SHUTTER

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. Modern shutters are of two principal types. The leaf shutter, positioned between or just behind the lens components, consists of a number of overlapping metal blades opened and closed either by spring action or electronically. The focal-plane shutter, located directly in front of the image plane, consists of a pair of overlapping blinds that form an adjustable slit or window; driven mechanically by spring or electronically, the slit moves across the film in one direction, exposing the entire frame in its sweep. The width of the slit determines exposure time; the narrower the slit, the shorter the exposure. The actual travel time is fairly constant for all exposure times; a mechanism triggers the release of the second blind. Exposures as brief as 1/12,000 of a second are possible with the focal-plane shutter.