Modern Lens Design, 2nd Edition

Chapter 13Telephoto Lenses

13.1 The Basic Telephoto

The arrangement shown in Fig. 13.1, with a positive component followed by a negative component, can produce a compact system with an effective focal length F that is longer than the overall length L of the lens. The ratio of L/F is called the telephoto ratio, and a lens for which this ratio is less than unity is classified as a telephoto lens. Typical ratios range from 0.6 to 0.85. The smaller the ratio, the more difficult the lens is to design. Note that many camera lenses sold as telephoto lenses are simply long-focal-length lenses and are not true telephotos.

Many of the comments in Chap. 14 regarding retrofocus or reverse telephoto lenses are equally applicable to the telephoto lens. The ...