused state, the peaks) show various aberrations in the short range focused state B (0,3m), and d shows the aberrations at the d-line (5B ?, 6 nm), g
indicates the aberration for the g-line (435,8 nm), the broken line for astigmatism indicates the meridional image plane, and the solid line indicates the sagittal image plane.

Comparison of various aberrations shows that the present invention not only corrects various aberrations well over a range from infinity to short distances, but also extremely well corrects variations in lateral aberrations depending on the angle of view, resulting in excellent imaging performance. It can be seen that it has therefore,
The present invention has a focal length of 20 to 181 and an F number of 2.8 to 3.
．． Maintains a brightness of about 5 and the angle of view is 94° to 100°.
It is clear that the aberrations are corrected very well from infinity to short distances, and excellent imaging performance is maintained, even though the aberrations reach approximately 100°.

In addition, in the present invention, it is possible to provide an aspherical surface on one surface of the lenses constituting the rear group, which has the function of correcting aberration fluctuations due to close-range focusing. It is also possible to provide a spherical surface; in this case,
This aspherical surface has a shape in which the positive surface pressure bending force becomes weaker toward the periphery than the apex. As described above, this aspherical shape is equivalent to the characteristic shape of the present invention in which the negative surface stress rupture force becomes weaker toward the periphery than the apex.

Furthermore, in the case where the aspheric surface of the present invention is provided in the lens located closest to the image side in the rear group G□, it is desirable to slightly change the lens configuration on the object side from the aperture S.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to configure the structure with a small number of lenses, maintain a compact shape, secure sufficient bank focus, improve operability etc., widen the angle of view, and reduce the deterioration of short-range performance. In particular, fluctuations in spherical aberration, coma aberration, and field curvature are extremely well suppressed, and the aberration balance between coma aberration and field curvature is also good, and aberrations are corrected extremely well from infinity to short distances. A retrofocus type lens with high performance can be achieved.

[Brief explanation of the drawing]

FIGS. 1 to 3 are lens configuration diagrams of first to third embodiments of the present invention, respectively, and FIG. 4 (5) is a lens configuration diagram of the first to third embodiments of the present invention. FIG. 4(b) is a diagram of various aberrations in a state where the focus is on an object point at infinity in the first embodiment, and FIG. FIG. 1al is a diagram of various aberrations in a state where the focus is on an infinite relic point in the second embodiment of the present invention, and FIG. It is an aberration diagram. Figure 6 1a)
6 is a diagram of various aberrations in a state where the focus is on an object point at infinity in the third embodiment of the present invention, and FIG. It is. [Explanation of symbols of main parts] First lens group...G.

Claims (1)

[Scope of Claims] 1) A retrofocus lens comprising, in order from the object side, a first lens group G_1 having a negative refractive power and a second lens group G_2 having a positive refractive power, The two lens groups are the front group G_2_F and the rear group G_ with positive refractive power.
2_R, and the rear group G_2_R is composed of a positive lens and a negative lens, and has a shape in which the negative surface refractive power becomes stronger as it gets closer to the periphery than the apex, or it has a positive surface refractive power as it gets closer to the periphery than the apex. At least one surface has an aspherical surface shaped to weaken the force, an aperture is arranged closer to the object side than the aspherical lens in the rear group G_2_R, and the rear group G_2_R is arranged along the optical axis toward the object side. A retrofocus lens that focuses on close objects by extending it out. 2) The retrofocus type lens according to claim 1, wherein the retrofocus type lens satisfies the following conditions. 0<|AS-S|/f<0.2...(1) However, f: Focal length of the entire system AS-S: Light between an aspheric surface at the most peripheral effective diameter and a reference spherical surface having a predetermined apex radius of curvature Difference in axial direction. 3) The retrofocus type lens according to claim 2, wherein the retrofocus type lens satisfies each of the following conditions. -1.0≦α≦1.0...(2) 0.7≦f_2_R/≦6.0...(3) However, α: Injects into the lens surface closest to the object side of the rear group G_2_R of the second lens group The angle that a paraxial ray from an object point at infinity on the axis makes with the optical axis is a value determined by using the paraxial ray tracing formula with initial values α_1 = 0 and h_1 = f. α_1: Angle between the paraxial ray incident on the front lens surface and the optical axis. h_1: Height of incidence of paraxial rays incident on the frontmost lens surface. f_2_R: Focal length of the rear group G_2_R of the second lens group.