retrofocus lens

Definition: Type of wide-angle lens designed so that the space between the back of the lens to the focal plane is longer than the focal length. * This is required e.g. in single-lens reflex cameras in order to leave space for the mirror to move. * Consists of negative power lens groups in front which gather the image and a positive power group at the rear which extends the back focal distance. * Also known as inverted telephoto lens. Retrospective splitting of hairs Single-lens reflex cameras are often credited with being the reason for the invention of the inverted telephoto lens. In fact it was invented for cinematography: the Technicolor process of the 30's used a beam-splitting unit behind the lens to create colour separations: the back focal distance needed was large. Taylor Hobson 40mm f/2 retrofocus lens was born, with all the modern features of a very large front element and complex rear group which relays the virtual image from the front groups to the focal plane.

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