This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
To Find Magnification Of A Telephoto System. To find the magnification of a telephoto system, divide the distance of the negative lens from the ground-glass by the focal length of the negative lens, and add 1. Expressed in formula form:
Illustration No. 94 - Goerz Telephoto Lens See Paragraph No. 930.
Illustration No. 100 - B. & L. Projection Lens See Paragraph No. 946.
Illustration No. 97 - Telephoto Demonstration See Paragraph No. 931.
M =( E / f2) + 1
M = (12 / 3) + 1=5.
Exposure In Telephotography. The greater the magnification, the less the rapidity of the telephoto lens. To find the speed or F. number of the telephoto system, divide the F. number of the positive lens by the magnification. Thus, if the positive is being worked at f.ll with 4 times magnification, the real rapidity is f.44, and exposure must be given on this basis.
936. The exposures obtained by above rule are indicative, and not imperative, and are often greatly modified by the condition of the atmosphere. As a rule an exposure shorter by about one-third than the theoretical one will be found correct in the United States.