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.
Zoom lenses vary in terms of complexity, with some products featuring over 30 lenses elements. Generally, however, even the most complex products follow the same basic design. Individual lens elements are either fixed or designed to slide axially along the body of the lens. To keep the image focus sharp, changes in zoom lens magnification require compensation for any movement in the focal plane. Such compensation may be achieved mechanically or optically. Mechanical compensation involves moving the complete optical lens assembly as lens magnification changes. Optical compensation involves adjusting the focal plane to move as little as possible while the lens is zoomed.
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.
Zoom lenses may be designed for laser or other high-power applications. For example, a laser anemometer system may use a zooming correction lens to eliminate the astigmatism caused by the cylindrical windows that enclose the flow fields. Multi-beam zoom lenses may be used in laser printers. Zoom lens implants (zoom implants) may be used in laser vision correction surgery. In terms of features, some zoom lenses provide adjustable divergence, collimation, or magnification. Other optical lenses offer motorized zoom, focus, divergence, or collimation features. Specialty and proprietary zoom lenses are also available.