JPH0271220A - High power compact zoom lens - Google Patents

Abstract

PURPOSE:To vary the power from the wide angle end to the telephoto end by providing a first lens group having a positive refracting power, a second lens group having a negative refractive index, and a third lens group having a positive refracting power which are arranged in order from the object side and distributing the refracting power in prescribed conditions. CONSTITUTION:The first lens group having a positive refracting power, the second lens group having a negative refracting power, and the third lens group having a positive refracting power are provided in order from the object side. Refracting powers of respective groups are proportionally distributed in accordance with inequalities I, II, and III where f1, f2, f3, and r3-1 are the focal length of the first lens group, that of the second lens group, that of the third lens group, and the radius of curvature of the object-side face of the third lens group respectively. In this case, inequalities I and II prescribe the refracting power distribution of respective lens groups, and the inequality II prescribes the refracting power of the concave facing the second lens group of the third lens group which is provided to correct various aberrations more proportionally. Thus, >=2.5 variable power ratio and a satisfactory lens performance are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a zoom lens with a short back focus suitable for lens shutter cameras.

(Prior technology) In recent years, there has been a demand for small cameras that can change the angle of view, such as lens shutter cameras that do not require changing lenses, and are equipped with bifocal cameras or zoom lenses with a magnification ratio of about 2x. cameras are beginning to become popular.

Conventionally, this type of zoom lens can typically be classified into the following three types.

(B) Two-group type This type consists of a first lens group with a positive focal length and a second lens group with a negative focal length, and one of its typical examples is disclosed in Japanese Patent Application Laid-Open No. 57-201213. There is an invention of an ultra-compact zoom lens described in a publication. With this type, the lens system is very easy to create, but since the light passing position in the lens system changes between the wide-angle side and the telephoto side, it becomes difficult to balance aberrations such as chromatic aberration and distortion. The maximum magnification ratio is about 2 times.

(b) Three-group type This consists of the 1121st lens group with a positive focal length, the second lens group with a negative focal length, and the third lens group with a positive focal length, and is a typical example. As, Unexamined Japanese Patent Publication No. 5
8-184916, JP 58-199312, JP 6239812, JP 62-18
There is an invention of a zoom lens described in Publication No. 7315 and the like.

A feature of this type is that a third lens group is provided to correct the various aberrations that occur in the two-group type (a).However, in these conventional technologies, the variable power ratio is At most, it is about twice as large, and the advantage of providing the third lens group, which has a larger lens outer shape than the two-group type (a), is not utilized at all.

(c) 4-group type This is already common in zoom lenses for eye reflex cameras, in order to obtain a high zoom ratio, the 1121st group has a positive focal length, and the 1121st group has a negative focal length. a second lens group having a distance; a third lens group having a positive focal length;
and a fourth lens group having a negative focal length, and typical examples include the zoom lens inventions described in, for example, Japanese Patent Laid-Open No. 60-57814 and Japanese Patent Laid-Open No. 63-43115. Characteristics of this type of lens include that it has a smaller amount of movement than the types (a) and (b) above, and can have a large variable power ratio.

However, since it has four lens groups, it is structurally complex and expensive, making it difficult to manufacture. Also, the total length of the lens system at the wide-angle end is as described in (a),
It is longer than type (b) and is not suitable for compact cameras with excellent portability.

(Problems to be Solved by the Invention) In the proposals of the prior art mentioned above, if the lens system is simple and compact, the magnification ratio is limited to about 2x, and if an attempt is made to obtain a zoom lens with an even higher magnification, , has a long overall length and a complex structure.

The present invention is a three-group type zoom lens that has a zoom ratio of 2.5 times or more from the wide-angle end to the telephoto end, has good lens performance, and has a simplified zoom mechanism similar to the two-group zoom system. The purpose is to provide the following.

(Means for solving the problems) In order to achieve the above problems, the zoom lens of the present invention has the following features:
It has, in order from the object side, a 1121st group having a positive refractive power, a second lens group having a negative refractive index, and a third lens group having a positive refractive power, and the 1121st group and the second lens group move toward the object while monotonically decreasing the distance between
This allows you to change the magnification from the wide-angle end to the telephoto end. The third lens group may be fixed or may be movable relative to the first and second lens groups. When moving the third lens group, it is possible to satisfactorily correct the movement of the image plane caused by zooming.

The third lens group helps correct lateral chromatic aberration, distortion, and coma, which increase when the zoom ratio is increased while maintaining the two-group zoom system, thus ensuring a zoom ratio of 2.5x or more. make it possible to

In addition to the above basic configuration, the zoom lens of the present invention preferably satisfies the following conditions.

(1) 0.85< f2 /f+ <0.9. f2<
0(2) 5.0 < f 3 / f + <&
0(3) 0.33<r':+-+/fs
<0.5. (r3-+ is a concave surface, r3-1<0) However, f1, f2, and r3 are the focal lengths of the 1121st lens group, the second lens group, and the third lens group, respectively.

r3-8 is the radius of curvature of the third lens group on the object side.

(Function) The present invention can easily correct distortion and chromatic aberration, which are difficult to correct when trying to obtain a zoom ratio of 2x or more with a 2-group type, using a 3-group type with 9 lenses. This is made possible by a lens configuration that is extremely compact, especially at the wide-angle end. This is achieved by arranging the refractive powers of each lens group in a well-balanced manner, and conditions (1) and (2) define the refractive power distribution of each lens group.

Weakening the refractive power of each lens group is advantageous for correcting aberrations, but the amount of movement of each lens group increases when changing the magnification, making it difficult to make the lens compact. If the lower limit of condition (1) is exceeded, the amount of movement of the first and second lens groups during zooming will become smaller, making it more compact even at the telephoto end. The spherical aberration and coma aberration become particularly large on the wide-angle side, and are difficult to correct using other lens groups. Exceeding the upper limit is advantageous in terms of aberration correction, but the overall length of the lens system becomes longer, and the distance between the principal points of the first lens group and the second lens group at the telephoto end becomes smaller.
Mechanical space for a shutter, aperture, etc. cannot be secured.

Condition (2) defines the refractive power distribution between the first lens group and the third lens group having positive refractive power.

The optical path changes due to zooming, and the aberrations generated in the third lens group also cause aberration fluctuations, but if the refractive power is within the range defined by condition (2), the effect is small.

Since the third lens group is composed of a single lens with positive refractive power, it is likely to generate negative distortion, and at wide-angle, positive distortion, which is a characteristic of the two-group type, occurs in the front lens group. It is effective in correcting. If the upper limit of condition (2) is exceeded,
Further, the refractive power borne by the third lens group is weakened, and although fluctuations in aberrations occurring in the third lens group are reduced, it becomes difficult to correct aberrations that are insufficiently corrected in the front group. Moreover, if the lower limit is exceeded, the amount of aberration generated in the third lens group becomes large, making it difficult to correct it with other lens groups.

Furthermore, one of the effects of providing the third lens group is the correction of lateral chromatic aberration, which is insufficiently corrected with the two-group type when the zoom ratio exceeds 2x and the magnification increases. Since the off-axis rays passing through the lens located closest to the image side of the second lens group change between the wide-angle side and the telephoto side, the third lens group corrects the chromatic aberration of magnification on the wide-angle side and the telephoto side in a well-balanced manner. Condition (3)
defines the refractive power of the concave surface of the third lens group facing the second lens group in order to further correct various aberrations such as distortion, lateral chromatic aberration, and astigmatism in a well-balanced manner.
If the upper limit is exceeded, positive distortion will be undercorrected. If the lower limit is exceeded, the astigmatism becomes large and it becomes difficult to correct the panlas of field curvature, resulting in excessive field curvature especially on the wide-angle side, which is difficult to correct with other components.

(Example) The basic configuration of the zoom lens of the present invention is shown in FIGS. 1(a) and 1(b).
) as shown. In addition, each example is shown in Fig. 2, Fig. 4,
6, 8, and 10, the first lens group 1 consists of a positive meniscus lens with a convex surface facing the object side in order from the object side, and a surface with a strong curvature facing the object side. The second lens group 2 consists of a negative lens having a negative lens, a positive lens having a surface with strong curvature on the image side with a cemented or slight air gap, and a cemented lens of a positive lens and a negative meniscus lens.
The third lens group 3 consists of a positive lens having a surface with strong curvature on the image side, a negative lens having a surface with strong curvature on the object side, and a negative meniscus lens having a surface with strong curvature on the object side.
consists of a single positive meniscus lens with the convex surface facing the image side.

Next, each example will be shown. nd and vd are the refractive index and Abbe number of d-1ine.

Note that although glass material is used for the lenses of the third lens group, it is easy to replace them with acrylic (PMMΔ) resin lenses, thereby providing an even cheaper lens system.

(Example 1) ■ (Example 3) (Example 2) (Example 4) (Example 5) (Effects of the invention) As described above, according to the present invention, the number of lenses constituting the lens is as small as nine. Despite the configuration, a zoom lens that is compact, has a zoom ratio of 2.5 times or more, and has various aberrations well corrected can be obtained.

[Brief explanation of the drawing]

FIG. 1(a) is a basic configuration diagram of Embodiment 1.2.3.4 of the present invention, FIG. 1(b) is a basic configuration diagram of Embodiment 5, FIG.
FIG. 4, FIG. 6, FIG. 8, and FIG. 10 are configuration diagrams of each embodiment. FIG. 3, FIG. 5, FIG. 7, FIG. 9, and FIG. 11 are diagrams of various aberrations of each example. Reference numeral 1: first lens group, 2: second lens group, 3: third lens group. Figure 5 Figure 6 Spherical Aberration Distortion Astigmatism Figure 8 Figure 10 Figure 11 0.2 0.2 Spherical Aberration Distortion Astigmatism 1: 6.6 -0.2 0.2 Spherical Aberration Distortion Aberration Astigmatism + 79.6 Spherical Aberration Distortion Aberration Astigmatism

Claims (1)

[Claims] In order from the object side, the first lens group has a positive refractive power, the second lens group has a negative refractive power, and the third lens group has a positive refractive power.
In a zoom lens having a lens group, (1) 0.85<|f_2|/f_1<0.9, f_2
<0 (2) 5.0<f_3/f_1<8.0 (3) 0.33<|r_3_-_1|/f_3<0.5
, (r_3_-_1 is a concave surface, r_3_-_1<0) However, f_1, f_2, f_3 are the focal lengths of the first lens group, second lens group, and third lens group, respectively, and r_3_
-_1 is a radius of curvature on the object side of the third lens group. A high-magnification compact zoom lens.