A telephoto lens has a focal length 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 only 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 make the background apear more blured than a normal lens, even though technically depth of field is only a function of aperture and it doesn't depend on the focal length of a lens. This is another effect of perspective change.
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.
- List different specific telephoto lenses here!
- This article contains information originally taken from the Wikipedia article "Telephoto_lens". You can see the authorship and revision history of that article here.
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