JPH0827428B2 - Retrofocus wide-angle lens - Google Patents

Description

The present invention relates to a retrofocus wide-angle lens incorporating a short-distance correction mechanism.

[Conventional technology]

In general, the focusing method of a retrofocus type photographic lens is often performed by moving the entire lens. However, since the whole lens system moves with a large weight, the total weight of the moving group including the lens barrel becomes heavy. Especially in the case of ultra wide-angle lenses and lenses with a very long back focus compared to the focal length, the negative refracting power of the front group increases, and the number of fake lenses that make aberration correction difficult also increases, and in general the front lens diameter also increases. As a result, the overall size and weight will be significantly increased. In particular, in the case of a camera lens or the like incorporating an automatic focusing mechanism, it is necessary to have good responsiveness, a small moving part and light weight, and a small moving amount. On the other hand, a retrofocus type lens having a two-group configuration consisting of a front group having a negative refracting power and a rear group having a positive refracting power is originally a type with a strong asymmetry, and therefore the object distance changes due to focusing and the photographing magnification increases. When it changes, various aberrations of the lens, especially astigmatism, field curvature, and coma, change remarkably as compared with other aberrations. Therefore, the following (a) ~
Efforts have been made to reduce the size and improve operability while correcting aberrations as in (d).

First, in order to correct the aberration change at the time of close-up photography, various retrofocus type photographic lenses have been proposed in which various mechanisms are added to move the entire lens group while extending the entire lens. For example, (a) Japanese Patent Publication No. 45-3987
It is disclosed in Japanese Patent No. 5 publication.

A second method is to fix two lenses from the object side and move all the lens groups on the image side to perform focusing, and (b) JP-A-61-140910. Is disclosed in.

A third method is a rear lens group moving-out method in which only the lens group closest to the image side is moved for focusing, and (c) Japanese Patent Laid-Open No. 55-1435.
No. 17, and (d) Japanese Patent Laid-Open No. 58-202414.

[Problems to be solved by the invention]

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

The method of adding a mechanism for moving a part of the lens groups while performing the first total extension has the drawback that the weight of the movable group is as heavy as the total extension and the total length is large. For example, (a) In the fully extended focusing of Japanese Patent Publication No. 45398398/1985, when the lens has a very long back focus compared to the super wide-angle lens or the focal length, the negative refracting power of the front group becomes strong. 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. Therefore, it is inconvenient to operate the system in which the short-distance correction is performed while performing the entire extension.

A method for focusing by fixing two lenses from the second object side and moving all lens groups on the image side of the second lens is disclosed in Japanese Patent Laid-Open No. 61-140910. However, the size of the focusing group, which is also a moving group, is large and heavy, and it is inconvenient to operate a camera lens or the like incorporating the automatic focusing mechanism as described above. In addition, this method of correcting various aberrations is not sufficient, and particularly in a super wide-angle lens having a large angle of view, changes such as coma aberration and short-distance photographing of field curvature are insufficiently corrected.

In the third rear lens group feeding system in which only the lens group closest to the image side is moved and focused, (c) in JP-A-55-143517, the movable lens group is small and an automatic focusing mechanism is incorporated. Although convenient as a lens for cameras, the angle of view is relatively small at 94 ° or less, and it has the drawback that changes in various aberrations at the time of short-distance shooting, especially changes in coma aberration are remarkable. Therefore, if the angle of view is further increased, it becomes very difficult to correct the aberration. Further, (d) In the case of Japanese Patent Application Laid-Open No. 58-202414, the weight of the focusing group can be reduced and the number of lenses can be reduced, which is convenient in operation, but as in (c), the angle of view can be reduced. Is as small as 84 ° or less, and changes in various aberrations at the time of close-up photography, especially coma, are remarkable, and it is difficult to achieve a wider angle.

Therefore, the present invention solves the above-mentioned drawbacks, maintains a wider angle of view and a compact shape, has good imaging performance during short-distance shooting, and has the same weight, size, and movement amount of the focusing group. An object of the present invention is to provide a rear focus type retrofocus wide-angle lens that can be reduced in size to improve operability.

[Means for solving problems]

In order to achieve the above object, the present invention has a retrofocus wide-angle lens system having a first lens group G ₁ having a negative refractive power and a second lens group G ₂ having a positive refractive power in order from the object side. This second lens group is a front lens group G _2F ,
And a rear group G _2R having a positive refractive power, the group G _2R Thereafter includes both a lens having a convex surface directed toward the image side and a positive lens component having a positive lens and a negative lens,
At least one surface has an aspherical surface with a shape in which the negative surface power becomes stronger toward the periphery of the vertex or a surface in which the positive surface power becomes weaker toward the periphery of the vertex, and the rear group G _2R A stop is arranged closer to the object side than the aspherical lens inside, and the rear lens group G _2R is adapted to focus on a short-distance object by moving the object toward the object side along the optical axis.

Then, in the present invention, the radius of curvature of the most object side and the most image side of the positive lens component in this rear group G _2R is respectively
If r _a and r _b , Is configured to satisfy.

In such a basic configuration of the present invention, it is preferable that the following conditions (2) to (4) are further satisfied in order to achieve sufficient aberration correction.

−1.0 ≦ α ≦ 1.0 (3) However, f: focal length of the entire system AS-S: difference in the optical axis direction between the aspherical surface at the periphery of the effective diameter and the reference spherical surface having a predetermined vertex radius of curvature.

α: The angle formed by the paraxial ray from the axial infinity object point incident on the most object side lens surface of the rear lens group G _2R of the second lens group with the optical axis in the paraxial crossing tracing formula, α = 0, h = f
The value obtained as.

α ₁ : The angle that the paraxial ray incident on the frontmost lens surface makes with the optical axis.

h ₁ : The incident height of paraxial rays incident on the front lens surface.

f _2R : Focal length of the rear lens group G _2R of the second lens group.

The surface refracting power referred to here is the difference between the incident angle and the exit angle of a certain ray of light incident on any one point of a certain refracting surface, that is, the deviation angle is the surface refraction of a minute surface near the refraction point. It is defined as a force, and when a parallel light beam incident near the refraction point converges after refraction, the refracting power of that surface is defined as a positive surface refracting power, and when diverging after refraction, the surface refracting power of that surface is a negative surface. Defined as refractive power.

Further, the aspherical surface in the present invention can be expressed as an aspherical surface shape in which the negative surface refractive power becomes stronger toward the periphery from the apex, and the positive surface refractive power becomes weaker toward the periphery from the apex. Can be expressed as, but in any case, the effect of the surface is substantially equivalent.

[Operation] In order to facilitate understanding before explaining the operation of the present invention,
First, the second one having the focusing function in the configuration of the present invention as described above.
Retrofocus when using a general spherical lens that does not include an aspherical lens (a lens having an aspherical surface whose negative surface refractive power increases toward the periphery from the apex) in the rear group G _2R in the lens group A qualitative description will be given of the principle of variation in aberration of the D-type lens, particularly variation in spherical aberration and coma.

Spherical aberration Rays a and b shown in Fig. 2A are incident on the lens system Ra.
nd ray (marginal ray), a is a ray from an object point at infinity, and b is a ray from an object point at a close range. And the second A
As shown in the figure, the rear lens group G _2R , which has the positive refracting power and is closest to the image side, is focused on the closest object point by extending in the object side direction, and is incident on the most object side surface of the rear lens group G _2R. When the Rand ray has a converging property (α value in the paraxial ray tracing equation is positive), the incident height of the Rand ray a incident on the rear group G _2R becomes high because it is extended. Therefore, by feeding the rear group G _2R having a positive refractive power toward the object side, the positive surface refractive power for the Rand ray a incident from the closest object point of the rear group G _2R is increased. Therefore, since the positive surface refracting power becomes stronger, the spherical aberration when focusing on an object point at infinity is more than the spherical aberration when focusing on an object point at infinity due to insufficient correction as shown in FIG. 2B as a whole. Change direction.

Also, when the Rand ray incident on the most object side surface of the rear group G _2R is divergent (when the α value in the paraxial ray tracing equation is negative), it is positive because of the completely opposite reason. By extending the rear group G _2R having a refractive power of, the spherical aberration changes in the overcorrection direction.

However, Rand incident this case, the amount and the refractive power of the magnitude of the residual aberrations with the rear group G _2R, the amount of movement of the large and small, the surface power of each surface in the rear group G _2R magnitude, in the rear group G _2R The degree of change in spherical aberration varies depending on the size of the incident ray and the lens shape of the rear group G _2R .

Coma Aberration Next, focusing on the oblique rays, when the rearmost group G _2R having the positive refracting power, which is closest to the image side, is extended toward the object to focus, the oblique rays of c in FIG. In comparison with the ray incident from the infinite object point, the oblique ray incident from the closest object point like the ray d in the figure is not parallel to the optical axis, and is incident on the rear group G _2R. The height becomes low, and the surface refractive power of the rear group G _2R for oblique rays becomes weak. Therefore, the positive surface refracting power is weakened, but to the extent that the upper ray and the lower ray in the oblique ray have different incident heights of the rays incident on the rear group G _2R , like the rays in FIG. The surface refracting power for oblique rays is different. Then, when the rear group G _2R having a positive refracting power is extended to the object side, the state varies depending on the size of the refracting power of each group, the surface refracting power of each surface, the amount of residual aberration, the amount of movement, etc. As shown in the figure, since the positive surface refracting power of the rear group G _2R is weakened, the upper and lower oblique rays bounce up, so that the symmetry of coma aberration is broken and the coma aberration is significantly changed in the outer coma direction.

Therefore, in general, focusing by the rear group extension method of a retrofocus type lens is caused by the above reasons.
Aberrations such as spherical aberration, coma and field curvature change remarkably.

As shown in the explanation of the qualitative principle as described above, when the rear lens group G _2R having a positive refractive power is extended to the object side for focusing, the rear lens group G _2R with respect to the most object side surface is height of incidence oblique light rays incident on the group G _2R becomes lower, surface power for oblique rays of the rear group G _2R is weakened. Therefore, as shown in FIG. 3A, in the upper ray and the lower ray in the oblique ray, the rear group G
_Since the incident height on the _2R is different, the degree of fluctuation is different, but the positive surface refracting power is weakened, so that each oblique ray bounces up and changes in the outward coma direction.

To solve the problem of such aberrations, in the present invention as described above, positive when unwinding the object side to the group G _2R after having a refractive power, positive surface refraction by reduction of the incidence height of the group G _2R post A lens having a negative surface refracting power, which has the opposite effect to the action of reducing the power, is introduced into the rear group G _2R to have an appropriate positive and negative refracting power arrangement. Then, these actions cancel each other out to reduce the fluctuations of the respective surface refracting powers. In other words, by introducing a lens having a strong negative surface power from a group G _2R after having positive surface power in the rear group G _2R, when the group G _2R After positive refractive power took to the object side, Rear group G
Positive as greater than the decrease of the positive surface power of negative decrease rear group surface power in G _2R in the rear group G _2R due to the decrease of the incidence height of the oblique light rays incident on _2R, negative refractive power By arranging, the positive refracting power for oblique rays does not change so much in the entire rear group even if the incident height for oblique rays in the entire rear group G _2R changes as before. Therefore, the fluctuation of coma can be suppressed to a minimum.

Therefore, as shown in FIG. 1, an aspherical lens having a difference in surface refracting power between the periphery and the center of the lens, that is, a spherical lens whose negative surface refracting power increases toward the periphery is introduced. As a result, the above effect can be supplemented by one lens. Here, e is an oblique ray from an object point at infinity, and f is an oblique ray from an object point at a close distance.

This method is particularly effective for the rays above the oblique rays and can suppress the fluctuation of coma aberration to a minimum.
Further, the amount of zag of the aspherical lens, that is, the amount of deviation (| AS−S |) from the reference spherical surface of the apex depends on the refractive power of the apex of the positive lens in the rear group, the refractive power of the apex of the aspherical lens, and the extension. The amount of movement is determined by the residual aberration of the lens system, etc., and varies depending on the location where the aspherical lens is introduced and the effective diameter, but only the rear group G _2R having a positive refractive power is extended toward the object side in parallel with the optical axis for focusing. In the present method, the aspherical shape is required so that the negative surface refractive power increases from the apex (center) of the lens toward the periphery for the above reasons.

As described above, the present invention introduces an aspherical lens in the rear lens group G _2R to prevent the above-mentioned deterioration of short-range performance (especially deterioration of coma aberration) and prevent and correct aberration fluctuations up to a close range. Is. That is, although it is possible to correct aberrations practically for light rays from an object point at infinity without introducing an aspherical surface, a retrofocus lens system that cannot completely correct aberration fluctuations when focusing at a short distance. In, in the rear lens group of the second lens group having the focusing function, an aspherical surface having a property of preventing deterioration of near distance performance of the main lens can be introduced to correct aberration at the time of focusing at a short distance. It becomes.

Therefore, the present invention is a method of mainly correcting distortion, image surface curvature, etc. by using an aspherical lens in the vicinity of the frontmost or rearmost part of the optical system, or by arranging in the vicinity of the diaphragm position to mainly spherical aberration or Unlike the conventional general aspherical lens such as the method of correcting sagittal coma flare, it is different in the idea of the aberration correction method.

By the way, particularly in a retrofocus wide-angle lens having a wide angle of view, maintaining sufficient back focus imposes many restrictions on aberration correction. Moreover, in the present invention, the rear focus system is adopted for the retrofocus wide-angle lens having a large screen of more than 100 °, and an attempt is made to focus from infinity to a close range. The movement space must be secured, which results in further restrictions. Therefore, according to the present invention, the first lens group G ₁ can be formed without increasing the number of lens components or increasing the front lens diameter.
And the rear lens group G _2R of the second lens group is configured to have an appropriate shape so that the principal points of the respective lenses are located closer to the image side.

However, in the retrofocus wide-angle lens adopting the rear focus system as in the present invention, it is necessary to particularly consider the shape of the rear group.

That is, as described above, since the aberration variation due to the rear focus system is largely related to the movement and shape of the focusing lens, the movement of the rear group G _2R of the second lens due to the focusing causes
The change in the incident height of the oblique ray incident on the rear group G _2R becomes a problem. Therefore, in a retrofocus wide-angle lens with a wide angle of view, it is necessary to consider aberration correction of oblique rays in particular.

Therefore, in the rear group G _2R of the second lens of the present invention,
It must have a suitable shape with the concave surface facing the diaphragm so that the oblique rays are refracted with the minimum deviation.

Further, when the incident angle of the axial ray incident on the rear group G _2R of the second lens is extremely large, the spherical aberration fluctuates remarkably during focusing. Therefore, it is necessary to have an appropriate shape for reducing this.

Therefore, in order to satisfy the above, in the present invention, in particular, the optimum shape of the positive lens component of the rear group G _2R , that is, the optimum shape factor (shape factor)
Condition (1) defines the principal point to the image side and secures the back focus, while correcting not only spherical aberration due to on-axis rays but also coma aberration due to off-axis rays and field curvature. are doing.

However, if the upper limit of this conditional expression is exceeded, an attempt is made to perform good aberration correction while securing the back focus,
This is not preferable because it causes an increase in the number of lens components and an increase in the size of the entire lens system. Further, the positive lens component in the rear lens group G _2R of the second lens approaches a plano-convex shape, and the oblique ray passing through this lens is refracted with a large deviation angle, so that the coma aberration and the curvature of field at the time of focusing are reduced. It becomes difficult to satisfactorily correct with the aspherical lens of the present invention. Therefore, as a result, spherical aberration and outer coma are greatly generated, the field curvature fluctuates in the positive direction, and fluctuation due to the difference in the angle of view of coma also increases. On the other hand, when the value goes below the lower limit of the conditional expression, the back focus can be secured, but the spherical aberration is undercorrected, the coma aberration and the image plane curvature largely change, and the coma aberration also increases due to the difference in the angle of view. In addition, the edge thickness of the lens becomes extremely thin, which makes manufacturing difficult.

Furthermore, in order to achieve sufficient aberration correction, (2)
It is desirable to be configured to satisfy the conditions (4) to (4), and the conditions will be described in detail below.

When the value exceeds the upper limit of the condition (2), it becomes difficult to form an aspherical surface, so that there are problems due to processing tolerances and performance deterioration due to decentering, which deteriorates the productivity of the aspherical lens. In addition, spherical aberration increases due to the influence of higher orders (terms of 5th and higher) due to the Rand ray having a high incident height, and the optical performance deteriorates. Therefore, the zag amount of the aspherical lens is practically desirable in this range. Within the range of the condition (2), an aspherical lens whose negative surface refracting power for oblique rays increases as at least one surface becomes closer to the periphery than the apex is set in the rear group of the second lens group. By combining appropriate positive and negative surface refracting powers, it is possible to reduce the fluctuation of coma aberration up to the closest distance.

Regarding the 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 the condition (3), the rear group G _2R , which is the focusing group, is closest to the object side. The Rand ray incident on the surface at can be made parallel to the optical axis. When the Rand rays become parallel to each other, even if the rear group G _2R is extended to focus on a close-range object point, the incident height of the Rand rays incident on each surface of the rear group hardly changes. Therefore, the change in the surface refractive power of each surface with respect to the Rand ray is suppressed to be small, and the fluctuation of the spherical aberration due to the extension for focusing is significantly reduced. Therefore, α in the above condition (3)
When the value exceeds 1.0, the rear lens group G having the positive refractive power described above for focusing from an infinite object point to a short-distance object point.
_{When 2R} is extended in the object direction, the Rand ray incident on the most object-side surface of the rear group G _2R is remarkably converged, and therefore, the extension of the Rand ray is significantly increased by extending the Rand ray. The refracting power of is significantly increased, and the spherical aberration is significantly changed in the direction of undercorrection. Also, the α value is −
When it is less than 1.0, it is the exact opposite of the above reason, the incident height of the Rand ray incident on the most object side surface of the rear group G _2R is low, the positive surface refraction to the Rand ray is weakened, and the spherical aberration is corrected. Change significantly in the excess direction.

In the present invention, the image plane curvature may fluctuate in the negative direction when focusing on a short distance. In this case, the above α value should not be set to 0 completely, but should be set in a direction in which the light rays converge slightly. Is desirable. In that case, by setting the α value as described above, the balance with the characteristics of the image plane can be improved. Therefore, the image quality tends to substantially increase, and it is possible to better balance the α value for aberration correction and the image plane characteristic. On the other hand, when the image surface curvature fluctuates in the positive direction when focusing on a short distance, the α value can be set slightly in the direction in which the light rays diverge, and the balance with the image surface characteristics can be improved. Is.

Here, regarding the shape of the aspherical lens in the present invention, by increasing the negative surface power of the peripheral portion and setting the negative surface power of the apex (center) to 0 or a value close to 0,
It is also possible to correct high-order spherical aberration due to Rand rays with a high incident height.

Incidentally, in the present invention, since the negative surface refracting power of the aspherical lens introduced into the rear group G _2R becomes larger toward the periphery, a high incident height Rand ray enters the aspherical lens, When the incident angle becomes large, the spherical aberration may change to a tendency of overcorrection even if the α value is convergent (plus) due to the influence of higher-order aberration. This is because the surface refracting power is negatively inclined toward the periphery of the aspherical lens, so that the surface refracting power of the entire rear lens group is also close to negative toward the periphery, and is negative at the position of high incident light. The lens surface having the surface refracting power is extended, so that there is a tendency for overcorrection. Such a tendency of high-order spherical aberration can be incorporated into the coma aberration correction, which is the main object of the present invention, so that the coma aberration and the spherical aberration can be well balanced.

Then, the angle α formed by the paraxial ray from the axial infinity object point incident on the most object-side lens surface of the rear group G _{2R and} the optical axis,
In the paraxial ray-tracing equation, it is well known as a value α multiplied by the refractive index of the medium on the object side immediately before the surface, and is calculated by the following ray-tracing equation. That is, the initial values α ₁ , h ₁ of the light ray incident on the first surface closest to the object are α ₁ = 0,
It is a value obtained by the following formula, where h ₁ = f (composite focal length of lens system).

α _K ′ = α _K + h _K φ _K α _{K + 1} = α _K ′ h _{K + 1} = h _K −e _K ′ d _K ′ where α _K ≡N _K U _K α _K ′ ≡N _K ′ U _K ′ ≡N _{K + 1} U _{K + 1} φ _K = (N _K ′ −N _K ) / r _k e _k ′ ＝ d _K ′ / N _K ′ r _k : radius of curvature of the k-th surface h _K : k-th surface Incident height φ _K : Surface refractive power of the apex of the k-th surface U _K : Angle of the paraxial incident ray to the k-th surface with respect to the optical axis d _K : Spacing of the apex between the k-th surface and the (k + 1) -th surface N _K , N _{K + 1} : refractive index for d-line The above paraxial tracking equation is described in detail in, for example, Yoshiya Matsui, “Lens Design Method” (Kyoritsu Shuppan), pages 19 to 20.

If the focal length of the rear group G _2R is determined with an appropriate refractive power arrangement that satisfies the condition (4), coma and field curvature can be corrected well. When exceeding the upper limit of condition (4), if the excessively large focal length of the rear group G _2R i.e., it is possible to reduce the residual aberrations of the rear group G _2R, the group G _2R after for focusing It is very difficult to realize as a lens for a single-lens reflex camera that has an increased amount of movement and has a limited back focus. When the rear lens group G _2R has a large focal length, that is, when the rear lens group G _2R has a small refractive power, the surface refractive power of each surface in the rear lens group G _2R is weakened, and the coma aberration, especially the upper coma aberration at a short distance is combined. It becomes easy to reduce the fluctuation during focusing. However, since the amount of movement becomes large, the incident height of the oblique ray incident on the divergent first lens group G ₁ changes greatly when the object point is focused at a short distance. Therefore, the change in the coma on the lower side also becomes larger. The fluctuation of the coma aberration on the lower side should be sufficiently small in the divergent first lens group G ₁ . This is because the aspherical surface in the rear group G _2R has a limit in correcting the fluctuation of the lower coma aberration. On the other hand, when the value goes below the lower limit of the condition (4), there is an advantage that the amount of movement of the rear lens group G _2R decreases, but it becomes difficult to correct aberrations, and the rear lens group becomes difficult.
It increases the number of G _2R of the lens, thereby also increasing the weight. Then, since the focal length of the rear group G _2R becomes small and the refracting power of each surface becomes large, variation in aberration due to extension for focusing is also increased. Even if the variation of the aberration can be suppressed, the deviation of the aspherical surface from the spherical surface at that time becomes large, and the upper limit of the condition (2) is exceeded.

Therefore, in the present invention, coma aberration, spherical aberration, and spherical aberration can be reduced by setting appropriate values such as conditions (1) to (4).
Image curvature can be kept small. Furthermore, rear group G _2R
The number of lenses in the inside can be reduced, and the weight is reduced.
A configuration in which the amount of G _2R movement is small may be possible. In particular, in the case of a camera lens or the like incorporating an automatic focusing mechanism, the movable portion for focusing is small and light, and it is necessary to reduce the moving amount thereof, so the present invention is very effective.

Further, in the present invention, it is desirable that the present invention is further configured to satisfy the following conditions.

However, r _d : radius of curvature of the positive lens component located closest to the image side of the front lens group G _2F of the second lens group on the object side.

r _e : the image-side radius of curvature of the positive lens component located closest to the image side in the front lens group G _2F of the second lens group.

It is very effective to configure the present invention so as to satisfy the above condition (1). However, if the present invention is further configured to satisfy the condition (5), the aberration will be increased in the case of widening the angle of view. This is advantageous.

If the condition (6) is satisfied, the second
The burden of aberration correction on the rear group of the lens group is reduced, and good aberration correction can be performed.

Further, in the present invention, in order to make the rear group G _2R as small and lightweight as possible, in order to make the rear lens group G _2R as small as possible, an aspherical lens having an aspherical shape, and a positive lens and a negative lens are cemented together. It is desirable to be composed of three lenses with a lens component having a shape. And
When a lens having an aspherical surface is made of plastic, asphericalization is easier than that of a glass lens, mass production efficiency is good, and it is very effective.

Further, setting the apex refractive power of a lens having an aspherical surface to a value close to 0 is advantageous in preventing changes in the focal length, back focus, and axial chromatic aberration due to temperature changes of the plastic.

Specifically, it is more effective if the apex refractive power φ of the lens having an aspherical surface satisfies the following condition.

f _ASP : Paraxial focal length (mm) of a lens having an aspherical surface.

Furthermore, it is most advantageous to make the lens on the image side aspherical for the correction of image field curvature and coma aberration, but it is easy to touch when considering safety because it uses a plastic lens. It is desirable to install it where it cannot be found.

The first surface of the lens having an aspherical surface is astigmatism,
In order to suppress the influence of image plane curvature to a minimum, it is desirable that the aperture is concave.

In the present invention, it is desirable that the positive lens component in the second lens group as described above is cemented with a positive lens and a negative lens to form a meniscus shape with a convex surface facing the image side as a whole. It is preferable to be configured to satisfy the condition of.

| r _a |> | r _c |, r _c <0, r _a <0 (8) ν _p −ν _n ≧ 25 (9) r _c : to the image side in the rear group of the second lens group The radius of curvature of the cemented surface in the positive meniscus lens component with the convex surface facing.

ν _p : Abbe number of the positive lens in the positive meniscus lens component with the image side facing the convex surface in the rear group of the second lens group.

ν _n : Abbe number of the negative lens in the positive meniscus lens component with the convex surface facing the image side in the rear group of the second lens group.

If the condition (8) is satisfied, the cemented surface in the positive meniscus lens component with the convex surface facing the image side in the rear group of the second lens group has a concentric shape with respect to the diaphragm position. In particular, the occurrence of coma and field curvature can be reduced, and the curvature of lateral chromatic aberration can also be suppressed to a relatively small level.

If configured so as to satisfy the condition (9), but the variation in lateral chromatic aberration generated by the movement of the rear group of the second lens group by focusing can be corrected well and further, [nu _p -
It is desirable to configure so as to satisfy ν _n ≧ 28.

If the refractive power of the front lens group G _2F of the second lens group of the present invention is positive, the positive refractive power of the second lens group as a whole will be the rear lens group G _2F.
Since it can be shared with the _2R , the lens mechanism can be simplified and the imaging function can be improved.

〔Example〕

Each embodiment according to the present invention will be described below. Each of the embodiments is a retrofocus type wide-angle lens that achieves an angle of view of 100 ° while maintaining an F number of 3.5 at a focal length of 18 mm.

4 and 5 respectively show the first and second embodiments of the present invention in order. The second embodiment also has a lens configuration similar to that of the first embodiment shown in FIG.

The rear group G _2R in each example has, in order from the object side, an aspherical lens having an aspherical surface on the image side and a meniscus lens component having a positive refracting power as a whole. Has a positive meniscus lens having a convex surface directed toward the image side, and a negative meniscus lens joined to the positive meniscus lens having a convex surface directed toward the image side.

The aspherical equations regarding the shape of the aspherical lens for aberration correction due to short-distance focusing in the rear group G _2R in each example are as follows.

Here, h is the height in the vertical direction from the optical axis, K is the conical constant, A ₂ , A ₄ , A ₆ , A ₈ and A ₁₀ are aspherical coefficients, and r is the paraxial radius of curvature.

This aspherical surface in each example is provided on the image-side convex surface of the lens located closest to the object side in the rear group G _2R , and has an aspherical surface whose positive surface power becomes weaker toward the periphery of the vertex. There is. As described above, this aspherical shape is equivalent to the characteristic shape in the present invention, that is, the shape in which the negative surface refractive power becomes stronger toward the periphery of the apex. The aspherical lens in the first example has a positive refracting power as a whole, and the aspherical lens in the second example has a negative refracting power as a whole.

As shown in FIGS. 4 and 5, the notch portion of the thick positive lens located closest to the object side of the front lens group G _2F of the second lens group in each embodiment is for determining the light ray. Is.

Tables 1 and 2 below show specifications of the first and second embodiments according to the present invention. In the table, the numbers on the left end represent the order from the object, and the refractive index n and the Abbe number ν are d lines (λ = 587.
It is a value for 6). R is the distance from the image plane to the object, β is the magnification, and E-n in the aspherical shape is
^Expressed as 10- ⁿ .

The condition corresponding values in the above-described first and second embodiments of the present invention are shown.

The aspherical lens in the first example is composed of a plastic lens having an aspherical shape on the image side, and the aspherical lens in the second example is a glass lens having an aspherical shape on the image side and this lens. It is formed of a compound lens that is compounded with a thin plastic layer along the aspherical surface.

Further, the diaphragm S in the first and second embodiments is the second
It is arranged at a position of 0.500 from the image side surface (third surface) of the thick positive lens which is located closest to the object in the lens group.

Each lens that constitutes the rear lens group G _2R has a diaphragm S
On the other hand, since it has a meniscus shape with a concave surface facing, the shape is very advantageous especially for reducing off-axis aberrations and aberration fluctuations due to focusing at a short distance.

Aberration diagrams of the first to second examples are shown in order in FIGS. 6 to 7, respectively.

In the aberration diagrams of FIGS. 6 to 7, (a) shows various aberrations in the infinity in-focus state, (b) shows various aberrations in the short-distance in-focus state (0.3 m), and d represents the d line (587.6 nm). ), G is the g line (4
35.8 nm), the broken line in astigmatism indicates the meridional image plane, and the solid line indicates the sagittal image plane. The broken line (SKEW) in the lateral aberration in the aberration diagram is the lateral aberration in the direction perpendicular to the meridional direction.

From the comparison of each aberration, the present invention shows that not only various aberrations are favorably corrected from infinity to a short distance, but also variation in lateral aberration due to the angle of view is extremely favorably corrected, and excellent imaging performance is obtained. It turns out that it has. Therefore, the present invention maintains the brightness of F number 3.5 at the focal length of 18 mm, and despite the angle of view reaching about 100 °, the aberration is excellently corrected from infinity to the short distance, and the excellent imaging performance is obtained. It is clear that they are maintaining.

In each of the embodiments, the positive meniscus lens component with the convex surface facing the image side in the rear group of the second lens unit is made up of positive and negative components. It is also possible to improve.

〔The invention's effect〕

As described above, according to the present invention, while maintaining a compact shape, operability and the like can be improved and a wide angle can be achieved, and deterioration of short-distance performance, in particular, variation of spherical aberration, coma aberration and curvature of field is extremely excellent. In addition, the aberration balance between coma and field curvature is improved, and a retrofocus wide-angle lens having excellent performance in which aberration is corrected extremely well from infinity to a short distance can be achieved.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram according to the present invention, FIG. 2A is an optical path diagram showing the principle of spherical aberration generation, FIG. 2B is an aberration diagram showing comparison of spherical aberration generation, and FIG. 3A is comparison of coma aberration generation. Optical path diagram, FIG. 3B is an aberration diagram showing a comparison of coma aberration generation, FIG. 4 is a lens configuration diagram of a first embodiment according to the present invention, and FIG. 5 is a lens configuration diagram of a second embodiment according to the present invention. FIG. 6 (a) is a diagram showing various aberrations of an object point at infinity in focus in the first embodiment of the present invention, and FIG. 6 (b) is in focus at a short distance object point in the first embodiment of the present invention. FIG. 7 (a), which shows various aberrations under the condition
Is a diagram of various aberrations when focused on an object point at infinity in the second embodiment of the present invention, and FIG. 7B is a diagram of aberrations when focused on an object point at a short distance according to the second embodiment of the present invention. It is a figure. [Explanation of Signs of Main Parts] First lens group …… G ₁ Front group of second lens group …… G _2F (Second lens group G ₂ ) Rear group of second lens group …… G _2R (Second lens Group G ₂ )

Claims (3)

[Claims] 1. A first lens element having a negative refractive power in order from the object side.
A retrofocus type lens having a lens group G ₁ and a second lens group G ₂ having a positive refractive power, wherein the second lens group is a front group G _2F and a rear group G having a positive refractive power. _2R , the rear group G _2R includes a lens having a convex surface facing the image side, and a positive lens component having a positive lens and a negative lens, and a negative surface refraction toward the periphery from the apex. At least one surface has an aspherical surface having a shape in which the force becomes strong or a positive surface refractive power becomes weaker toward the periphery of the apex, and is closer to the object side than the aspherical lens in the rear group G _2R. A diaphragm is disposed, the rear group G _2R performs focusing on a short-distance object by extending toward the object side along the optical axis, and the most object side and the most image of the positive lens component in the rear group G _2R. If the radii of curvature on the sides are r _a and r _b , respectively, A retro-focus wide-angle lens characterized by satisfying 2. The retrofocus type lens according to claim 1, wherein the retrofocus type wide-angle lens satisfies the following conditions. However, f: focal length of the entire system AS-S: difference in the optical axis direction between the aspherical surface at the periphery of the effective diameter and the reference spherical surface having a predetermined vertex radius of curvature. 3. The retrofocus wide-angle lens according to claim 2, wherein the retrofocus wide-angle lens satisfies the following conditions. −1.0 ≦ α ≦ 1.0 (3) Where α is the initial value in the paraxial ray-tracing equation, which is the angle formed by the paraxial ray from the axial infinity object point incident on the most object-side lens surface of the rear group G _2R of the second lens group in the paraxial ray-tracing equation. Α ₁ = 0, h ₁ =
The value obtained as f. α ₁ : The angle that the paraxial ray incident on the frontmost lens surface makes with the optical axis. h ₁ :: Incident height of paraxial ray incident on the frontmost lens surface. f _2R : Focal length of the rear lens group G _2R of the second lens group.