Mono=focal lens system with four lenses, for use in photocopiers

摘要

The system uses a series of four lenses, in which the first and fourth lenses are of convex form, and the second and third lenses are of concave form. The first and third lenses are made from plastic. The second and fourth lenses are made from glass, in order to compensate for alterations in the focal characteristics of the plastic lenses, due to thermal expansion or contraction. The focal length of the complete system (f) is related to the focal length (f1) of the first lens by the inequality: 0.3f < f1 < 0.7f.