Sometimes reverse telephoto or inverted telephoto. A lens design used for wide-angle lenses on SLRs.
It isnt possible to construct a wide-angle (short focal length) lens for an SLR camera using conventional means, because the physical obstruction of the mirror prevents you from getting close enough to the film surface. Old SLRs got around this by requiring the user to lock up (move up) the mirror when attaching a wide angle lens - a thoroughly unsatisfactory solution as it means you cant use the normal viewfinder. So such lenses shipped with supplementary viewfinders that fit into the accessory shoe.
Modern wide-angle lenses avoid all this through clever design. They are inverted telephoto lenses - optical systems that result in very long back focus distances - which permits a wide-angle lens to be attached to an SLR.
Coincidentally the inverted telephoto design also reduces a form of vignetting, called cos4 vignetting, to which rangefinder and some medium format cameras are particularly vulnerable. This problem occurs when the rear lens element is very close to the surface of the film and so light hitting the edges of the film does so at a very oblique angle. Since inverted telephoto lenses allow the rear element to be a reasonable distance away from the film this light falloff problem is essentially eliminated.
The primary drawback to the inverted telephoto design is that all those extra lens elements can reduce sharpness and image quality unless the lens is very precisely engineered from high-quality materials. (ie: expensive) This is why very wide-angle lenses for SLRs tend to be extremely costly.
cf. mirror lockup, single lens reflex (SLR), vignetting, wide angle lens.
Entry last updated 2002-05-02. Term 1036 of 1487.
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