In photography and cinematography, a telephoto lens is a lens where the focal length is significantly longer than the focal length of a normal lens. For a 35 mm camera with a 36 mm by 24 mm format, the normal lens is 50 mm and a lens of focal length 70 mm or more is considered telephoto. On the 6 x 6 cm format (on 120 film) the normal lens is 80 mm, focal length above 100 mm are considered telephoto.
Telephoto lenses are best known for making distant objects appear larger. This effect is similar to moving closer to the object, but is not the same, since a telephoto does not introduce additional perspective distortion. Telephoto lenses also have less depth of field than shorter lenses.
Strictly speaking, telephoto lenses must contain a telephoto group , which allows the lens to be physically shorter than its focal length. Without this group, a longer than normal lens should be referred to as long focus. although common nomenclature simply refers to all long focus lenses as telephoto. Compare with the opposite effect used in retrofocus lenses, which have greater clearance from the rear element to the film plane than their focal length would permit with a conventional optical design.
See angle of view for an example of an image taken by a telephoto lens.
See also: film format, normal lens
