Definition: (1) Noun (also as adjective: ~ lens): optical design in which the focal length is greater than the physical length of the mounted lens. (2) Noun (also as adjective: ~ lens): lens with much longer than normal focal length e.g. 300mm where normal is 50mm. (3) Adjective: ~ power: ratio between the focal length and the back focal length of a telephoto design lens. * Also known as telephoto ratio. * Catadioptric lenses have the highest telephoto ratios thanks to doubling the light-path onto itself. Not all teles are photo Early designs of long focal length lenses were like telescopes -- about as long as their focal length: the best of them, e.g. from Leitz, used the simplest of constructions -- a doublet or triplet lens, offering excellent flare control and high contrast. The discovery that a negative power group at the rear of the lens could produce an overall shorter optic was tempered by an increase in aberrations, which needed more elements for correction, but telephoto designs are now the norm.

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