A zoom lens or vario lens is a camera lens with continuously variable focal length. This allows the photographer to zoom in and zoom out, to adjust the field of view (and so how much is included in the photograph) without changing their position.
|tele lens with variable focal length, made by Zeiss in the mid-1920s (Image rights)|
Early 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 zoom lens Zoomar was introduced by Voigtländer. It was the first zoom lens for 35mm cameras.
Early variable focal length lenses were known as vario lenses and had quite simple construction. Every change of focal length necessitated re-focussing 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. Originally developed for cinematography, zoom lenses quickly became very popular for still photography.
Autofocus can automatically compensate focus shift when zooming, enabling simpler lens construction especially for compact cameras.