Telephoto lens

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In photography and cinematography, a telephoto lens is a lens whose focal length is significantly longer than the focal length of a normal lens. For a 35 mm camera with a 36 mm x 24 mm format (43.3 mm diagonal), the normal lens is 50 mm and a lens of focal length 70 mm or more is considered telephoto. On the 6 cm x 6 cm format (84.9 mm diagonal) (120 film) the normal lens is 80 mm, and focal lengths above 100 mm are considered telephoto.

Telephoto lenses are best known for making distant objects appear magnified. This effect is similar to moving closer to the object, but is not the same, since perspective is a function solely of viewing distance. Two images taken from the same location, one with a wide angle lens and the other with a telephoto lens, will show identical perspective. Telephoto lenses also have less depth of field at a given aperture than shorter lenses.

Optical designs of telephoto lenses must contain a telephoto group, which allows the lens to be physically shorter than its focal length. A lens with a conventional design and a focal length longer than a normal lens should properly be referred to as long focus. Still, common nomenclature simply refers to all long-focus lenses as telephoto.

This same property is achieved with mirrors in catadioptric lenses.

Compare with the opposite effect used in retrofocus lenses (sometimes designed as inverted telephotos), which have greater clearance from the rear element to the film plane than their focal length would permit with a conventional optical design.