A zoom lens is a mechanical assembly of 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, video, motion picture cameras, projectors, some
binoculars, microscopes, telescopes, telescopic sights, and other optical
instruments
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 super-zoom or hyper-zoom 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. Currently, photographic zoom lenses beyond
about 3× are not considered to have a quality on par with prime lenses, 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 lenses, those
with a narrow angle of view. Some zoom lenses are telephoto, some are
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 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.
Video Production Glossary
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