JPH05273466A - Zoom lens of rear focusing system - Google Patents

Abstract

PURPOSE:To miniaturize a lens as employing a rear focusing system and providing a high variable power ratio and a large aperture ratio by comprising the lens in which the travel conditions of sec-cond and fourth groups in the refracting power and variable power of five lens groups are set and the fourth group is moved when focusing is applied. CONSTITUTION:This lens is comprised of a first lens group I of positive refracting power, a second lens group II of negative refracting power, a third lens group III of positive refracting power, a fourth lens group IV of positive refracting power, and a fifth lens group V of negative refracting power located in the above sequence from an object side. Zooming is performed by moving the second and fourth lens groups II, IV, and also, the focusing is performed by moving the fourth lens group IV. A zoom lens of rear focusing system can be obtained which satisfies conditions 0.8<¦f5/f3¦<2.1, 1.2<beta5<1.6 assuming the focal distance of the third and fifth lens groups as f3, f5, the image-forming magnification of the fifth lens group whose object distance is infinite as beta5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear focus type zoom lens, which is particularly used for a photographic camera or a video camera, and has a high zoom ratio of about 8 to 10 and a large aperture ratio of about F2. The present invention relates to a rear focus type zoom lens that has a small size while having it.

[0002]

2. Description of the Related Art Hitherto, various zoom lenses for photographic cameras, video cameras, etc. have been proposed which employ a so-called rear focus system in which focusing is performed by moving a lens unit other than the first lens unit on the object side. Has been done.

Generally, in a rear focus type zoom lens, the effective diameter of the first lens group is smaller than that of a zoom lens in which the first lens group is moved to perform focusing, and it is easy to downsize the entire lens system. Close-up photography, especially very close-up photography, is facilitated, and since the relatively small and lightweight lens group is moved, there is a feature that the driving force of the lens group is small and quick focusing is possible.

As such a rear focus type zoom lens, for example, in JP-A-63-247316, a positive first lens group, a negative second lens group, and a positive third lens group are sequentially arranged from the object side. , Has a positive fourth lens group, and has a second
The lens group and the fourth lens group are moved to perform zooming, and
A zoom lens for moving a lens group to perform focusing is disclosed.

In Japanese Patent Laid-Open No. 58-160913, a first group having a positive refractive power, a second group having a negative refractive power, are arranged in this order from the object side.
The third lens unit has a positive refracting power and the fourth lens unit has a positive refracting power, and there are four lens units. The first lens unit and the second lens unit are moved to perform zooming, and the image plane changes due to zooming. Is performed by moving the fourth group. Then, focusing is performed by moving one or more lens groups among these lens groups.

Further, in JP-A-58-129404 and JP-A-61-258217, a positive first lens group, a negative second lens group, a positive third lens group, and a positive lens group are sequentially arranged. Disclosed is a zoom lens that includes a fourth lens unit and a negative fifth lens unit, and moves the fifth lens unit or a plurality of lens units including the fifth lens unit to perform focusing. Japanese Unexamined Patent Publication No. 60-6914 discloses a zoom lens having a refracting power arrangement similar to that described above, and the position of the focus lens group on the optical path is constant regardless of zooming for a specific finite distance. Such a zoom lens is disclosed.

Further, in Japanese Patent Application Laid-Open No. 4-13109,
In a similar refractive power arrangement, a zoom lens is disclosed in which the fifth lens component has a relatively weak refractive power and the fourth lens unit or the first lens unit and the third lens unit perform focusing.

[0008]

Generally, when a rear focus system is adopted in a zoom lens, the entire lens system is downsized and quick focusing becomes possible as described above, and further features such as close-up photography are obtained. ..

On the other hand, on the other hand, the aberration variation at the time of focusing becomes large, and it becomes very difficult to obtain high optical performance while miniaturizing the entire lens system over the entire object distance from an object at infinity to a near object. The problem arises.

In particular, in a zoom lens having a large aperture ratio and a high zoom ratio, it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance.

The first lens group having a positive refractive power, the second lens group having a negative refractive power, the third lens group having a positive refractive power, the fourth lens group having a positive refractive power, and the fifth lens group having a negative refractive power. A rear-focus type zoom lens composed of five lens groups is advantageous in that the total lens length is shortened, but in order to effectively shorten the total lens length, the refractive power of the second lens group should be increased. Since the amount of movement of the lens unit is reduced and the refracting power of the fifth lens is increased to shorten the length of the third lens unit and thereafter, the Petzval sum increases in the negative direction, and especially the field curvature during zooming. However, there is a problem that it becomes difficult to correct the fluctuation of

The present invention, while adopting the rear focus system, prevents a further increase in size of the entire lens system when aiming for a large aperture ratio and a high zoom ratio, and also provides a total zoom range from the wide-angle end to the telephoto end. An object of the present invention is to provide a rear-focus type zoom lens having a simple structure with good optical performance over the object range from infinity objects to short-distance objects over a double range.

[0013]

A zoom lens according to the present invention comprises a first lens group having positive refracting power in order from the object side.
The second lens group having a negative refracting power, the third lens group having a positive refracting power, the fourth lens group having a positive refracting power, and the fifth lens group having a negative refracting power are included, and zooming is performed as described above. The second and fourth lens groups are moved and the focusing is performed by moving the fourth lens group and the third and fourth lens groups are moved.
When the focal lengths of the lens unit and the fifth lens unit are f ₃ and f ₅ , respectively, and the imaging magnification of the fifth lens unit at an infinite object distance is β ₅ , 0.8 <| f ₅ / f ₃ | <2.1 ... (1) 1.2 <β ₅ <1.6 ... (2) The conditional expressions are satisfied.

[0014]

1 is a schematic view of an embodiment showing the paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention.

In the figure, I is the first lens group having a positive refractive power, I
I is a second lens group having a negative refractive power, III is a third lens group having a positive refractive power, IV is a fourth lens group having a positive refractive power, and V is a fifth lens group having a negative refractive power. SP is an aperture stop, which is arranged in front of the third lens group III.

At the time of zooming from the wide-angle end to the telephoto end, the second lens group is moved to the image plane side as indicated by the arrow, and the image plane variation due to zooming is also corrected by moving the fourth lens group. ..

Further, a rear focus type in which focusing is performed by moving the fourth lens group on the optical axis is adopted. The solid curve 4a and the dotted curve 4b of the fourth lens shown in the figure show the image plane variation due to the zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and a near object, respectively. The movement locus for correction is shown.

The first lens group, the third lens group, and the fifth lens group are stationary during zooming and focusing.

In the present embodiment, the fourth lens group is moved to correct the image plane variation due to zooming, and the fourth lens group is moved to perform focusing. In particular, as shown by the curves 4a and 4b in the figure, the lens is moved so as to have a convex locus toward the object side during zooming from the wide-angle end to the telephoto end. As a result, the space between the third lens group and the fourth lens group is effectively used, and the total lens length is effectively shortened.

In the present embodiment, for example, when focusing on an object at infinity from a near object at the telephoto end,
This is performed by repeating the fourth lens group forward as indicated by a straight line 4c in the figure.

In this embodiment, as compared with the case where the first group lens is extended and the focusing is performed in the conventional four-group zoom lens, the rear focus type as described above is adopted to increase the effective lens diameter of the first lens group. Is effectively prevented.

By arranging the aperture stop immediately before the third lens unit, the variation of aberration due to the movable lens unit is reduced, and by shortening the distance between the lens units in front of the aperture stop, the front lens diameter can be reduced. Achieved easily.

Then, by specifying the optical constants of each lens group as in the conditional expressions (1) and (2), a high zoom ratio having good optical performance over the entire zoom range and over the entire object distance. Got a ratio zoom lens.

Next, the technical meaning of each conditional expression will be described.

Conditional expression (1) is a condition relating to the ratio of the focal lengths of the third lens group and the fifth lens group, and mainly for shortening the lens length after the third lens group while maintaining good optical performance. belongs to. If the lower limit of conditional expression (1) is exceeded and the refracting power of the fifth lens group becomes too strong, the negative Petzval sum increases and it becomes difficult to correct the field curvature. On the other hand, if the upper limit is exceeded and the refracting power of the fifth lens group becomes too weak, it becomes difficult to achieve a sufficient reduction in the total lens length.

Conditional expression (2) relates to the magnification of the fifth lens group and is for obtaining a predetermined optical performance while shortening the total lens length. When the lower limit value is exceeded and the magnification of the fifth lens unit becomes small, it becomes difficult to reduce the total lens length. On the other hand, if the magnification exceeds the upper limit and the magnification becomes large, it becomes an advantageous configuration for shortening the total lens length, but it becomes difficult to confirm the predetermined back focus and the distance between the exit pupil and the image plane becomes short. That is, the telecentricity is considerably deteriorated, and it becomes difficult to apply this zoom lens to a video camera.

The zoom lens of the present invention can be achieved by satisfying the above conditions, but it is desirable to further satisfy the following conditions in order to obtain better optical performance.

That is, the focal lengths of the first group are set to fi,
When the focal lengths at the wide-angle end and the telephoto end are f _W and f _T , respectively,

[0029]

[Outside 3] 0.5 <f ₃ / f ₄ <1.2 (4) Conditional expression (3) relates to the refracting power of the second lens group, effectively reduces a variation in aberration due to zooming, and effectively achieves a predetermined zoom ratio. It is for getting. If the refractive power of the second lens unit becomes too strong beyond the lower limit, it is easy to downsize the entire lens system, but the Petzval sum increases in the negative direction, the field curvature increases, and the aberration fluctuation associated with zooming increases. Is getting bigger. If the refractive power of the second lens group becomes too strong beyond the upper limit, the aberration variation due to zooming will decrease, but the amount of movement of the second lens group to obtain a predetermined zoom ratio will increase, and the total lens length will increase. It ’s not good because

Conditional expression (4) relates to the refracting powers of the third lens group and the fourth lens, and is for shortening the lens length after the third lens group while maintaining a predetermined optical performance.

If the refracting power of the three groups becomes too strong beyond the lower limit, it is advantageous for shortening the total lens length, but it becomes difficult to satisfactorily correct spherical aberration and coma and it becomes difficult to secure back focus. It is not good because it does. If the upper limit is exceeded and the refractive power of the third lens group becomes too weak, the shortening of the total lens length will be insufficient.

If the total lens length is shortened under the conditional expressions (1) and (2), the Petzval sum tends to increase in the negative direction. On the other hand, it is preferable that the fifth lens group is composed of one concave lens and that the condition N _S1 > 1.6 (5) is satisfied when the refractive index of the fifth lens unit is N _S1. Good.

Further, in order to effectively achieve the shortening of the lens length in the present invention, when the magnification at the telephoto end of the second lens group with respect to the object point at infinity is B _2T and the magnification ratio of the entire system is Z,

[0034]

[Outside 4] It is better to satisfy the following condition.

If the lower limit of conditional expression (6) is exceeded, the amount of movement of the second lens group required for zooming will become large, and shortening of the total lens length will not be achieved sufficiently. Although the amount of movement required for zooming the group is small, the speed required for moving the fourth lens group at the time of zooming in the vicinity of the telephoto end increases, and the load on the lens driving means such as the actuator increases. So not good.

Further, in the present invention, by disposing at least one convex lens whose both surfaces are aspherical surfaces in the fourth lens group, it is possible to reduce the fluctuation of spherical aberration during zooming.

Furthermore, in the present invention, by introducing at least one aspherical surface into the fifth lens group, it is possible to remove high-order flare around the screen.

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 from the object side, and Ni and νi are the values of the i-th lens in order from the object side, respectively. The refractive index of glass and the Abbe number.

Table 1 shows the relationship with each conditional expression in each numerical example. In addition, R20 and R21 in Numerical Examples 1 to 5 and 7, and R19 and R20 in Numerical Example 6
Is a glass material such as a face plate.

The aspherical shape has an X axis in the optical axis direction, an H axis in the direction perpendicular to the optical axis, a positive light traveling direction, R ₀ is a paraxial radius of curvature, and K, B, C, D, and E are aspherical shapes, respectively. Assuming the spherical coefficient,

[0041]

[Outside 5] It is expressed by the formula.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

[0046]

[Table 5]

[0047]

[Table 6]

[0048]

[Table 7]

[0049]

[Table 8]

[0050]

According to the present invention, as described above, the lens configuration is such that the refractive powers of the five lens groups and the moving conditions of the second and fourth groups in zooming are set and the fourth group is moved during focusing. By adopting the above, it is possible to reduce the size of the entire lens system, achieve good aberration correction over the entire zoom range with a zoom ratio of about 8 to 10, and have high optical performance with little aberration fluctuation during focusing. F number 1.8-
A rear focus type zoom lens having a large aperture ratio of about 2.0 can be achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing a paraxial arrangement of a zoom lens according to the present invention and a movement locus of each lens.

FIG. 2 is a lens cross-sectional view of Numerical Example 1 according to the present invention.

FIG. 3 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 1.

FIG. 4 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 1.

FIG. 5 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 1.

FIG. 6 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 2.

FIG. 7 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 2.

FIG. 8 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 2.

FIG. 9 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 3.

FIG. 10 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 3.

FIG. 11 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 3.

FIG. 12 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 4.

FIG. 13 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 4.

FIG. 14 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 4.

FIG. 15 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 5.

16 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 5. FIG.

FIG. 17 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 5.

FIG. 18 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 6.

FIG. 19 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 6.

FIG. 20 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 6.

FIG. 21 is a diagram of various types of aberration at the wide-angle end of the zoom lens according to Numerical Example 7.

FIG. 22 is a diagram of various types of aberration in the intermediate region of the zoom lens according to Numerical Example 7.

FIG. 23 is a diagram of various types of aberration at the telephoto end of the zoom lens according to Numerical Example 7.

[Explanation of symbols]

 I First lens group II Second lens group III Third lens group IV Fourth lens group ΔM Meridional image plane ΔS Sagittal image plane d d line g g line

Claims (7)

[Claims] 1. A first lens element having a positive refractive power in order from the object side.
A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a positive refractive power, and a fifth lens group having a negative refractive power, Zooming is performed by moving the second and fourth lens groups and focusing is performed by moving the fourth lens group, and the focal lengths of the third lens group and the fifth lens group are f ₃ , f ₅ , respectively. When the imaging magnification of the fifth lens group when the subject distance is an infinite distance is β ₅ , 0.8 <| f ₅ / f ₃ | <2.1 ... (1) 1.2 <β ₅ <1 .6 ... (2) A rear focus type zoom lens characterized by satisfying the following conditional expression. 2. When the focal length of the second lens group is f ₂ and the focal lengths of the entire system at the wide-angle end and the telephoto end are f _W and f _T , respectively, The rear focus type zoom lens according to claim 1, wherein the following conditional expression is satisfied. 3. When the focal lengths of the third lens group and the fourth lens group are f ₃ and f ₄ , respectively, a conditional expression of 0.5 <f ₃ / f ₄ <1.2 (4) The rear focus type zoom lens according to claim 1, wherein the zoom lens is satisfied. 4. The fifth lens group is composed of one concave lens, and when the refractive index of the lens is N _S1 , N _S1 > 1.6 ... (5). Rear focus type zoom lens. 5. When the magnification at the telephoto end when the object distance of the second lens group is infinity is β _2T and the zoom ratio is Z, then The rear focus type zoom lens according to any one of claims 1 to 4, wherein the following condition is satisfied. 6. The rear focus type zoom lens according to claim 1, wherein the fourth lens group has at least one convex lens whose both surfaces are aspherical surfaces. 7. The rear focus type zoom lens according to claim 1, wherein the fifth lens group includes a lens having at least one aspherical surface.