Telephoto vs. Ordinary Lenses: A Tutorial Paper

Abstract

Long-focus lenses are often called telephoto lenses, but this name should be restricted to those objectives which comprise a positive front component spaced apart from a negative rear component, such that the total length from front vertex to film plane is less than the focal length of the whole objective. In 1891, telephoto lenses were made by mounting a negative achromat behind a normal objective, making possible a very long focal length even with a relatively short camera bellows extension; however, the low aperture, field curvature and distortion were such that by 1905 manufacturers started making fixed-focus telephoto lenses with a moderate aperture and field. The simple thin-lens theory of the telephoto lens shows that the negative lens will be as weak as possible if it is situated midway between the positive lens and the focal plane. Telephoto lenses are currently popular for the longer focal lengths on single-lens reflex cameras. A survey of the principal types of telephoto lens is given, with examples of some actual lenses currently available.