JPH08327896A - Lens system - Google Patents

Abstract

PURPOSE: To provide a lens system of inner focusing method shortening a photographing distance on the side of the closest range and excellently correcting the aberration over all photographing range from infinity to the closest range. CONSTITUTION: This lens system is composed, in order from the object side, of a first lens group Gr1 having a positive refractive power (a first positive lens Ll, a second positive lens L2 and a third negative lens L3), a second lens group Gr2 having a negative refractive power (a fourth negative lens L4, a fifth lens L5 joined to the fourth lens L4, a sixth negative lens L6 and a seventh lens L7 joined to the sixth lens L6), a third lens group Gr3 having a positive refractive power (a eighth positive meniscus lens L8 whose convex surface confronts the image side) and focusing is performed by moving the second lens group Gr2 in the direction of optical axis. The following three relations are satisfied. 0.40<fl/f<0.52, 0.23<|f2/f|<0.35, 0.35<r/f3<0.55.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lens system used in a photographic camera, a video camera or the like, and more particularly to an inner focus type lens system suitable for auto focus.

[0002]

2. Description of the Related Art Conventionally, in the field of lens systems, as a focusing method, there is an entire extension method in which the entire lens group is moved in the optical axis direction to perform focusing, and only the central lens group is moved in the optical axis direction. An inner focus method for performing focusing and a rear focus method for performing focusing by moving only the image side lens unit in the optical axis direction are known.

Among them, the total payout method has a problem that the operability is deteriorated because the diameter of each lens group is large and the weight is large, especially when the focal length of the lens system is long. In recent years, interchangeable lenses for single-lens reflex cameras are required to have high-speed focusing corresponding to autofocus. However, the whole feeding method cannot cope with the speeding up of the autofocus because the heavy lens group has to be moved.

Therefore, in the above-mentioned lens system, the inner focus system and the rear focus system are adopted. Examples of the inner focus method are lens systems described in JP-A-54-147606 and JP-A-5-297272, and examples of the rear focus method are JP-A-54-55474 and JP-A-5-257474. 5-2
There is a lens system described in Japanese Patent No. 7164.

[0005]

However, the inner focus type and rear focus type lens systems described in the above publication have the following problems.

Generally, in order to shorten the time required for focusing, the weight of the lens group that moves during focusing is reduced to reduce the load required for moving the lens group, or from infinity to the closest point of the moving lens group. It is necessary to reduce the movement amount corresponding to the entire shooting range.

In order to reduce the weight of the moving lens group, a lens group having a small lens outer diameter may be used for focusing. Such a lens group having a small lens outer diameter generally has a strong refractive power. Further, in order to reduce the moving amount of the moving lens group, the refractive power of the lens group may be strengthened. Therefore, in order to reduce the time required for focusing, it is necessary to move the lens group having a strong refractive power in the optical axis direction.

However, when a lens unit having a strong refracting power is moved in the optical axis direction, the aberration changes greatly. If the aberration changes greatly due to focusing,
It becomes impossible to obtain a good image in the entire shooting range from infinity to the closest point.

Further, in the inner focus type or rear focus type lens system, since the lens unit closest to the object side is fixed, the amount of movement of the lens unit that moves during focusing is limited. In other words, the inner focus type and rear focus type lens systems essentially tend to have a long shooting distance on the closest side.

In view of the above problems, the present invention provides an inner focus type lens system in which the photographing distance on the closest side is shortened and the aberration is favorably corrected over the entire photographing range from infinity to the closest distance. To aim.

[0011]

In order to achieve the above object, the lens system according to claim 1 has, in order from the object side, a first lens group having a positive refractive power and a first lens group having a negative refractive power. In a lens system that includes two lens groups and a third lens group having a positive refractive power and moves the second lens group in the optical axis direction to perform focusing, the third lens group has a positive refractive power. It is characterized by comprising one meniscus lens having a refractive power and having a convex surface facing the image side, and satisfying the following three expressions.

0.40 <f1 / f <0.52 (1) 0.23 <| f2 / f | <0.35 (2) 0.35 <r / f3 <0.55 (3) Further, in the lens system according to claim 2, in the lens system according to claim 1, the first lens group includes, in order from the object side,
The first lens having a positive refracting power, the second lens having a positive refracting power, and the third lens having a negative refracting power are included. The second lens group includes, in order from the object side, a negative lens. A fourth lens having a refractive power, a fifth lens having a positive refractive power and cemented to the fourth lens, a sixth lens having a negative refractive power, and a fifth lens having a positive refractive power. And a seventh lens cemented to six lenses.

[0013]

EXAMPLES Examples of the present invention will be described below.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

[Table 3]

In each embodiment, ri (i = 1, 2, 3, ...
・ ・) Is the radius of curvature of the i-th surface counted from the object side, Ti
(I = 1, 2, 3, ...) Is the i-th axial upper surface distance counted from the object side, n is the refractive index of the d-line, ν is the Abbe number, FL is the focal length of the entire system, Fno. Indicates an open F number. Also, among the axial upper surface intervals, T6 and T12 are the focusing position at infinity from the left and the closest point (shooting distance 3 m).
Corresponding to the focusing position of.

FIG. 1 to FIG. 3 are configuration diagrams showing the lens arrangement when the photographing distance of the lens system of the embodiment of the present invention is infinity. All the lens systems of the examples are, in order from the object side,
It includes a first lens group Gr1 having a positive refractive power, a second lens group Gr2 having a negative refractive power, a third lens group Gr3 having a positive refractive power, and a diaphragm SP. In each of the lens systems of the examples, the first lens group Gr1 and the second lens group Gr2 are used during focusing from infinity to the closest point.
Is an inner focus type lens system in which the second lens group is moved to the image side (indicated by an arrow in each figure) so that the distance between the second lens group Gr2 and the third lens group Gr3 is decreased. is there.

The first lens group Gr1 includes, in order from the object side,
A biconvex first lens L1 having a positive refractive power, a biconvex second lens L2 having a positive refractive power, and a biconcave third lens L3 having a negative refractive power. ,,.

The second lens group Gr2 includes, in order from the object side,
A meniscus-shaped fourth lens L4 having a negative refractive power and a concave surface facing the image side, and a meniscus-shaped fourth lens L4 having a positive refractive power and a concave surface facing the image side, and a convex surface on the image side surface of the fourth lens L4. Of the fifth lens L5, a biconcave sixth lens L6 having a negative refracting power, and a meniscus shape having a positive refracting power with a concave surface facing the image side. And a seventh lens L7 having a convex surface cemented to the image side surface.

The third lens group Gr3 has a positive meniscus power, and a meniscus eighth lens L8 having a convex surface directed toward the image side.
(Meniscus lens).

Each embodiment satisfies the above conditional expressions (1) to (3). Table 4 shows the values corresponding to the conditional expressions of the examples.

[0023]

[Table 4]

The conditional expressions (1) to (3) will be described below in order.

0.40 <f1 / f <0.52 (1) Here, f1 is the focal length of the first lens group Gr1, and f is the focal length of the entire system.

Conditional expression (1) defines the refractive power of the first lens group Gr1. By satisfying conditional expression (1), it is possible to correct various aberrations in a well-balanced manner and shorten the total lens length.

If the upper limit of conditional expression (1) is exceeded, the refracting power of the first lens group Gr1 becomes too weak, the overall lens length becomes long, and the operability of the lens system deteriorates.

On the other hand, if the lower limit of conditional expression (1) is exceeded, the refractive power of the first lens group Gr1 becomes too strong, and it becomes difficult to correct axial chromatic aberration and lateral chromatic aberration.

0.23 <| f2 / f | <0.35 (2) where, f2 is the focal length of the second lens group Gr2.

Conditional expression (2) defines the refractive power of the second lens group Gr2. By satisfying conditional expression (2), it is possible to shorten the photographing distance on the closest side and reduce the movement amount of the second lens group Gr2 due to focusing.

When the upper limit of conditional expression (2) is exceeded, the refracting power of the second lens group Gr2 becomes too weak, and the moving amount of the second lens group Gr2 during focusing becomes large, so that the time required for focusing is increased. It will be long. Further, the total length of the lens becomes long and the operability of the lens system deteriorates.

On the contrary, if the lower limit of conditional expression (2) is exceeded, the refracting power of the second lens group Gr2 becomes too strong, and the fluctuations of coma, axial chromatic aberration, and lateral chromatic aberration during focusing become large. Good performance cannot be maintained on the closest side.

The third lens group Gr3 of the present invention is composed of 81 meniscus eighth lenses L8 having positive refractive power and having a convex surface facing the image side. The above configuration is effective for correcting variations in spherical aberration and curvature of field due to movement of the second lens group during focusing. Furthermore, since the eighth lens L8 has a concave surface facing the second lens group Gr2, spherical aberration and chromatic aberration that occur during focusing can be corrected.

0.35 <r / f3 <0.55 (3) where, f3: focal length of eighth lens L8 (third lens group Gr3), r: object of eighth lens L8 The radius of curvature of the side surface is.

Conditional expression (3) defines the radius of curvature of the object side surface of the eighth lens L8 (third lens group Gr3).

If the upper limit of conditional expression (3) is exceeded, the amount of change in spherical aberration due to focusing becomes too large, and spherical aberration worsens at infinity. It is difficult to correct this spherical aberration by the first lens group Gr1 and the second lens group Gr2.

On the other hand, when the value goes below the lower limit of conditional expression (3), the amount of change in lateral chromatic aberration due to focusing becomes too large, and the third lens group Gr3 cannot be composed of one lens. An increase in the number of lenses causes an increase in cost and weight, which is not desirable. If the lower limit of conditional expression (3) is exceeded, the amount of change in spherical aberration due to focusing becomes too large, and spherical aberration worsens on the closest side. This spherical aberration is also difficult to correct by the first lens group Gr1 and the second lens group Gr2.

4 to 9 are aberration diagrams of the respective examples,
4 is an aberration diagram on the infinity side of Example 1, FIG. 5 is an aberration diagram on the closest side (shooting distance 3 m) of Example 1, FIG. 6 is an aberration diagram on the infinity side of Example 2, and FIG. An aberration diagram of the closest side (shooting distance 3 m) of Example 2, FIG. 8 is an aberration diagram of the infinite side of Example 3, and FIG. 9 is an aberration diagram of the closest side (shooting distance 3 m) of Example 3. Represent each. Each aberration diagram corresponds to a spherical aberration diagram, an astigmatism diagram, a distortion diagram, and a lateral chromatic aberration diagram in order from the left.

In each spherical aberration diagram, the open F number is 4.59, the solid line d is the spherical aberration for the d line, the chain line g is the spherical aberration for the g line, and the chain double-dashed line C is the spherical aberration for the C line. SC represents a sine condition.

In each of the astigmatism diagram, the distortion diagram, and the chromatic aberration of magnification diagram, the half value of the maximum incident field angle to the optical system is ω =
Set to 3.2 °.

In each of the astigmatism diagrams, M represents astigmatism in the meridional plane, the solid line d represents the astigmatism with respect to the d line, the chain line g represents the astigmatism with respect to the g line, and the chain double-dashed line C represents the C line. Astigmatism is shown. Further, S represents astigmatism on the sagittal surface, dotted line d represents astigmatism with respect to d line, dotted line g represents astigmatism with respect to g line, and dotted line C represents astigmatism with respect to C line.

In each of the lateral chromatic aberration diagrams, the solid line g shows the lateral chromatic aberration with respect to the g line, and the dotted line C shows the lateral chromatic aberration with respect to the C line.

[0043]

As described above, the lens system according to the present invention suppresses fluctuations in aberrations during focusing, is well corrected for aberrations over the entire shooting range from infinity to the closest distance, and is also the closest distance side. It is possible to provide a lens system with a short shooting distance.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a lens arrangement of a lens system of Example 1 with an infinite shooting distance.

FIG. 2 is a configuration diagram showing a lens arrangement in which a shooting distance of a lens system of Example 2 is infinite.

FIG. 3 is a configuration diagram showing a lens arrangement in which a shooting distance of a lens system of Example 3 is infinite.

FIG. 4 is an aberration diagram on the infinity side of Example 1.

FIG. 5 is an aberration diagram of a closest side (shooting distance 3 m) of Example 1.

6A and 6B are aberration diagrams of Example 2 on the infinity side.

FIG. 7 is an aberration diagram of Example 2 on the closest side (shooting distance: 3 m).

FIG. 8 is an aberration diagram for Example 3 on the infinity side.

FIG. 9 is an aberration diagram of Example 3 on the closest side (shooting distance: 3 m).

[Explanation of symbols]

 Gr1: First lens group Gr2: Second lens group Gr3: Third lens group L1: First lens L2: Second lens L3: Third lens L4: Fourth lens L5: Fifth lens L6: Sixth lens L7: 7th lens L8: 8th lens

Claims (2)

[Claims] 1. A first lens group having a positive refracting power, a second lens group having a negative refracting power, and a third lens group having a positive refracting power, which are arranged in this order from the object side. Second
In the lens system for performing focusing by moving the lens group in the optical axis direction, the third lens group includes one meniscus lens having a positive refracting power and a convex surface facing the image side.
A lens system characterized by satisfying the formula. 0.40 <f1 / f <0.52 0.23 <| f2 / f | <0.35 0.35 <r / f3 <0.55 where fi (i = 1, 2, 3): The focal length of the i-th lens group, f: the focal length of the entire system, r: the radius of curvature of the object side surface of the meniscus lens, 2. The first lens group, in order from the object side,
The first lens having a positive refracting power, the second lens having a positive refracting power, and the third lens having a negative refracting power are included, and the second lens group includes, in order from the object side, a negative lens. A fourth lens having a refracting power, a fifth lens having a positive refracting power and cemented to the fourth lens, and a sixth lens having a negative refracting power.
The lens system according to claim 1, comprising a lens and a seventh lens having a positive refractive power and cemented to the sixth lens.