JPH08179196A - Retrofocus type lens - Google Patents

Abstract

PURPOSE: To obtain a retrofocus lens which employs a rear focus type having high optical performance over the entire object distance range. CONSTITUTION: This lens consists of a 1st group L1 which has two or three lens elements and positive or negative refracting power on the whole and a 2nd group L2 which has two or three lens groups moving on the optical axis and positive refracting power on the whole in order from an object side. For focusing on a short-distance body from an infinite-distance body, the 1st group L1 is fixed and the respective lens groups in the 2nd lens group L2 are moved to the object side while varied in mutual interval.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus type lens suitable for a 35 mm camera, a video camera or the like having a shooting angle of view of about 75 ° and an F number of about 1.8 and a back focus longer than the focal length. A rear focus type retrofocus lens using a rear focus type in which the lens system is appropriately configured so that good optical performance can be obtained over the entire object distance when focusing is performed by moving the rear lens group in the system on the optical axis. Is.

[0002]

2. Description of the Related Art Conventionally, a retrofocus (reverse telephoto type) lens in which a lens group having a weak positive refractive power or a negative refractive power precedes as a wide-angle photographing lens having a back focus longer than a focal length. Have been proposed.

For example, Japanese Patent Publication No. 60-34730 proposes a bright retrofocus type lens having a compact lens structure having a photographing field angle of about 75 ° to 85 ° and an F number of about F2.0. In Japanese Patent Laid-Open No. 61-144616, spherical aberration and sagittal flare are corrected well by using an aspherical surface, and the shooting angle of view is about 75 ° and the F number is F.
We have proposed a bright retrofocus lens with a brightness of about 1.4. In Japanese Patent Publication No. 55-10049, the shooting angle of view is 85 °.
, A bright retrofocus lens with an F number of about F1.4 is proposed. Further, various focusing methods are used in the retrofocus type lens.

For example, in Japanese Patent Publication No. 58-5, a first group having a negative refractive power and a second group having a positive refractive power are provided in order from the object side.
There is proposed a retrofocus lens in which both the lens groups are moved to the object side while changing the distance between the first group and the second group when focusing from an object at infinity to a near object. In Japanese Patent Publication No. 58-26001, a first group having negative refracting power and a second group having positive refracting power are provided in order from the object side, and the first group is fixed when focusing from an object at infinity to a near object. As a proposal, a retrofocus lens that moves only the second group to the object side is proposed.

In Japanese Laid-Open Patent Publication No. 55-143517, only one or two lenses having a positive refracting power, which are arranged closest to the image side in the entire lens system, are used for focusing from an object at infinity to a near object. We have proposed a retrofocus lens that moves to the side. In Japanese Laid-Open Patent Publication No. 63-163317, only two or three lenses having negative or positive refractive power, which are arranged closest to the image side in the entire lens system, are used for focusing from an object at infinity to a near object. We have proposed a retrofocus lens that moves to the object side while changing the distance.

[0006]

Generally, a retrofocus type lens has an asymmetric lens structure in which a lens unit having a negative refractive power is arranged in the front and a lens unit having a positive refractive power is arranged in the rear. As a result, the amount of various aberrations such as spherical aberration, coma aberration, distortion, and astigmatism increases, and the absolute value of the negative refractive power of the front lens group is increased to secure a long back focus. Therefore, the amount of various aberrations is further increased, and it is generally difficult to correct these various aberrations in good balance.

Further, the retrofocus type lens tends to increase the diameter of the front lens when the amount of light around the screen is increased. It is generally undesirable to move a large, heavy front lens for focus adjustment. Particularly, as a photographing lens for a camera with an automatic focus adjustment mechanism, if the front lens is used for focus adjustment, the load on the focus adjustment actuator becomes large, which makes quick driving difficult. On the other hand, there is a method of using a rear focus type in which a relatively small lens group in the rear of the lens system is moved to perform focusing.

Generally, in the rear focus type, the amount of extension of the focusing lens group is smaller than that in the overall focus type in which the entire lens system is extended, and the focusing lens group is relatively small and lightweight, so that focusing can be performed with a small driving force. Therefore, it is suitable for a camera or the like having an automatic focus detection device. Further, since the total lens length is always constant even when focusing is performed, there is an advantage that it is easy to hold the photographing device and is less likely to cause camera shake.

However, in the rear focus system, aberration fluctuations increase when the focusing lens group is moved, as compared with the whole focus system in which the entire lens system is extended,
It becomes difficult to satisfactorily correct aberrations over the entire object distance. This tendency is remarkable especially in so-called retrofocus type lenses in which the front group is composed of a lens group having negative refractive power and the rear group is composed of positive refractive power. For example, when focusing is performed, an outward coma aberration occurs in a short-distance object. As a result, the astigmatism is increased, and the optical performance is significantly deteriorated.

The present invention has two lens groups, a first lens group having a positive or negative refractive power and a second lens group having a positive refractive power, and has a back focus of a retrofocus type lens whose focal length is longer than the focal length. By properly setting the lens configuration of the second lens group, which is moved when moving, while maintaining the advantages of the rear focus type, it is possible to satisfactorily correct aberration fluctuations during focusing over the entire object distance from infinity objects to short-distance objects. It is an object of the present invention to provide a retrofocus lens having high optical performance, a shooting angle of view of about 75 °, and an F number of about 1.8 and a back focus longer than the focal length.

[0011]

The retrofocus type lens of the present invention has two or three lenses in order from the object side, and moves on the optical axis with the first group having a positive or negative refractive power as a whole. Which has two or three lens groups and has a positive refractive power as a whole, and when focusing from an object at infinity to a short-distance object, the first group is fixed, and It is characterized in that the respective lens groups are moved to the object side while changing the distance between them.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 to 8 are numerical examples 1 of the present invention described later.
9 is a lens cross-sectional view of FIGS. 9 to 16 are various aberration diagrams of Numerical Examples 1 to 8 of the present invention. In the aberration diagram, (A) is an object at infinity, and (B) is an object distance of which the focal length is 1
The case of a short distance when 0 times is shown.

In the figure, L1 has two or three lenses, and the first group (front group) has a weak positive or negative refractive power as a whole, and L2 is two groups that move on the optical axis during focusing. Alternatively, a second group (rear group) having three lens groups and having a positive refracting power as a whole, and SP is a diaphragm.

The first lens unit L1 is a negative meniscus first lens having a convex surface directed toward the object side, and a second positive lens element having convex lens surfaces on both sides.
The second lens unit L2 has two lenses, and the second lens unit L2 has a negative meniscus third lens element having a convex surface directed toward the object side, a negative fourth lens element having a concave surface on both lens surfaces, and a positive lens element having a convex surface on both lens surfaces. Fifth
Lens, aperture stop, negative meniscus sixth lens with convex surface facing the image side, positive meniscus seventh lens with convex surface facing the image side, and negative eighth lens with concave surface facing the object side It has a cemented lens cemented with a lens, a meniscus-shaped positive ninth lens with a convex surface facing the image side, and a meniscus-shaped positive tenth lens with a convex surface facing the image side. ing. When focusing from an object at infinity to an object at a short distance, the first lens unit L1 is fixed, and each lens of the second lens unit is moved toward the object side while changing the distance between at least one lens in the second lens unit as indicated by the arrow in the figure. Is moving to.

According to the present invention, the entire lens system has a relatively weak positive or negative refractive power in order from the object side, and the first group composed of two or three lenses and the first lens group having a positive refractive power. Two
The second group is made up of two or three lens groups that are movable along the optical axis. When focusing from an object at infinity to a near object, the first group is fixed and each lens group of the second group is moved so as to be close to the first group while changing the distance between them. As a result, an extremely good retrofocus type lens that overcomes the conventional drawbacks is realized.

In particular, in the present invention, the load of the focus adjusting drive member is reduced by fixing the first group having a relatively large outer diameter and heavy, which is arranged on the most object side of the retrofocus type lens, during focusing. At the same time, it improves the operability of filters that are commonly used in wide-angle lenses.

Further, according to the present invention, it is possible to reduce fluctuations of various aberrations during focusing, particularly coma aberration and astigmatism, which are difficult to be corrected by such a retrofocus type lens, so that a near object at infinity can be obtained. The second group is divided into two or three lens groups so that a good image can always be obtained even when focusing on a distance object, and the interval between these lens groups is appropriately changed to focus. It is configured to operate. In a retrofocus lens, generally, in order to maintain the back focus sufficiently long, a lens group having negative refractive power is arranged on the object side of the lens system, and a lens group having positive refractive power is arranged on the image side. Although the lens has a refractive power arrangement, in order to satisfactorily correct coma and astigmatism, which are asymmetrical aberrations at that time, a lens group arranged on the object side and a lens group arranged on the image side are preferably arranged. It is necessary to properly set the shape and arrangement of the. However, since it is necessary to move a part of the lens group for focusing here, there arises a problem that it becomes impossible to maintain an appropriate arrangement from an object at infinity to a near object. come.

Therefore, in the present invention, a part of the lens units is moved at the same time during focusing, and at the same time, the distance between the lens units is partially changed, so that the increase of the asymmetrical aberration is satisfactorily corrected. The symmetry aberration, that is, the increase in spherical aberration and the like is also well corrected.

In the present invention, the asymmetrical aberration is satisfactorily corrected by providing the sixth lens with at least one aspherical surface. The focal lengths of the first lens group and the entire system are f1,
When f, the condition of 5 <| f1 / f | (1) is satisfied.

Conditional expression (1) defines the ratio of the fixed focal length f1 of the first lens group to the focal length f of the entire lens system during focus adjustment. By limiting the refracting power of the group to be relatively weak, asymmetrical aberrations are mainly corrected well.

If the refractive power of the first lens unit is increased outside the range satisfying conditional expression (1), the following problems will occur.
When the focal length f1 of the first lens group is negative and has a small absolute value, the asymmetry of the entire lens system becomes strong, and it becomes difficult to correct coma aberration, astigmatism, and the like. Fluctuations in field curvature during adjustment are large. On the contrary, when the focal length f1 of the first lens unit is positive and has a small absolute value, it becomes difficult to maintain the reverse telephoto type refractive power arrangement of the entire lens system, and a sufficiently long back focus is maintained. Becomes difficult to do. In addition, since the focal length of the second lens unit becomes large, the driving amount of the lens unit for focus adjustment increases, which is not suitable for downsizing.

In Numerical Embodiments 1 and 2 shown in FIGS. 1 and 2, the second lens group is, in order from the object side, the second lens group having a negative refractive power, the second lens group having a positive refractive power, and the second lens group having a positive refractive power. It has three lens groups of 2f group, and increases the distance between the 2d group and the 2e group when focusing from an object at infinity to a near object,
Each lens group is moved to the object side so as to reduce the distance between the 2e-th group and the 2f-th group.

The second group d and the second group f are integrally moved at the time of focusing in order to simplify the mechanism. 2nd f
At least one lens surface of the group is an aspherical surface, and mainly asymmetrical aberration is favorably corrected. An aperture stop SP is provided between the second group e and the second group f to prevent the lens outer diameters of the front lens and the rear lens from increasing, and the overall size of the lens system is reduced.

In Numerical Embodiments 3 and 4 of FIGS. 3 and 4, the second lens group in order from the object side is a second lens group having a positive refractive power, a second lens group having a negative refractive power, and a second lens group having a positive refractive power. It has three lens groups of 2c group, and decreases the distance between the 2a group and the 2b group and increases the distance between the 2b group and the 2c group when focusing from an object at infinity to a short distance object. Thus, each lens group is moved to the object side.

The second group a and the second group c are moved integrally during focusing to simplify the mechanism. Second b
At least one lens surface of the group is an aspherical surface to favorably correct asymmetrical aberration. The aperture SP is the second group a and the second
It is provided between the lens unit b and the lens unit b to prevent the lens outer diameters of the front lens and the rear lens from increasing, and to downsize the entire lens system.

In Numerical Embodiments 5 and 6 of FIGS. 5 and 6, the second lens group has, in order from the object side, two lens groups, a second lens group having a positive refractive power and a second lens group having a positive refractive power. At the time of focusing from an object at infinity to an object at a short distance, each lens group is moved to the object side so as to reduce the distance between the 2i-th group and the 2j-th group.

At least one lens surface of the 2j-th lens group is an aspherical surface, and mainly asymmetrical aberration is favorably corrected. An aperture stop SP is provided between the 2i-th and 2j-th lens groups to prevent the lens outer diameters of the front lens and the rear lens from increasing and to downsize the entire lens system.

In Numerical Embodiments 7 and 8 of FIGS. 7 and 8, the second lens group has, in order from the object side, two lens groups, a second lens group having a negative refractive power and a second lens group having a positive refractive power. , When focusing from an object at infinity to a near object, the second group g and the second group h
Each lens group is moved to the object side so as to increase the distance from the group.

At least one lens surface of the second h group is an aspherical surface, and mainly asymmetrical aberration is satisfactorily corrected. By providing the aperture stop SP in the second h group, the overall lens system is downsized while preventing the lens outer diameters of the front lens and the rear lens from increasing.

Next, numerical examples of the present invention will be shown. In the numerical examples, ri is the radius of curvature of the i-th lens surface in order from the object side, di is the i-th lens thickness and air gap in order from the object side, and ni and vi are the i-th lens in order from the object side, respectively. Is the refractive index and Abbe number of the glass. When the aspherical shape is the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis, the traveling direction of light is positive, R is the paraxial radius of curvature, and B, C, D, and E are aspherical coefficients, respectively.

[0031]

[Equation 1] It is expressed by the formula. Also, "D-0X" means "10 ^-X ".

[0032]

[Outside 1]

[0033]

[Outside 2]

[0034]

[Outside 3]

[0035]

[Outside 4]

[0036]

[Outside 5]

[0037]

[Outside 6]

[0038]

[Outside 7]

[0039]

[Outside 8]

[0040]

As described above, according to the present invention, there are two lens groups, a first lens group having a positive or negative refractive power and a second lens group having a positive refractive power, and the back focus is longer than the focal length. By properly setting the lens configuration of the second lens group that is moved during focusing in a long retrofocus lens, while maintaining the advantages of the rear focus type, focusing can be performed over the entire object distance from infinity objects to short-distance objects. It is possible to achieve a retrofocus type lens which has a high optical performance in which the fluctuation of aberrations is satisfactorily corrected and has a shooting field angle of about 75 ° and an F number of about 1.8 and a back focus longer than the focal length.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 of the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 2 of the present invention.

FIG. 3 is a lens sectional view of Numerical Example 3 of the present invention.

FIG. 4 is a lens cross-sectional view of Numerical Example 4 of the present invention.

FIG. 5 is a lens cross-sectional view of Numerical Example 5 of the present invention.

FIG. 6 is a lens cross-sectional view of Numerical Example 6 of the present invention.

FIG. 7 is a lens cross-sectional view of Numerical Example 7 of the present invention.

FIG. 8 is a lens cross-sectional view of Numerical Example 8 of the present invention.

FIG. 9 is a diagram showing various types of aberration in Numerical Example 1 of the present invention.

FIG. 10 is a diagram of various types of aberration of Numerical example 2 of the present invention.

FIG. 11 is a diagram showing various types of aberration in Numerical Example 3 of the present invention.

FIG. 12 is a diagram of various types of aberration in Numerical example 4 of the present invention.

FIG. 13 is a diagram of various types of aberration of Numerical example 5 of the present invention.

FIG. 14 is a diagram of various types of aberration of Numerical example 6 of the present invention.

FIG. 15 is a diagram of various types of aberration in Numerical example 7 of the present invention.

FIG. 16 is a diagram of various types of aberration of Numerical Example 8 of the present invention.

[Explanation of symbols]

 L1 1st group L2 2nd group SP diaphragm dd line g g line S.I. C Sine condition ΔS Sagittal image plane ΔM Meridional image plane

Claims (18)

[Claims] 1. An optical system having two or three lenses in order from the object side, and having a first group having positive or negative refractive power and two or three lens groups moving on the optical axis as a whole, As a positive refractive power second lens unit, when focusing from an infinitely distant object to a short-distance object, the first lens unit is fixed, and the respective lens units in the second lens unit are varied in distance from each other. A retro focus lens that is moved to the side. 2. The second lens group has three lens groups, a second lens group having a positive refractive power, a second lens group having a negative refractive power, and a second lens group having a positive refractive power, in order from the object side. When focusing from an object at infinity to a near object, the second group a and the second group b
2. The retrofocus type lens according to claim 1, wherein each lens group is moved toward the object side so that the distance between the lens group and the second group is decreased and the distance between the second group b and the second group c is increased. 3. The retrofocus type lens according to claim 2, wherein the second group a and the second group c move integrally during focusing. 4. The retrofocus type lens according to claim 2, wherein at least one lens surface of the second group b is an aspherical surface. 5. The retrofocus type lens according to claim 2, wherein an aperture stop is provided between the second group a and the second group b. 6. The second lens group, in order from the object side, has three lens groups, a second lens group having a negative refractive power, a second lens group having a positive refractive power, and a second lens group having a positive refractive power, and a second lens group having a positive refractive power. When focusing from an object at infinity to a near object, the second group d and the second group e
2. The retrofocus type lens according to claim 1, wherein each lens group is moved toward the object side so as to increase the distance between the lens group and the second e group and decrease the distance between the second e group and the second f group. 7. The retrofocus type lens according to claim 6, wherein the second group d and the second group f are integrally moved during focusing. 8. The retrofocus type lens according to claim 6, wherein at least one lens surface of the second f group is an aspherical surface. 9. The retrofocus type lens according to claim 6, wherein an aperture stop is provided between the second group e and the second group f. 10. The focus from an infinite object to a short-distance object, wherein the second lens group has two lens groups, in order from the object side, a second lens group having a negative refractive power and a second lens group having a positive refractive power. At this time, each of the lens groups is moved toward the object side so as to increase the distance between the second group g and the second group h, and the retrofocus lens according to claim 1. 11. The retrofocus lens according to claim 10, wherein at least one lens surface of the second h group is an aspherical surface. 12. The retrofocus type lens according to claim 10, wherein an aperture stop is provided in the second h group. 13. The focusing from an infinitely distant object to a short-distance object, wherein the second lens group has two lens groups in order from the object side, that is, a second lens group having a positive refractive power and a second lens group having a positive refractive power. At this time, each of the lens groups is moved toward the object side so as to reduce the distance between the second i-th group and the second j-th group, and the retrofocus type lens according to claim 1. 14. The retrofocus lens according to claim 13, wherein at least one lens surface of the second j-th group is an aspherical surface. 15. The retrofocus lens according to claim 13, wherein an aperture stop is provided between the second i-th group and the second j-th group. 16. A meniscus having, in order from the object side, two lens groups, a first lens group having a positive or negative refractive power and a second lens group having a positive refractive power, the first lens group having a convex surface directed toward the object side. Negative first lens, positive lens second with two convex lens surfaces
The second lens unit has two lenses, the second lens unit has a negative meniscus lens having a convex surface directed toward the object side, a negative fourth lens element having concave lens surfaces, and a positive fifth lens element having both convex lens surfaces. , Aperture stop, negative meniscus sixth lens with convex surface facing the image side, positive meniscus seventh lens with convex surface facing the image side, and negative eighth lens concave on the object side And a cemented lens having a cemented surface, a meniscus-shaped positive ninth lens having a convex surface facing the image side, and a meniscus-shaped positive tenth lens having a convex surface facing the image side, When focusing from an object at infinity to an object at a short distance, the first group is fixed, and each lens of the second group is moved to the object side while changing the distance between at least one lens in the second group. A retro focus lens featuring. 17. The retrofocus lens according to claim 16, wherein the sixth lens has at least one aspherical surface. 18. The focal lengths of the first lens group and the entire system are f
18. When 1 and f are satisfied, the condition 5 <| f1 / f | is satisfied.
The retrofocus lens according to any one of the above.