entrance pupil

Definition: The hole seen from the front of the lens through which light enters the camera. * The virtual image of the aperture as seen from the object side: its diameter is used to calculate the f/number of the optic. Its position also is that for panoramic projection, in which the position of the image does not change if the lens is rotated about a vertical axis through the entranc pupil. * Note: the entrance and exit pupils of multi-component photographic lenses are usually not the same size, particularly inverted telephoto type wide-angle lenses. * Also known as effective aperture.

Related Terms: exit pupil, f/number

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