Varifocal Telephoto Lens

In the introduction to the varifocal telephoto lens, it was assumed that the focus was at infinity. This calculation examines the lens positions necessary to focus at different distances. You will note that if you change the focal length, both lenses must be repositioned. This is a disadvantage of the varifocal type lens; if you change the magnification, you must refocus the lens. In a true zoom lens, more elements are used and their positions are changed so that the image on the film remains in focus when you change focal length.

The distances can be calculated using the thin lens equation and the equivalent focal length using Gullstrand's equation.