Zoom lens
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 factors, 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 are trans-standard zooms covering a range from wide-angle to telephoto.
Trans-standard zoom lenses have displaced the fixed
prime lens as the popular one-lens selection on many contemporary cameras.
The situation is further complicated by the fact that some digital cameras allow cropping and enlarging of the resultant image once the limits of the zoom lens has been reached, in order to emulate the effect of a longer focal length zoom lens. This is commonly known as
digital zoom and results in a much lower quality image than optical zoom as no information or detail is gained in the process; what detail there is simply spread over an image with larger pixel dimensions. Arguably, digital zoom causes a loss of information as a result of the parts of the image that are cropped out. Some digital cameras perform this without the enlarging step, which is known as digital crop. This is intended to reduce file size and in the process increase continuous shooting speeds.
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Views at different focal lengths |
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 retained near-sharp focus while the effective focal length of the lens assembly was changed, was patented in
1902 by Clile. C. Allen (). The first industrial production was the
Bell and Howell Cooke "Varo" 40-120mm Lens for 35mm movie cameras introduced 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.
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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). 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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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
1 and L
3) with a negative (diverging) lens (L
2) between them, with an absolute focal length less than half that of the positive lenses. Lens L
3 is fixed, but lenses L
1 and L
2 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 L
2 moves from the front to the back of the lens, the lens L
1 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.
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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-numbers. 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 refocusing once the focal length has changed. As most still cameras are auto-focus these days, it hardly presents a problem. Some zoom lenses that have large zoom ratios often trade some unexpected abberations to achieve high image quality.
For example, it is quite difficult to design a lens that focuses from a quite wide angle to telephoto with a zoom range of 10x or more without the lens exhibiting significant barrel distortion at the wide angle setting. Modern lens design has reduced this to very low levels, such that it only becomes visible if a straight edge appears at the edge of the image. The price paid is that at the extreme telephoto setting of the lens, the apparant focal length changes alarmingly as the lens is focused on nearer and nearer objects. The apparent focal length can more than half as the lens is focused from infinity to a few feet.
In addition to its photographic use, the
afocal part of a zoom lens can be used as a
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.
*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
*
Telephoto lens*
Prime lens*
Wide-angle lens*
Superzoom*
Zoom from the Milky Way to a quark*
In-Depth Discussion of Zoom vs. Prime Camera Lenses*
Renaming zoom lenses
