JPH0651199A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To provide the rear focus type zoom lens which has long back focus and is applicable to a multiplate type video camera. CONSTITUTION:This rear focus type zoom lens consists of four groups, i.e., a 1st lens group I which has positive refracting power, a 2nd lens group II which has negative refracting power, a 3rd lens group III which has positive refracting power, and a 4th lens group IV which has positive, refracting power; and the power is varied from a wide-angle end to a telephoto end by moving the 2nd lens II from an object side to an image plane side, image plane variation accompanying the power variation is corrected by moving the 4th lens group IV, and the lens is put in focus by moving the 4th lens group IV. Then 3.5< F3/F4 holds, where Fi is the focal length of an (i)th lens group.

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 has a zoom ratio of 1 used in video cameras and the like.
The present invention relates to a rear focus type zoom lens having 0 or more zoom lenses, and particularly having a long back focus.

[0002]

2. Description of the Related Art Conventionally, in a zoom lens such as a photographic camera or a video camera, focusing is performed by moving a lens unit other than the first lens unit on the object side. Various types of so-called rear focus type have been proposed.

In general, the rear focus type is carried out by moving a relatively small and lightweight lens group, and therefore has 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 Japanese Patent Laid-Open No. 63-44614, a first group having a positive refractive power and a second group having a negative refractive power for zooming are arranged in order from the object side. In a so-called 4-group zoom lens composed of four lens groups, a third lens group having a negative refractive power for correcting an image plane variation due to zooming, and a fourth lens group having a positive refractive power, Move to focus. However, this zoom lens needs to secure a moving space for the third lens unit, which tends to increase the total lens length.
In Japanese Patent Application Laid-Open No. 58-136012, the variable power portion is composed of three or more lens groups, and some of these lens groups are moved for focusing.

In Japanese Patent Laid-Open No. 63-247316, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a positive refractive power are arranged in this order from the object side. It has four lens groups of the fourth lens group, and the second lens group is moved to perform zooming, and the fourth lens group is moved to perform image plane variation and focusing due to zooming.

In Japanese Patent Laid-Open No. 58-160913, the first group having a positive refractive power, the second group having a negative refractive power, the third group having a positive refractive power, and the positive refractive power are arranged in this order from the object side. No. 4 of the fourth lens group, the first group and the second group are moved to perform zooming, and the image plane variation accompanying zooming is performed by moving the fourth group. And one or two of these lens groups
Focusing is performed by moving one or more lens groups.

In Japanese Patent Laid-Open No. 62-24213, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a positive refractive power are arranged in this order from the object side. It has four lens groups of the fourth lens group, the second lens group is moved to perform zooming, and the image plane variation due to zooming is corrected by moving the fourth lens group to perform focusing.

[0008]

Generally, when a rear focus system is adopted in a zoom lens, the entire lens system is downsized as described above, quick focusing is possible, and further close-up photography is facilitated. To be

On the other hand, on the other hand, the fluctuation of aberration 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 infinite object to a short-distance object. The problem arises.

By the way, most of the video cameras for consumer use are currently single-chip type, but a multi-plate type video camera is preferable to obtain higher quality images.

However, in the case of the multi-plate type, since a color separation prism or the like has to be arranged behind the taking lens, a relatively long back focus is required as compared with a single-plate type zoom lens for a consumer video camera. Will occur.

The present invention, while adopting the rear focus method, achieves a good overall object distance from the wide-angle end to the telephoto end while preventing an increase in size of the entire lens system when achieving a large aperture ratio and a high zoom ratio. It is an object of the present invention to provide a rear-focus type zoom lens having a simple structure with excellent optical performance and long back focus.

[0013]

The features of the present invention are that the first lens group having a positive refractive power, the second lens group having a negative refractive power, and the positive refractive power are arranged in order from the object side. And a fourth lens group having a positive refracting power, and moving the second lens group from the object side to the image plane side to change from the wide-angle end to the telephoto end. 2. When the magnification is doubled and the image plane variation due to the magnification change is corrected by moving the fourth lens group and the fourth lens group is moved for focusing, and the focal length of the i-th lens group is Fi. 5 <F ₃ / F ₄ (1) is satisfied.

Further, the third lens group has an aperture stop, the lens group in front of the aperture stop in the third lens group has a negative refracting power, and the rear lens group gives a positive refracting power. Is desirable to secure the back focus.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a lens sectional view of a zoom lens according to the present invention.

I is the first lens group having a positive refractive power, I
I is a second lens group having a negative refractive power and moving along the optical axis for zooming, III is a third lens group having a positive refractive power, and IV is a positive refractive power and zooming It is a fourth lens group for correcting the image plane variation due to, and satisfies the above-mentioned conditional expression. SP is an aperture stop.

In the present embodiment, the fourth lens unit is moved to correct the image plane variation due to zooming and to perform focusing. In particular, during zooming from the wide-angle end to the telephoto end, the lens is moved so as to have a convex locus toward the object side. 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 this embodiment, for example, when focusing on an object at infinity from a near object at the telephoto end,
This is done by moving the fourth lens group forward.

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

In this embodiment, all spherical lenses are used, but by using a lens having an aspherical surface, it becomes possible to further improve the performance and brighten Fno. In particular, by using a lens having an aspherical surface in the third lens group, it is possible to reduce the number of lenses, and at the same time, it is possible to effectively correct spherical aberration and the like by the aspherical surface. Further, by using a lens having an aspherical surface in the fourth lens group, it becomes possible to effectively correct coma and the like.

By setting the conditional expression (1) as described above, it is possible to prevent the lens system from becoming large in size and to have a good optical performance over the entire object distance from the wide-angle end to the telephoto end. We have obtained a rear focus type zoom lens with a long and simple structure.

Next, the technical meaning of the above conditional expressions will be described.

Conditional expression (1) relates to the focal lengths of the third lens unit and the fourth lens unit, and in order to maintain the good optical performance by sufficiently lengthening the back focus while achieving the compactness after the diaphragm. belongs to. Conditional expression (1)
If the focal length of the third lens unit becomes shorter than the lower limit of, it becomes difficult to correct the variation of spherical aberration due to zooming or during focusing. In addition, it becomes difficult to secure a sufficient back focus, and the amount of movement of the fourth lens unit becomes large, which causes a large variation in aberration during zooming and focusing.

Further, in this embodiment, it is preferable that the following conditional expression is satisfied.

[0025] It back focus L _W at the wide angle end for the infinite object distance, when the image height and I _W, the F-number at the wide-angle end and _{Fno W, 2 <L W /(2.I} W · Fno _W ) <4- (2).

Alternatively, when the focal lengths of the second lens group and the third lens group are F ₂ and F ₃ , respectively, 0.0005 <| F ₂ / F ₃ | <0.15- (3).

Conditional expression (2) is a conditional expression regarding the back focus. If the lower limit of conditional expression (2) is exceeded and the back focus becomes short, it becomes impossible to dispose a color separation prism or the like, or the angle of the light beam incident on the prism becomes tight, and color shading occurs. On the contrary, when the back focus becomes longer than the upper limit and the back focus becomes long, the effective diameter of the fourth lens group becomes large and the lens becomes heavy, which causes a problem that smooth focusing cannot be performed.

Conditional expression (3) relates to the focal lengths of the second lens group and the third lens group. When the lower limit of conditional expression (3) is exceeded and the focal length of the second lens group becomes short, the Petzval sum becomes too large, and it becomes difficult to correct aberrations such as image plane tilt. On the contrary, if the focal length of the second lens unit becomes longer than the upper limit and the amount of movement of the second lens unit increases, the front lens diameter becomes too large and the focal length of the third lens unit becomes short, sufficient back focus cannot be obtained. The problem arises. Further, in the present embodiment, a diaphragm is provided in the third lens group, and a negative refracting power is given to the front of the third lens group and a positive refracting power is given to the rear thereof, so that the back focus is easily lengthened.

Next, numerical examples of the present invention will be shown. Note that the i-th curvature radius R ₁ includes, in order from the object side in the numerical examples, D _i is the i-th lens thickness or air separation in order from the object side, N _i and [nu _i i-th from the respective object side The refractive index and Abbe number of the glass of the th lens.

Further, R27 to R29 in Numerical Embodiment 1
Indicates color separation optical heat, face plate, and the like.

[0031]

[Table 1]

[0032]

[Outer 1]

[0033]

[Outside 2]

[0034]

[Outside 3]

[0035]

[Outside 4]

[0036]

According to the present invention, while the refractive power of each lens group is set and the fourth lens group is moved during focusing as described above, the lens system as a whole can be downsized. A rear focus type zoom lens having a large aperture ratio with a zoom ratio of 10 times or more and a good aberration correction over the entire zoom range and with little fluctuation in aberration during focusing and a sufficiently long back focus can be achieved. it can.

[Brief description of drawings]

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

FIG. 2 is a diagram of various types of aberration at the wide-angle end according to Numerical Example 1.

FIG. 3 is a diagram of various types of aberration at the telephoto end according to Numerical Example 1.

FIG. 4 is a lens cross-sectional view of Numerical Example 2 according to the present invention.

FIG. 5 is a diagram of various types of aberration at the wide-angle end according to Numerical Example 2.

FIG. 6 is a diagram of various types of aberration at the telephoto end according to Numerical Example 2.

FIG. 7 is a lens cross-sectional view of Numerical Example 3 according to the present invention.

FIG. 8 is a diagram of various types of aberration at the wide-angle end according to Numerical Example 3.

FIG. 9 is a diagram of various types of aberration at the telephoto end according to Numerical Example 3.

FIG. 10 is a lens cross-sectional view of Numerical Example 4 according to the present invention.

FIG. 11 is a diagram of various types of aberration at the wide-angle end according to Numerical Example 4.

FIG. 12 is a diagram of various types of aberration at the telephoto end according to Numerical Example 4.

[Explanation of symbols]

 d d line g g line ΔM meridional image surface ΔS sagittal image surface I First lens group II Second lens group III Third lens group IV Fourth lens group

Claims (4)

[Claims] 1. A first lens element having a positive refractive power in order from the object side.
The second lens group includes a lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The lens unit is moved from the object side to the image plane side to perform zooming from the wide-angle end to the telephoto end, and the image plane fluctuation due to zooming is corrected by moving the fourth lens unit and focusing is performed. A rear focus type zoom lens which satisfies the condition 3.5 <F ₃ / F ₄ when the focal length of the lens group is Fi. 2. A diaphragm is provided in the third lens group,
2. The rear focus type zoom lens according to claim 1, wherein the lens group in front of the diaphragm in the lens group has a negative refracting power, and the lens group in the rear side has a positive refracting power. 3. The back focus at the wide-angle end is set to L
_{W, 2 <L W / (} 2 · I W · Fno W) < rear focus type zoom lens according to claim 1 which satisfies the 4 The condition when the image height and I _W, the F-number and Fno _W. 4. When the focal lengths of the second lens group and the third lens are F ₂ and F ₃ , respectively, the condition 0.0005 <| F ₂ / F ₃ | <0.15 is satisfied. The rear focus type zoom lens according to the above item 1.