JP3015192B2 - Rear focus zoom lens - Google Patents

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, and more particularly to a rear focus type zoom lens which is used for a photographic camera or a video camera and has a high zoom ratio of about 8 to 10 or a large aperture ratio of about F2. The present invention relates to a rear-focus type zoom lens that has a small size while being held.

[0002]

2. Description of the Related Art Conventionally, various types of zoom lenses such as a photographic camera and a video camera adopt a so-called rear focus type in which a lens group other than the first lens group on the object side is moved to perform focusing. Have been.

In general, a rear focus type zoom lens has a smaller effective diameter of the first lens group than a zoom lens which performs focusing by moving the first lens group, so that the entire lens system can be easily miniaturized. Since close-up photography, especially very close-up photography, is facilitated, and the relatively small and lightweight lens group is moved, the driving force of the lens group is small and quick focusing can be performed.

As such a rear focus type zoom lens, for example, Japanese Patent Application Laid-Open No. 63-247316 discloses a positive first lens group, a negative second lens group, and a positive third lens group in this order from the object side. , Having a positive fourth lens group,
The zooming is performed by moving the lens group and the fourth lens group.
A zoom lens that performs focusing by moving a lens group is disclosed.

In Japanese Patent Laid-Open Publication No. 58-160913, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and
The zoom lens has four lens groups, a third lens group having a positive refractive power and a fourth lens group having a positive refractive power. The first and second groups are moved to perform zooming, and the image plane varies with zooming. Is performed by moving the fourth group. Then, focusing is performed by moving one or more of these lens groups.

Further, Japanese Patent Application Laid-Open Nos. 58-129404 and 61-258217 disclose a positive first lens unit, a negative second lens unit, a positive third lens unit, and a positive third lens unit. A zoom lens that includes a fourth lens group and a negative fifth lens group and performs focusing by moving the fifth lens group or a plurality of lens groups including the fifth lens group is disclosed. Japanese Patent Application Laid-Open No. 60-6914 discloses a zoom lens having the same refractive power arrangement as described above, and the position of the focus lens group on the optical trajectory for a specific finite distance becomes constant regardless of zooming. Such a zoom lens is disclosed.

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

[0008]

Generally, when a rear focus system is employed in a zoom lens, the entire lens system can be reduced in size as described above, quick focusing can be achieved, and further advantages such as close-up photographing can be easily obtained. .

On the other hand, however, aberration fluctuations during focusing become large, and it becomes very difficult to obtain high optical performance while reducing the size of the entire lens system over the entire object distance from an object at infinity to a close object. The problem arises.

Particularly, a zoom lens having a large aperture ratio and a high zoom ratio has a problem that it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance.

A first lens group having a positive refractive power, 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. The rear focus type zoom lens composed of five lens units is advantageous in shortening the overall length of the lens. However, in order to effectively reduce the overall length, the refractive power of the second lens unit must be increased. Since the amount of movement of the lens unit is reduced and the refractive power of the fifth lens unit is increased to shorten the third and subsequent lens units, the Petzval sum increases in the negative direction, and particularly, the field curvature during zooming However, there is a problem that it becomes difficult to correct the fluctuations of the data.

The present invention employs a rear focus system to achieve a large aperture ratio and a high zoom ratio, while preventing a further increase in the size of the entire lens system. It is an object of the present invention to provide a rear-focus type zoom lens having a simple configuration having excellent optical performance over a double range and over an entire object distance from an object at infinity to an object at a short distance.

[0013]

A zoom lens according to the present invention comprises a first lens unit having a positive refractive power in order from the object side,
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. Focusing is performed by moving the fourth lens group while moving the second and fourth lens groups. The focal lengths of the third lens group and the fifth lens group are f ₃ and f ₅ , respectively. The imaging magnification of the fifth lens group at infinite distance is β
_5. The focal length of the second lens group is f ₂ , wide-angle end and telephoto end
Assuming that the focal lengths of the entire system at f _W and f _T are respectively 0.8 <| f ₅ / f ₃ | <2.1 (1) 1.2 <| β ₅ | <1.6 (( 2)

[Outside 3] This satisfies the following conditional expression.

[0014]

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

In the figure, I is a first lens unit having a positive refractive power,
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 denotes 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 shown by the arrow, and the image plane fluctuation accompanying the zooming is corrected by moving the fourth lens group. .

Also, a rear focus system is employed in which the fourth lens group is moved on the optical axis to perform focusing. A solid line curve 4a and a dotted line curve 4b of the fourth lens shown in the same figure show the image plane fluctuation caused by zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance, respectively. The movement locus for correction is shown.

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

In the present embodiment, the fourth lens group is moved to correct the image plane fluctuation caused by 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 zoom lens is moved so as to have a convex trajectory toward the object side when zooming from the wide-angle end to the telephoto end. Thereby, the space between the third lens unit and the fourth lens unit is effectively used, and the overall length of the lens is effectively reduced.

In this embodiment, for example, when focusing from an object at infinity to an object at a short distance at the telephoto end,
This is performed by repeating the fourth lens group forward as indicated by the straight line 4c in FIG.

In the present embodiment, the effective lens diameter of the first lens group is increased by adopting the rear focus method as described above in comparison with a conventional four-group zoom lens in which the first group lens is extended and focused. Is effectively prevented.

By arranging the aperture stop immediately before the third lens group, aberration variation due to the movable lens group is reduced, and the distance between the lens groups in front of the aperture stop is shortened to reduce the diameter of the front lens. Achieved easily.

Then, the above-mentioned conditional expressions (1), (2) ,
By specifying the optical constants of each lens unit as in (3) , a zoom lens having a high zoom ratio having good optical performance over the entire zoom range and over the entire object distance is obtained.

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 unit and the fifth lens unit, and is mainly for maintaining good optical performance while shortening the lens length after the third lens unit. belongs to. If the refractive power of the fifth lens group becomes too strong beyond the lower limit of conditional expression (1), the negative Petzval sum increases, and it becomes difficult to correct the field curvature. On the other hand, if the refractive power of the fifth lens group becomes too weak beyond the upper limit, it becomes difficult to sufficiently reduce the overall length of the lens.

Conditional expression (2) relates to the magnification of the fifth lens unit and is intended to obtain a predetermined optical performance while shortening the overall length of the lens. If the magnification of the fifth lens group is reduced below the lower limit, it becomes difficult to reduce the overall length of the lens. On the other hand, when the magnification is increased beyond the upper limit, it is advantageous for shortening the overall length of the lens, but it becomes difficult to confirm a predetermined back focus or the distance between the exit pupil and the image plane becomes short. That is, the telecentricity deteriorates considerably, and it becomes difficult to apply this zoom lens to a video camera.

Condition (3) relates to the refractive power of the second lens unit, and
Effectively achieves a predetermined zoom ratio while minimizing aberration fluctuation due to magnification
In order to obtain Refractive power of the second group beyond the lower limit
Becomes too strong, it is easy to downsize the entire lens system
However, the Petzval sum increases in the negative direction and the field curvature increases.
As the magnification becomes larger, the aberration fluctuation accompanying the magnification change becomes larger.
In addition, if the refractive power of the second lens unit becomes too strong beyond the upper limit, it will change.
Aberration fluctuation due to magnification is reduced, but to obtain a predetermined zoom ratio
The amount of movement of the second lens unit increases, and the overall length of the lens increases.
Not so good. The zoom lens of the present invention is 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 length of the i-th lens unit is fi.
When

0.5 <f ₃ / f ₄ <1.2 (4)

Conditional expression (4) relates to the refractive power of the third lens unit and the fourth lens unit, and is for shortening the lens length of the third lens unit and thereafter while maintaining a predetermined optical performance.

If the refractive power of the third lens unit becomes too strong beyond the lower limit, it is advantageous for shortening the overall length of the lens, but it becomes difficult to satisfactorily correct spherical aberration and coma aberration, and it becomes difficult to secure the back focus. Is not good. On the other hand, if the refractive power of the third lens group becomes too weak beyond the upper limit, the reduction of the overall length of the lens becomes insufficient.

When the total lens length is reduced under the conditional expressions (1) and (2), the Petzval sum tends to increase in the negative direction. In order to cope with this, it is preferable that the fifth lens group is composed of one concave lens and that the condition of N _S1 > 1.6 (5) is satisfied when the refractive index of the lens is N _S1. Good.

Further, in order to effectively achieve the reduction of the lens length in the present invention, when the magnification at the telephoto end of the second lens unit with respect to the object point at infinity is β _2T and the magnification ratio of the whole system is Z,

[0034]

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

If the lower limit of conditional expression (6) is exceeded, the amount of movement of the second lens unit required for zooming will be large, and the overall length of the lens will not be sufficiently shortened. Although the amount of movement required for zooming the group is small, the speed required for movement of the fourth lens group during zooming near the telephoto end increases, and the load on lens driving means such as an actuator increases. Not so good.

In the present invention, by arranging at least one convex lens whose both surfaces are aspherical in the fourth lens group, the fluctuation of spherical aberration at the time of zooming can be reduced.

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

Next, numerical examples of the present invention will be described. 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 spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass.

Table 1 shows the relationship with each conditional expression in each numerical example. 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 surface has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis, a positive traveling direction of light, R ₀ is a paraxial radius of curvature, and K, B, C, D, and E are aspherical. Assuming spherical coefficients,

[0041]

[Outside 5] It is represented by the following expression.

[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 arrangement for setting the refractive power of the five lens units and the moving conditions of the second and fourth units in zooming and moving the fourth unit during focusing. By adopting the optical system, it is possible to achieve high aberration performance while achieving good aberration correction over the entire zoom range with a zoom ratio of about 8 to 10 while reducing the size of the entire lens system, and with little aberration fluctuation during focusing. F-number 1.8 ~
A rear focus zoom lens having a large aperture ratio of about 2.0 can be achieved.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram illustrating various aberrations of the zoom lens illustrated at Numerical Example 1 at the wide-angle end.

FIG. 4 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 1.

FIG. 5 is a diagram illustrating various aberrations of the zoom lens illustrated at Numerical Example 1 at a telephoto end.

FIG. 6 is a diagram illustrating various aberrations at the wide-angle end of the zoom lens described in Numerical Example 2;

FIG. 7 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 2;

FIG. 8 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 2;

FIG. 9 is a diagram illustrating various aberrations at the wide-angle end of the zoom lens described in Numerical Example 3;

FIG. 10 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 3;

FIG. 11 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 3;

FIG. 12 is a diagram illustrating various aberrations of the zoom lens at a wide angle end according to Numerical Example 4;

FIG. 13 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 4;

FIG. 14 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 4;

FIG. 15 is a diagram illustrating various aberrations of the zoom lens at a wide angle end according to Numerical Example 5;

FIG. 16 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 5;

FIG. 17 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 5;

FIG. 18 is a diagram illustrating various aberrations at the wide-angle end of the zoom lens described in Numerical Example 6;

FIG. 19 is a diagram illustrating various aberrations in the intermediate region of the zoom lens described in Numerical Example 6;

FIG. 20 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in Numerical Example 6;

FIG. 21 is a diagram illustrating various aberrations of the zoom lens at a wide angle end according to Numerical Example 7;

FIG. 22 is a diagram showing various aberrations in the intermediate region of the zoom lens shown in Numerical Example 7;

FIG. 23 is a diagram illustrating various aberrations at the telephoto end of the zoom lens described in 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

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (6)

(57) [Claims] 1. A first lens 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, 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. The focal lengths of the third lens group and the fifth lens group are respectively f ₃ , f ₅ , When the object distance is infinite, the imaging magnification of the fifth lens group is β ₅ , the focal length of the second lens group is f ₂ ,
The focal lengths of the entire system at the wide-angle end and the telephoto end are denoted by f _W and f _T , respectively.
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 lengths of the third lens unit and the fourth lens unit are f ₃ and f ₄ , respectively, the following conditional expression is satisfied: 0.5 <f ₃ / f ₄ <1.2 (4) 2. The rear focus type zoom lens according to claim 1, wherein the zoom lens is satisfied. 3. The fifth lens group according to claim 1, wherein 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 zoom lens. 4. When the magnification at the telephoto end of the second lens group at infinity of the object distance is β _2T and the variable power ratio is Z. 4. A rear focus type zoom lens according to claim 1, wherein the following condition is satisfied. 5. The rear focus type zoom lens according to claim 1, wherein said fourth lens group includes at least one convex lens whose both surfaces are aspherical. 6. The rear focus type zoom lens according to claim 1, wherein said fifth lens group includes a lens having at least one aspheric surface.