Zoom lenses are mechanical assemblies of lens elements that provide the ability to vary focal length and angle of view. In optical systems, shorter focal lengths provide greater optical power. In telescopy and photography, longer focal lengths are associated with the greater magnification of distant objects. Typically, zoom lenses are used with still, video, and motion picture cameras; video, movie, and slide projectors; some types of binoculars, microscopes, telescopes, and telescopic sights; and other optical instruments. True zoom lenses or parafocal lenses maintain their focus when the focal length changes. Varifocal lenses lose their focus during zooming.




Specifications for zoom lenses include diameter or width, length, height, field of view, focal length, wavelength, magnification, and working distance. The diameter or width is that of the entire mechanical assembly. The length and height are also measurements for the entire zoom lens, rather than for a constituent lens. The field of view is the angular or linear distance that zoom lenses can observe. The focal length is the distance that light is brought to a focus. Wavelength and magnification are also important specifications to consider. The working distance is measured between the front of the zoom lens assembly and the object when in focus.

