Any lens, convex or concave, has a surface with a certain curvature, which is a part of a large sphere. Such a spherical lens cannot converge parallel light rays at a single focal point completely. This phenomenon exists for certain extent in any lenses as far as they are constructed with spherical elements only by any optical theory. So, there is a limit of the following improvements in case of such spherical lens construction;
1. high definition of the large aperture lenses, compensating spherical aberrations.
2. correction of distorted image with wide angle lenses (correction of distortion).
3. compact and high quality zoom lenses.
Canon developed aspherical lens surface to solve this problem. Aspherical lens elements have a special non-spherical surface to converge central and peripheral light rays at a single focal point perfectly. The FD55mm f/1.2 AL was marketed in 1971 as the first interchangeable lens for 35mm SLR camera in the world, employing an aspherical lens element. Canon uses four different type of aspherical lens elements now depending on the purpose;
1. a ground and polished glass aspherical lens element.
2. a molded glass aspherical lens element.
3. a molded plastic aspherical lens element produced by a high-precision molding technology.
4. a replica aspherical lens element, ultraviolet-light-hardening resin layer on a spherical glass lens element.
Large diameter ground and polished aspherical lens elements are used for "L" series lenses to achieve sharp definition.
