As an observation or photographic camera is often used indoors, where light levels are generally lower than outdoor, a lens with a low F-number is favored. Further, in order to view or photograph an area wider than the field of view of most cameras, a super wide angle lens is required.
Normally, a wide angle lens generates a negative distortion aberration, thereby imparting a size reduction effect on the image in the peripheral areas of the angle of view due to the distortion. However, when using a super wide angle lens having a very large angle of view, the distortion aberration is exacerbated to the extent that it becomes an even more serious problem, bringing the utility of such a lens into question. Therefore, a super wide angle lens having less distortion and which effectively corrects the image aberrations over the entire angle of view is needed.
A type of super wide angle lens is disclosed in Japanese Laid Open Patent Application No. 9-80303 (1997). However, with the lens described in this publication, there is need for improvement in the correction of aberrations. Especially in an observation or photographic camera, it is common for the lens to be inexpensive in order for the product to be inexpensive. In this case, there is the risk that colors may blur in the image as a result of insufficient correction of chromatic aberrations and so forth.
Furthermore, in order to devise a wide viewing angle, the radius of curvature of the concave surface of the frontal lens element(s) is generally made small with respect to the inner diameter of the lens element, thereby creating a tendency for the lens element surface to be shaped similar to that of a half sphere. This creates a problem in manufacturing the lens element, in that work such as lens grinding, and so forth, becomes very difficult.