VARIABLE Foci.
The telephoto lens, as already stated, possesses the peculiar advantage of giving not only one focus, but many, and the means by which this property is obtained may be ex plained as follows : If a plano-convex and a plano-concave lens of the same kind of glass be placed in contact with each other, it is obvious that their action on light coming from a great distance will be practically the same as that of a parallel plate of glass. Their combined focus may be considered to be infinitely far away, and the plano-convex being a positive lens, and the pIano-concave a negative, form a telephoto system with the longest imaginable focus. Now, should the two lens_es be separated by degrees the focus will be gradually shortened, until a point arrives when the plano-concave lens is in such a position that the light is brought by the plano-convex lens to its normal focus, and thus by varying the distance between the two lenses of the telephoto system the various foci are obtained. It will be seen, therefore,
the length of focus of the complete tele photo lens depends upon the degree of separa tion between its negative and positive com ponents. The closer they approach each other, the longer the focus and the greater the mag nification ; while the further apart they are placed, the shorter becomes the focus, and the lesser the magnification, until the image al together ceases to be enlarged. To find the magnification obtainable, divide the camera extension by the focal length of the negative lens and add 1 ; to hind the necessary camera for any given magnification, multiply the focal length of the negative lens by the magnification, minus 1.