1. Field of the Invention
The present invention relates generally to a zoom lens and, more particularly, to a compact wide-angle zoom lens suitable for use with a 35 mm format camera.
2. Related Background Art
In recent years, a super wide-angle lens and a wide-angle lens employed as an interchangeable lens for a 35 mm format still camera have become smaller and smaller in size and higher and higher in performance. Especially for actualizing a small-sized wide-angle zoom lens at a low price, the optimum lens type is a so-called negative/positive two-group zoom lens. A variety of types have hitherto been proposed. Further, it has recently been possible to attain a miniaturization, a higher performance and a drop in costs by incorporating an aspherical lens surface. Examples of such known zoom lenses are shown in Japanese Patent Application Laid-open Nos. 62-94812 and 63-136013, and U.S. Pat. No. 5,076,677.
The wide-angle zoom lens disclosed in Japanese Patent Application Laid-open No. 62-94812 stated above, however, presents the following defect. This wide-angle zoom lens is, though small in the number of constructional lenses and also relatively small in total thickness of the lens system, relatively large in total length at the wide-angle end. A diameter of the front lens element increases. It is difficult to install a .phi.52 mm filter that has already been generalized as a standard filter size. The following is an additional defect. If forcibly installed, this brings about a remarkable reduction in amount of marginal rays at the wide-angle end. The principal ray having the maximum field angle is eclipsed during near distance photographing. Besides, in an aspect of aberrations also, it is insufficient to compensate a coma, an astigmatism and a curvature of field in the vicinity of the maximum field angle at the wide-angle end. It is also insufficient to compensate a chromatic aberration of magnification and a downward coma (hereafter called a [downward chromatic coma]) due to a difference between wavelengths of rays.
The super wide-angle zoom lens disclosed in Japanese Patent Application Laid-open No. 63-136013 stated above exhibits such an advantage that this zoom lens has the maximum field angle as large as 92.degree. or more. The super wide-angle zoom lens is, however, constructed of a Great number of lenses and is large both in size and in diameter of the front lens element. This zoom lens is therefore insufficient in terms of reducing the size and the costs as well. A filter size is also large. Further, in the aspect of aberrations, the compensations of the curvature of field and the astigmatism are not enough from the wide-angle end to a state of intermediate focal length.
In addition, U.S. Pat. No. 5,076,677 by the same applicant as the one of the present invention discloses a small-sized wide-angle zoom lens having a field angle of approximately 76.degree. at the wide angle and a small diameter of the front lens element. If the field angle at the wide-angle end increases up to as large as 84.degree. with this lens construction, a total length at the wide-angle end outstandingly elongates. The diameter of the front lens element is also dilated, which in turn causes an increase in the filter size. Further, the amount of marginal rays is conspicuously reduced. Eventually, this conduces to a defect in which the eclipse occurs. Besides, in terms of aberrations, there are undesirably worsened particularly the curvature of field, the astigmatism, the downward coma, the downward chromatic coma and the distortion.
Hence, it is difficult to actualize the small-sized wide-angle zoom lens exhibiting a good performance with such refracting power arrangements and lens constructions of the known zoom lenses.