JPH0416764B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a small zoom lens, and particularly to a small zoom lens that does not have a large back focus and is suitable for use in a lens shutter type camera. Recently, as lens-shutter cameras have become more multifunctional, there has been a demand for this type of highly portable camera to be able to use a zoom lens.
However, most conventional zoom lenses are designed for single-lens reflex cameras with a reflector at the rear of the lens, and because they are designed with a long back focus, they have problems with compactness, and the entire camera This leads to an increase in size. The present invention aims to provide a compact zoom lens that has a simple structure, does not have a large back focus, and is suitable for a highly portable camera such as a lens shutter type camera. That is, in the zoom lens of the present invention, the lens groups are arranged in this order from the object side: a first lens group having a positive focal length, a second lens group having a positive focal length, and a third lens having a negative focal length. , at the wide-angle end, by bringing the first lens group and the second lens group close together and moving the third lens group near the image plane, the combined focal length of the entire system is approximately the same as that of the first lens group and the second lens group. Get the shortest focal length by just equaling the composite focal length. At the telephoto end, by bringing the second and third lens groups close together, their respective refractive powers cancel each other out, making the combined focal length of the entire system almost the same as the first lens group.
The focal length of only the lens group is made equal to the maximum focal length, and between the shortest focal length and the longest focal length, the first and third lens groups are moved monotonically toward the object along the optical axis. The second lens group is moved so as to correct the focal shift due to zooming. A zoom lens with such a configuration changes magnification while maintaining a positive/positive/negative refractive power distribution from the object side, so that the refractive power distribution is always telephoto during zooming, and the overall length of the lens is shortened. It is possible to do this. The focal length of the first lens group is f ₁ , the focal length of the second lens group is f ₂ , the focal length of the third lens group is f ₃ , the shortest value of the combined focal length of the entire system is f _w , and the longest value is f When _T , it is desirable that the zoom lens system of the present invention satisfies the following conditions. f _T ／2 ＜ f ₁ ＜ 2f _T ……(1) 1.3 ＜ f ₁ + f ₂ ／f ₂ ＜2.5 ……(2) −f ₂ ／2 ＜ f ₃ ＜−2f ₂ ……(3) Condition ( 1) is related to the focal length of the first lens group; if the lower limit is exceeded, the partial systems of the second and third lens groups must become strongly negative lenses at the telephoto side, and spherical aberration and field curvature will be reduced. There is a strong tendency to exceed the value, and it becomes impossible to correct it by selecting other conditions. On the other hand, when the upper limit is exceeded, the contribution of the first lens group to zooming becomes smaller. Condition (2) relates to the variable power ratio, and in order to obtain a sufficient variable power ratio, the lower limit must not be exceeded. If the upper limit is exceeded, the refractive power of the second lens group becomes too strong, and the spherical aberration on the wide-angle side tends to be undervalued, making it difficult to correct it. Condition (3) relates to the focal length of the third lens group; exceeding the lower limit is effective in shortening the overall length, but the spherical aberration on the telephoto side tends to be excessive. On the other hand, if the upper limit is exceeded, it becomes difficult to offset the refractive power with the second lens group, and efficient zooming becomes impossible. Specifically, the zoom lens system of the present invention includes, in order from the object side, a first lens group having a positive focal length, a second lens group having a positive focal length, and a third lens group having a negative focal length. It has a three-group configuration, and as the zoom is changed from the wide-angle end to the telephoto end, the first and third lens groups move monotonically from the image side to the object side along the optical axis, and the second lens group moves as follows: A zoom lens system in which the first lens group is closest to the first lens group at the wide-angle end, the third lens group is the closest to the third lens group at the telephoto end, and the first lens group moves to compensate for focal shift due to zooming at focal lengths in between. The lens group consists of at least one positive lens and one negative lens, the second lens group consists of at least one positive lens, and the third lens group consists of at least one positive lens and one negative lens.
The lens groups consist of at least one negative lens with a strongly concave surface facing the object side, and the movement path of each lens group is shown in FIG. Further, the first and second embodiments are shown in the following table, and cross-sectional views of the lenses are shown in FIGS. 2 and 3.

【table】

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is a movement path diagram of each lens group of the zoom lens of the present invention, FIGS. 2 and 3 are sectional views of the first and second embodiments, respectively, and FIGS. 4 and 5 are cross-sectional views of the first and second embodiments, respectively. FIG. 3 is an aberration curve diagram of a first example and a second example.

Claims (1)

[Claims] 1. It has a three-group configuration: the first lens group with a positive focal length, the second lens group with a positive focal length, and the third lens group with a negative focal length, in order from the object side, from the wide-angle end to the telephoto end. As the magnification changes, the first and third lens groups move monotonically from the image side to the object side along the optical axis, and the second lens group approaches the first lens group at the wide-angle end, and at the telephoto end. A zoom lens system that is closest to the third lens group at the end and moves to compensate for focal shift due to zooming at the focal length between, and the first lens group has at least one positive lens and one negative lens. Consisting of a lens, the second
The lens group consists of at least one positive lens,
The third lens group is a small three-group zoom lens consisting of at least one negative lens with a strongly concave surface facing the object side.