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in photography, device by which the lens aperture of a camera is opened to admit light and thus expose the film. Adjustable shutters control exposure time, or the length of time during which light is admitted. Optimum exposure time varies according to lighting conditions, movement of subject, and other factors and may be either selected in advance by the photographer or, in the case of automatic cameras, set by the camera itself on a signal from a built-in exposure-metering system. The mechanical shutter can usually be set only for indicated speeds throughout its range; some electronic shutters have a continuous operating range