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Encyclopedia > Zoom lens
A Canon Inc. 70-200mm lens
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A Canon Inc. 70-200mm 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. Image File history File links Download high resolution version (699x1593, 202 KB)The Canon EF 70-200mm F2. ... Image File history File links Download high resolution version (699x1593, 202 KB)The Canon EF 70-200mm F2. ... Canon Inc. ... A lens. ... The focal point F and focal length f of a positive lens, a negative lens, a concave mirror, and a convex mirror. ... Categories: Stub | Lenses by type | Optics ... A camera is a device used to take photographs. ... Look up Video in Wiktionary, the free dictionary Video is the technology of capturing, recording, processing, transmitting, and reconstructing moving pictures, typically using celluloid film, electronic signals, or digital media. ... A camera is a device used to take pictures (usually photographs), either singly or in sequence, with or without sound recording, such as with video cameras. ... Binoculars Binocular telescopes, or binoculars, are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, one to be viewed through each of the users eyes. ... 50 cm refracting telescope at Nice Observatory. ... ...


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. A Canon 28-300mm modern professional zoom. ...


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. In photography and cinematography, a telephoto lens is a lens whose focal length is significantly longer than the focal length of a normal lens. ... In photography and cinematography, a wide-angle lens is a lens whose focal length is shorter than the focal length of a normal lens. ...


Trans-standard zooms have replaced the fixed prime lens on many contemporary cameras. Categories: Stub | Lenses by type | Optics ...


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. Digital zoom is a method of zooming on a digital camera either by increasing the size of the pixels in the image or by interpolating between them. ...

Views at different focal lengths

Contents

Image File history File links Zoom. ... Image File history File links Zoom. ...


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. An optical telescope is a telescope which is used to gather, and focus light, for directly viewing a magnified image, making a photograph, etc. ... Magnification is the process of enlarging something only in appearance, not physical size. ... The premises of the Royal Society in London (first four properties only). ... 1834 was a common year starting on Wednesday (see link for calendar). ... A patent is a set of exclusive rights granted by a state to a person for a fixed period of time in exchange for the regulated, public disclosure of certain details of a device, method, process or composition of matter (substance) (known as an invention) which is new, inventive, and...


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 (U.S. Patent 696788). 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. 1902 (MCMII) was a common year starting on Wednesday (see link for calendar). ... Abraham Zapruders Bell & Howell Zoomatic movie camera, in the collection of the US National Archives Founded in 1907 and headquartered in Skokie, Illinois, the Bell & Howell Company merged with Böwe Systec Inc in 2003 to become Böwe Bell & Howell. ... 1932 (MCMXXXII) was a leap year starting on Friday (the link will take you to a full 1932 calendar). ... 1959 (MCMLIX) was a common year starting on Thursday of the Gregorian calendar. ... Simulated 35 mm film with soundtracks _ The outermost strips (on either side) contain the SDDS soundtrack as an image of a digital signal. ...


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. An illustration of a modern personal computer. ... A ray traced scene. ...


Design

A simple zoom lens system
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. Image File history File links Simple zoom lens. ...


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.

Movement of lenses in an afocal zoom system
Movement of lenses in an afocal zoom system

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. Image File history File links Movement of lenses in a zoom lens. ... The term servo can refer to: Servomechanism - usually just shortened to servo, is a device used to effect mechanical motion for a specified distance. ...


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, 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. Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. ... Aberration in optical systems (lenses, prisms, mirrors or series of them intended to produce a sharp image) generally leads to blurring of the image. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...


Applications

In addition to its photographic use, the afocal part of a zoom lens can be used as 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. Afocal projection is a method of astrophotography in which photographs are taken by holding the camera over the telescope eyepiece, with the camera taking the place of your eye. ... An optical telescope is a telescope which is used to gather, and focus light, for directly viewing a magnified image, making a photograph, etc. ... Magnification is the process of enlarging something only in appearance, not physical size. ... Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other physics experiments. ... Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. ...


References

  • 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

See also

In photography and cinematography, a telephoto lens is a lens whose focal length is significantly longer than the focal length of a normal lens. ... Categories: Stub | Lenses by type | Optics ... In photography and cinematography, a wide-angle lens is a lens whose focal length is shorter than the focal length of a normal lens. ...

External links

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Shutterbug: Sigma’s 120-300mm f/2.8 APO HSM Zoom Lens (1491 words)
It’s the 120-300mm f/2.8 APO HSM zoom from Sigma, and it is the first pro-quality zoom lens with both a 300mm top end and a fast fixed f/2.8 aperture.
How they manage to build this complicated lens (it has 18 elements in 16 groups!) and keep it this hand holdable is beyond me. The finish is Sigma’s slick satin fl finish, and the huge knurled rubber focus and zoom rings are a nice touch.
Autofocus was brilliant—the lens banged into focus when pointed in any direction—and focus speeds seemed very consistent with what I had been experiencing with my good older 300mm f/2.8 L lens.
Zoom lens - Wikipedia, the free encyclopedia (1080 words)
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.
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).
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.
  More results at FactBites »

 
 

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