Thursday, June 19, 2008

Zoom Lens: Applications

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. 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.

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