Zoom lens
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A zoom lens is a mechanical assembly of lenses whose focal length can be changed, as opposed to a prime lens, which has a fixed focal length. They are commonly used with still and video cameras, some binoculars and telescopes, and other optical instruments.
Zoom lenses are sometimes described by the ratio of their longest and shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a "4x" zoom. The term hyperzoom is used to advertise zoom lenses with unconventionally large focal length ratios, typically more than 4x and ranging up to 10x (e.g. 35 mm to 350 mm) and even 12x.
Photographic zoom lenses should not be confused with telephoto lenses, those with large focal lengths. While many zoom lenses provide telephoto capabilities, others are wide-angle zooms, that is, they have shorter than normal focal lengths, and still others cover a range from wide-angle to telephoto.
The situation is further confused by the fact that some digital cameras use electronic manipulation of the image to increase or decrease its size. This is commonly known as digital zoom and results in a much lower quality image than optical zooming.
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History
Early forms of zoom lenses were used in optical telescopes to provide continuous variation of the magnification of the image, and this was first reported in the proceedings of the Royal Society in 1834. Early patents for telephoto lenses also included moveable lens elements which could be adjusted to change the overall focal length of the lens. Lenses such as these are now called varifocal lenses, in that as the focal length is changed, the position of the focal plane also moves, requiring readjustment of the focussing of the lens after each change.
The first real zoom lens, which retains sharp focus while the effective focal length of the lens assembly is changed, was patented in 1902 by C.C. Allen. The first attempt to make a practical, useful zoom lens that corrected for optical aberrations was the Bell and Howell Cooke Varo lens 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.
Design
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There are many possible designs for zoom lenses, the most complex ones having upwards of thirty individual lens elements, and multiple moving parts. Most however follow the same basic design. Generally they consist of a number of individual lenses that may be either fixed, or slide axially along the body of the lens. As the magnification of a zoom lens changes, it is necessary to compensate for any movement of the focal plane to keep the focussed image sharp. This compensation may be done by mechanical means (moving the complete lens assembly as the magnification of the lens changes), or optically (arranging the position focal plane to vary as little as possible as the lens is zoomed). Some early zoom lenses, referred to as varifocal lenses, did not preserve focus as the zoom setting was changed. This simplified the lens design, but required the user to re-focus the lens after zooming.
A simple scheme for a zoom lens divides the assembly into two parts: a focussing lens similar to a standard, fixed-focal-length photographic lens, preceded by an afocal zoom system, an arrangement of fixed and movable lens elements that does not focus the light, but alters the size of a beam of light travelling through it, and thus the overall magnification of the lens system.
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In this simple optically-compensated zoom lens, the afocal system consists of two positive lenses of equal focal length (lenses L1 and L3) with a negative (diverging) lens (L2) between them, with an absolute focal length less than half that of the positive lenses. Lens L3 is fixed, but lenses L1 and L2 can be moved axially, and do so in a fixed, non-linear relationship. This movement is usually performed by a complex arrangement of gears and cams in the lens housing, although some modern zoom lenses use computer-controlled servos to perform this positioning.
As the negative lens L2 moves from the front to the back of the lens, the lens L1 moves forward and then backward in a parabolic arc. In doing so, the overall angular magnification of the system varies, changing the effective focal length of the complete zoom lens. At each of the three points shown, the three-lens system is afocal (neither diverging or converging the light), and so does not alter the position of the focal plane of the lens. Between these points, the system is not exactly afocal, but the variation in focal plane position can be very small (~±0.01 mm in a well-designed lens) and so this slight defocussing is not apparent.
An important issue in zoom lens design is the correction of optical aberrations (such as chromatic aberration) across the whole operating range of the lens; this is considerably harder in a zoom lens than a fixed lens, which need only correct the aberrations for one focal length. This problem was a major reason for the slow uptake of zoom lenses, with early designs being considerably inferior to contemporary fixed lenses, and usable only with a narrow range of f-numbers. Modern optical design techniques have enabled the construction of zoom lenses with good aberration correction over widely variable focal lengths and apertures.
References
- Clark, A.D. (1973), Zoom Lenses, Monographs on Applied Optics No. 7. Adam Hildger (London).
- Malacara, Daniel and Malacara, Zacarias (1994), Handbook of Lens Design. Marcel Dekker, Inc. ISBN 0-8247-9225-4