ABSTRACT
The use of a long focus lens, where the back focal distance of simple two- or three-element designs is similar to the equivalent focal length, gives a large image size, but this configuration also requires a long lens mount and focusing barrel or a long bellows extension. Various mechanical stabilizing methods have been used to obtain a stabilized optical image. The telephoto configuration of a front positive lens followed by a rear negative lens gives a more compact design, since the rear nodal point is located in front of the lens. For photographic use, the telephoto principle was proposed independently in 1891 by three designers: Dallmeyer of England, Dubosq of France and Miethe of Germany. These were variable power telephoto lenses and were the forerunners of the varifocal lens. The classical telephoto configuration is still used in lenses of long focal length for large-format view cameras.
