A zoom lens (also pancratic, varifocal or vario lens, especially in older texts) is a camera lens with continuously-variable focal length. This allows the photographer to zoom in (increase the focal length, magnifying the central part of the scene) and zoom out (decrease the focal length and magnification, including a wider angle of view in the picture), without changing their position.
Lenses with variable focal length were available for large format cameras. These were tele lenses consisting of a normal camera lens in the front of the lens tube combined with a dispersing lens near the tube's screw mount. Variable focal length could be achieved by adjusting the distance of the both lenses within the lens tube by means of a rack & pinion mechanism.
In 1959 Dr Frank G. Back's Zoomar lens was introduced by Voigtländer. This was the first zoom lens for 35mm cameras. Back's design introduced optical means to make the lens smaller, and other practical improvements.
Early varifocal lenses had quite simple construction. Every change of focal length necessitated refocusing the lens. Zoom lenses which hold focus throughout their focal range are much more complex and only became practical once lens coatings and special glass types became available. The first of these were developed for cinematography: Back himself had patented such a lens; but these were impractically large for still cameras, especially small hand cameras such as 35 mm SLRs.
Autofocus can automatically compensate focus shift when zooming, enabling simpler lens construction especially for compact cameras.
- US Patent 2902901, Reflex camera varifocal lens, filed June 1958 and granted September 1959 to Frank Back. At Espacenet, the patent search facility of the European Patent Office.
- US Patent 2454686, Varifocal lens for cameras, filed July 1946 and granted November 1948 to Frank Back; also at Espacenet.