Zoom lens
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2008-07-08T22:53:18Z
82.134.121.18
prime lens
[[Image:Nikkor 28-200 zoom.jpg|thumb|[[Nikkor]] 28-200 mm zoom lens, extended to 200 mm at left and collapsed to 28 mm focal length at right]]
A '''zoom lens''' is a mechanical assembly of [[lens (optics)|lens elements]] with the ability to vary its [[focal length]] (and thus [[angle of view]]), as opposed to a fixed focal length (FFL) lens (see [[prime lens]]). They are commonly used with [[still camera|still]], [[video camera|video]], [[motion picture camera|motion picture]] [[camera]]s, [[projector]]s, some [[binoculars]], [[microscope]]s, [[telescope]]s, and other [[optical instrument]]s.
==Applications==
[[Image:Powershot A720is.jpg|thumb|[[Canon PowerShot A720 IS]] 5.8-34.8 mm zoom on a compact digital camera, resulting in a 6x optical zoom]]
Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or "4×" zoom. The term [[superzoom]] or hyperzoom is used to describe photographic zoom lenses with very large focal length factors, typically more than 4× and ranging up to 10× and even 14×. This ratio can be as high as 100× in professional television cameras.<ref>{{cite web | title = DIGISUPER 100 xs | publisher = Canon Broadcast Equipment | url = http://www.canon.com/bctv/products/digi100xs.html}}</ref> Currently, photographic zoom lenses beyond about 3× are not considered to have a quality on par with [[prime lens]]es, and constant fast aperture zooms (usually {{f/}}2.8 or {{f/}}2.0) are typically restricted to this range.
Photographic zoom lenses should not be confused with [[telephoto lens]]es, those with a narrow angle of view. Some zoom lenses are telephoto, some are [[wide-angle lens|wide-angle]], and others cover a range from wide-angle to telephoto. Lenses in the latter group of zoom lenses, sometimes referred to as "normal" zooms, have displaced the fixed focal length lens as the popular one-lens selection on many contemporary cameras.
Some digital cameras allow cropping and enlarging of a captured image, in order to emulate the effect of a longer focal length zoom lens (narrower angle of view). This is commonly known as [[digital zoom]] and results in a lower quality image than optical zoom, as no [[optical resolution]] is gained. Many digital cameras, such as the [[Canon PowerShot A720 IS]] have both, combining them by first using the optical, then the digital zoom. The optical zoom in this case can be calculated by dividing 34.8/5.8 as it is written on the lens tube of the camera, resulting in the zoom factor 6.
In addition to its photographic use, the [[afocal projection|afocal]] part of a zoom lens can be used as a [[optical telescope|telescope]] of variable [[magnification]] to make an adjustable [[beam expander]]. This can be used, for example, to change the size of a [[laser]] beam so that the [[irradiance]] of the beam can be varied.
==History==
[[Image:Zoomar MG 1895a.jpg|thumb|The Voigtländer Zoomar, 36–82 mm {{f/}}2.8]]
Early forms of zoom lenses were used in [[optical telescope]]s 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 [[patent]]s for telephoto lenses also included movable lens elements which could be adjusted to change the overall focal length of the lens. Lenses such as these are now called '''[[#Varifocal lens|varifocal lenses]]''', in that as the focal length is changed, the position of the focal plane also moves, requiring readjustment of the focusing of the lens after each change.
The first true ''zoom'' lens, which retained near-sharp focus while the effective focal length of the lens assembly was changed, was patented in [[1902]] by Clile C. Allen ({{US patent|696788}}). The first industrial production was the [[Bell and Howell]] Cooke "Varo" 40–120 mm lens for 35mm movie cameras introduced in [[1932]]. The most impressive TV Zoom lens was the VAROTAL III from Rank Taylor Hobson from UK built in 1953. The Kilfitt 36–82 mm/2.8 Zoomar introduced in [[1959]] was the first zoom lens in regular production for still [[35mm film|35mm]] photography.
Since then, advances in optical design, particularly the use of [[computer]]s for optical [[ray tracing (physics)|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==
[[Image:Zoomlens1.svg|frame|right|A simple zoom lens system]]
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).
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.
[[Image:Zoomlens2.svg|frame|left|Movement of lenses in an afocal zoom system]]
In this simple optically compensated zoom lens, the afocal system consists of two positive (converging) lenses of equal focal length (lenses L<sub>1</sub> and L<sub>3</sub>) with a negative (diverging) lens (L<sub>2</sub>) between them, with an absolute focal length less than half that of the positive lenses. Lens L<sub>3</sub> is fixed, but lenses L<sub>1</sub> and L<sub>2</sub> 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 [[servo]]s to perform this positioning.
As the negative lens L<sub>2</sub> moves from the front to the back of the lens, the lens L<sub>1</sub> 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.
[[Image:Zoom prinzip.gif|frame|right|Simplified zoom lens in operation]]
An important issue in zoom lens design is the correction of optical aberrations (such as [[chromatic aberration]], and in particular, [[field curvature]]) 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-number]]s. Modern optical design techniques have enabled the construction of zoom lenses with good aberration correction over widely variable focal lengths and apertures.
Whereas lenses used in cinematography and video applications are required to maintain focus as the focal length is changed, there is no such requirement for still photography, or if a zoom lens is used as a projection lens. Since it is harder to construct a lens that does not change focus with the same image quality as one that does, the latter applications often have lenses that require refocussing once the focal length has changed (and thus strictly speaking are [[varifocal lens]]es, not zoom lenses). As most still cameras are [[autofocus]] these days, it hardly presents a problem.
Designers of zoom lenses with large zoom ratios often will trade one or more aberrations for higher image sharpness. For example, a greater degree of barrel distortion is tolerated in lenses that span the focal length range from wide angle to telephoto with a focal ratio of 10x or more than would be acceptable in a fixed focal length lens or a zoom lens with a lower ratio. Although modern design methods have been continually reducing this problem, barrel distortion of greater than one percent is common in these types of lenses. Another price paid is that at the extreme telephoto setting of the lens, the effective focal length changes significantly as the lens is focussed on nearer and nearer objects. The apparent focal length can more than halve as the lens is focussed from infinity to a few feet. To a lesser degree, this effect is also seen in fixed focal length lenses that move internal lens elements, rather than the entire lens, to effect changes in focal length.
==Varifocal lens==
Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually ''[[varifocal lens]]es'', which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than true parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation essentially the same as a true parfocal zoom.<ref> {{cite web|url=http://www.rogercavanagh.com/helpinfo/30_parfocal.stm |title=Parfrocal Lenses |accessdate=2007-11-18 |last=Cavanagh |first=Roger |date=2003-05-29 }}</ref>
==See also==
By focal length:
*[[Telephoto lens]]
*[[Prime lens]]
*[[Wide-angle lens]]
*[[Superzoom]]
Focus attributes:
*[[Parfocal lens]]
*[[Varifocal lens]]
By system:
*[[Canon_EF_lens_mount#List_of_EF_lenses|List of Canon EF lenses]]
==References==
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<references/>
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*Kingslake, R. (1960), "The development of the zoom lens". ''Journal of the SMPTE'' '''69''', 534
*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
*"[http://www.adaptall-2.com/articles/InsideZoomLens/InsideZoomLens.html What is Inside a Zoom Lens?]", ''Adaptall-2.com''.
==External links==
{{Commons|Zoom lens}}
*[http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html Zoom from the Milky Way to a quark]
[[Category:Photographic lenses]]
[[Category:Telescopes]]
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