Simple Telephoto Lens A simple telephoto lens can be made by modifying the Galilean telescope to project a real image on the film of a camera. In a varifocal zoom lens the focal length is varied by moving the lenses, but the camera must be refocused. With a true zoom lens, the focus does not have to be readjusted as the focal length is changed. The telephoto lens is shown mounted on a single lens reflex camera. Calculation for varifocal lens

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Varifocal Telephoto Lens Variation of focal length can be achieved with just two lens elements as demonstrated by the calculation below, but note that the lens to film distance changes. In a true zoom lens, more elements are used and their positions are changed so that the image on the film remains in focus. The distances can be calculated using the thin lens equation and the equivalent focal length using Gullstrand's equation. For front lens focal length and back lens focal length the equivalent focal length is If the lens separation is cm, then the lens to film distance for focus is cm Calculation to be added Vary focus

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