(a) Field of the invention
The present invention relates to a large-aperture wide-angle zoom lens system designed for a field angle larger than 70.degree. and a relative aperture of F/2.8.
(b) Description of the prior art
In the recent years, remarkable progress has been made in the field of zoom lens systems in which numerous researches have been made for development of new lens systems. However, the researches have been made chiefly for developing new lens systems having higher zooming ratios and are still insufficient for developing zoom lens systems having large relative apertures. Under the present circumstance, zoom lens systems are generally evaluated as those having relatively small relative apertures. Large relative apertures are demanded especially for the zoom lens systems having field angles larger than 70.degree. at the wide position. However, it is rather hard to realize such a zoom lens system.
As a zoom lens system which is similar to that according to the present invention, there is known the ones, for example, disclosed by Japanese Unexamined Published Patent Applications No. 113012/80 and No. 161718/82. However, each of these lens systems has a small relative aperture of F/4. Further, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 39759/78 has a relatively large relative aperture but is designed for a small zooming ratio of 1.528.
A zoom lens system comprising a front diverging lens group and a rear converging lens group, like that according to the present invention is focused generally by displacing the front lens group. In case of a wide-angle zoom lens system, however, curvature of field is remarkably varied by focusing the lens system from an object located at infinite distance onto another located at a short distance by displacing the front lens group as described above. Therefore, quality of an image formed by offaxial rays is remarkably degraded in a zoom lens system having a relative aperture on the order of F/2.8 though deviation of the image plane due to curvature of field is within the depth of focus in a zoom lens system having a relative aperture on the order of F/4. This fact constitutes hindrance to design of a zoom lens system having a large relative aperture.
There have conventionally been known zoom lens systems comprising two lens groups in which an airspace reserved in the front diverging lens group is varied for correcting curvature of field which is aggravated in conjunction with the displacement of the front diverging lens group for focusing. Zoom lens systems of this type is exemplified by the one disclosed by Japanese Unexamined Published Patent Application No. 32342/77. In this zoom lens system, however, magnification of the rear lens group has different magnification levels at the wide position, standard position and tele position respectively. Accordingly, the rear lens group causes different influences on aberrations in the entire lens system at different zooming conditions, thereby making different degrees of aberrations to be corrected at the individual conditions. This makes it difficult to find out an airspace and its variation ratio for favorably correcting aberrations in all the zooming conditions.
Moreover, there has already been known a zoom lens system comprising two lens groups in which the rear lens group is further divided into two groups, and an airspace reserved between these two groups is varied to correct aberrations which are aggravated by focusing the lens system on an object located at a short distance. However, this lens system actually comprises three lens groups in place of two lens groups, and therefore requires a complicated mechanism for zooming.