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A zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length, as opposed to a prime lens which has a fixed focal length. They are commonly used with still, video, and motion picture cameras, some binoculars and telescopes, and other optical instruments.

The first real zoom lens, which retained near-sharp focus while the effective focal length of the lens assembly was changed, was patented in 1902 by Clile. C. Allen (U.S. Patent 696788 ). The first industrial production was the Bell and Howell Cooke "Varo" 40-120mm Lens for 35mm movie cameras introduced in 1932. The Kilfitt 36-82mm/2.8 Zoomar introduced in 1959 was the first zoom lens in regular production for still 35mm photography.

Since then, advances in optical design, particularly the use of computers for optical ray tracing, has made the design and construction of zoom lenses much easier, and they are now used widely in professional and amateur photography.