perspective distortion

Definition: Variation of image from being an accurate representation of object caused by viewing image from an incorrect perspective e.g. when a small print taken with an ultra-wide angle (short focal length) lens is viewed at normal viewing distances. * For accurate perspective, the angle subtended at the viewpoint by a given object should equal the angle subtended by the object at the camera. * This effect is best treated as an optical illusion. From the ball to the eye-ball Correct viewing distance for a photograph -- i.e. to reproduce the taking perspective and avoid distortion -- equals the focal length of the lens multiplied by the enlargement factor e.g. with a 50mm taking lens and a print enlarged 10 times, the 'correct' viewing distance is 500mm. Therefore correct viewing distance varies with focal length of taking lens. Now, as prints are viewed or seen usually at a more or less constant reading distance, it follows that the perspectives of images are usually distorted -- this is, of course, what enables us to enjoy the variety given by the use of different focal length lenses.

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