JP2001108896A - Telephoto lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the fluctuation of various aberrations from an infinite distance to a short distance. SOLUTION: The telephoto lens is composed of a 1st lens group G1 with positive refracting power, a 2nd lens group G2 with negative refracting power, and a 3rd lens group G3 with positive or negative refracting power in order from the object side; and the 1st lens group G1 is composed of at least two positive lenses L1 and L2 and one negative lens L3, the 2nd lens group G2 is composed of a negative lens L4 and a positive lens L5 in order from the object side, and the 3rd lens group G3 is composed of a positive lens L6 and a negative lens L7 in order from the object side. This is telephoto lens is put in focus from an infinite distance to a short distance by moving the 2nd lens group G2 toward an image plane on the optical axis and the positive lens L6 in the 3rd lens group G3 toward the object on the optical axis.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a telephoto lens of an inner focus type in which various aberrations change little from infinity to a short distance.

[0002]

2. Description of the Related Art Generally, the entire focusing is a basic focusing method for a photographic lens or the like regardless of a telephoto lens. Particularly, in a telephoto lens, the longer the focal length, the larger the lens itself becomes and the more the focusing becomes. Since the amount increases rapidly, a burden is imposed on the focusing mechanism, making it difficult to obtain a sufficient shortest photographing distance, and causing a disadvantage that the balance of the lens is impaired and the operability is deteriorated.

In recent years, in order to solve such a problem, an “inner focus” or “rear focus” system in which a front lens group in the entire lens system is fixed and a middle lens group or a rear lens group is moved to perform focusing. Has become mainstream. In particular, the inner focus method does not change the overall length of the lens and only needs to move a relatively small lens for a short distance, so there is no instability of the camera, the focusing operation is light, and the operability can be greatly improved. it can.

In such an inner focus system, conventionally, in general, a three-group structure including a positive front group, a negative focusing group, and a positive or negative fixed group from the object side is generally used. However, in such a configuration, due to the movement of the focusing group,
Considering marginal rays and off-axis chief rays that exit the focusing group and enter the rear group, the ray height when these rays enter the rear group changes due to the movement of the focusing group. The height of incident light on the rear lens unit when focusing on a distance is reduced, and the fluctuation of the incident light height is one factor of fluctuation of aberration.

[0005] In view of these points, since the development of zoom lenses has recently increased, some telephoto lens systems have disclosed several focusing methods using two or more groups. For example, in JP-A-59-142511 and JP-A-60-418, the entire system is composed of positive, negative, and positive lens units from the object side, and the negative lens unit is used for image formation. By moving the positive lens group on the image side to the object side, focusing on a short-distance object is performed.

[0006] Further, in Japanese Patent Application Laid-Open Nos. 1-237611 and 7-301749, the entire system is composed of positive, negative, positive and negative lens groups from the object side, By moving the negative lens group to the image side and the image-side positive lens group to the object side, focusing on a short-distance object is performed, and both of them can reduce aberration fluctuation during focusing. It has been shown.

[0007]

However, in such a conventional telephoto lens, since the F value is brighter than F4, the lens diameter tends to be large and the overall length tends to be long.
Particularly, since the diameter of the front lens group becomes large, there is a problem that the diameter of the subsequent focusing lens group also becomes large. Each of these telephoto lenses considers the correspondence to autofocus (AF), and the size, weight,
Since it is necessary to keep the amount of movement small, the refractive index of each lens group, particularly the focusing lens group, is set to be relatively large, and the aberration fluctuation during focusing cannot be sufficiently suppressed.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and in a relatively dark telephoto lens having an angle of view of about 8 ° at about F5.6, various aberrations from infinity to a close distance of about 0.17 in magnification are provided. It is an object of the present invention to provide an inner focus type telephoto lens with little fluctuation of the lens.

[0009]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides, in order from the object side, a first lens having a positive refractive power.
The first lens group includes a lens group, a second lens group having a negative refractive power, and a third lens group having a positive or negative refractive power. The first lens group has at least two positive lenses and one negative lens. The second lens group is negative,
The third lens group is composed of two positive and negative lenses in order from the object side. The third lens group is formed by moving the second lens group in the image plane direction on the optical axis. By moving each positive lens in the direction of the object on the optical axis,
An object of the present invention is to provide a telephoto lens that performs focusing from infinity to a short distance and satisfies the following conditional expression. 1.0 <| f2 | / f <2.4 (1) Ri, Rii> 0 (2) where f1: focal length of the first lens group f2: focal length of the second lens group Ri, Rii: respectively Radius of curvature of image side of negative lens and positive lens of two lens groups

In the above telephoto lens, it is preferable that the following conditional expression is satisfied. 0.47 <f31 / f <0.9 (3) where, f: focal length of the entire system f31: focal length of the positive lens in the third lens group

In the above telephoto lens, it is more preferable that the following conditional expression is satisfied. V6 <55 (4) Ni, Nii> 1.7 (5) where V6: Abbe number of the positive lens in the third lens group Ni: refractive index of the negative lens in the first lens group Nii: second lens group Refractive index of medium negative lens

By configuring the telephoto lens according to the present invention as described above, the fluctuation of the height of the incident light beam to the third lens group is suppressed, and various aberrations during focusing, particularly chromatic aberration, spherical aberration, and astigmatism, are further reduced. In addition to reducing the fluctuation, it is possible to make the positive lens in the third lens group bear the focusing action.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments and examples of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a configuration diagram showing a state when focusing on an object at infinity according to a first embodiment of the present invention, FIG. 2 is an aberration curve diagram thereof, and FIG.
FIG. 4 is a configuration diagram showing a state when focusing on the close object, and FIG. 4 is an aberration curve diagram thereof.

In the present invention, as shown in FIG. 1, the first lens group G1 having a positive refractive power is arranged in order from the object side.
, A second lens group G2 having a negative refractive power, and a third lens group G3 having a positive or negative refractive power. The first lens group G1 includes at least two positive lenses L
The second lens group G2 includes a negative lens L4 and a positive lens L5 in order from the object side, and the third lens group G2 includes a positive lens L6 in order from the object side.
And a negative lens L7 sandwiching the stop S. And the second
By moving the lens group G2 toward the image plane on the optical axis and moving the positive lens L6 in the third lens group G3 toward the object on the optical axis, focusing from infinity to a short distance is performed.

In the above-mentioned lens configuration, the following conditional expressions (1) to (5) should be satisfied. 1.0 <| f2 | / f1 <2.4 (1) Ri, Rii> 0 (2) 0.47 <f31 / f <0.9 (3) V6 <55 (4) Ni, Nii (5) Where, f1: focal length of the first lens group G1 f2: focal length of the second lens group G2, Ri, Rii: radius of curvature (R8, R10) of the image surface of the negative lens L4 and the positive lens L5 of the second lens group G2 F: focal length of the entire system f31: focal length of the positive lens L6 in the third lens group G3 V6: Abbe number of the positive lens L6 in the third lens group G3 Ni: negative lens L3 in the first lens group G1 Index of refraction (N
3) Nii: the refractive index (N) of the negative lens L4 in the second lens group G2
4)

Conditional expression (1) relates to the ratio of the refractive power of the first lens group G1 to the refractive power of the second lens group G2. When the value of this equation exceeds the upper limit of 2.4, the refractive power of the second lens group G2 is changed. The force becomes very weak, and the second lens group G2 becomes the first lens group G1.
Becomes too close to the shape. In addition, lower limit 1.0
When the refractive index is lower than, the refractive power of the second lens group G2 becomes relatively too strong if the refractive power for keeping the overall length of the first lens group G1 relatively short is provided. Therefore, the second lens group G2 is located on the image side with respect to the first lens group G1, and the weight of the focusing group can be reduced, but the aberration variation of the focusing surface increases.

The second lens group G2 is composed of two lenses for correcting aberrations such as achromatism. As for the second lens group G2 having a negative refracting power, the generation of coma is suppressed by adopting the configuration of the negative lens L4 and the positive lens L5 in order from the object side within the range of the conditional expression (1). If the second lens group G2 is composed of the positive lens L4 and the negative lens L5 in order from the object side, the refracting power of the negative lens L5 is strengthened, and the light emitted from the positive first lens group G1 is positively emitted by the second lens group G2. Since the lens L5 is further subjected to a positive refraction action, the change in the ray angle between these positive and negative lenses becomes large, which particularly causes coma.

Further, by setting the directions of the image-side surfaces of the negative lens L4 and the positive lens L5 of the second lens unit G2 as shown in conditional expression (2), a relatively concentric surface with respect to the stop S is obtained. With this configuration, the change in the ray angle on each surface during focusing is small, and as a result, the variation in aberration can be suppressed.

In addition to these conditions, in this embodiment, since the first lens group G1 has a relatively small refractive power for correcting aberrations, the under lens generated due to the negative lens L3 in the first lens group G1. In order to correct the spherical aberration, the negative curvature of field and the chromatic aberration of the positive lens L in the third lens group G3,
It is necessary to determine the condition for 6 as in conditional expression (3).

When the value of the conditional expression (3) exceeds the upper limit of 0.9, chromatic aberration occurs and the positive curvature of field becomes excessive. When the value of the conditional expression (3) falls below the lower limit of 0.47, the spherical aberration under-tilt and the negative. This causes deterioration of astigmatism due to curvature of field and narrowing of the distance between the second lens group G2 and contact during focusing.

If the condition (4) is not satisfied, the occurrence of chromatic aberration in the lens itself can be suppressed, but the fluctuation of chromatic aberration due to focusing cannot be reduced. Finally, the conditional expression (5) shows the condition necessary for favorably correcting the field curvature. If the condition is not satisfied, the negative field curvature becomes too strong, and the spherical aberration cannot be balanced. .

[0022]

Next, preferred embodiments of the telephoto lens according to the present invention will be described. FIGS. 1, 5, and 9 are configuration diagrams showing infinity states of the first, second, and third embodiments of the present invention. FIGS. 3, 7, and 11 are close views of the first, second, and third embodiments. FIG. 2, FIG. 6, FIG. 10 are aberration curve diagrams at infinity of the first, second, and third embodiments, and FIG. 4, FIG. 8, and FIG. FIG. 3 is an aberration curve diagram in a close distance state of FIG.

Tables 1, 2, and 3 below show parameters of the first, second, and third embodiments, respectively.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

In these examples, f: focal length of the whole system Fn: F number 2ω: angle of view I: surface number R (I): radius of curvature of the I-th surface D (I): after the I-th surface Nd (I): Refractive index for d-line after the I-th surface Vd (I): Abbe number for d-line after the I-th surface, respectively.

[0028]

As described above, according to the telephoto lens according to the present invention, various kinds of aberrations, particularly chromatic aberration, spherical aberration, astigmatism, and the like, occur during focusing, compared with the conventional inner focus system using a single lens group. Variations in aberrations can be reduced, and good performance can be obtained from infinity to short distances.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a focused state at infinity according to a first embodiment of the present invention.

FIG. 2 is an aberration curve diagram of the same.

FIG. 3 is a configuration diagram showing the close-range focusing state.

FIG. 4 is an aberration curve diagram of the same.

FIG. 5 is a configuration diagram illustrating an infinity in-focus state according to a second embodiment of the present invention;

FIG. 6 is an aberration curve diagram of the same.

FIG. 7 is a configuration diagram showing the close-distance focusing state.

FIG. 8 is an aberration curve diagram of the same.

FIG. 9 is a configuration diagram illustrating an infinity in-focus condition according to a third embodiment of the present invention.

FIG. 10 is an aberration curve diagram of the same.

FIG. 11 is a block diagram showing the close focus condition.

FIG. 12 is an aberration curve diagram of the same.

[Explanation of symbols]

 G1: first lens group G2: second lens group G3: third lens group R (I): radius of curvature of surface I D (I): distance between surfaces after surface I ΔM: meridional ΔS: sagittal ω: half Angle of view

Claims (3)

[Claims] A first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive or negative refractive power. The first lens group includes at least two positive lenses and one negative lens, the second lens group includes two negative and positive lenses in order from the object side, and the third lens group includes from the object side. By sequentially moving the second lens group toward the image plane on the optical axis and the positive lens in the third lens group toward the object on the optical axis, the second lens group is moved from infinity. A telephoto lens which focuses on a short distance and satisfies the following conditional expression. 1.0 <| f2 | / f1 <2.4 (1) Ri, Rii> 0 (2) where, f1: focal length of the first lens group f2: focal length of the second lens group Ri, Rii: each The radius of curvature of the image-side surface of the negative and positive lenses of the two lens groups 2. The telephoto lens according to claim 1, wherein the following conditional expression is satisfied. 0.47 <f31 / f <0.9 (3) where, f: focal length of the entire system f31: focal length of the positive lens in the third lens group 3. The telephoto lens according to claim 1, wherein the following conditional expression is satisfied. V6 <55 (4) Ni, Nii> 1.7 (5) where V6: Abbe number of the positive lens in the third lens group Ni: refractive index of the negative lens in the first lens group Nii: second lens group Refractive index of medium negative lens