Diaphragm.

The mechanical or electromechanical light-blocking device used to control the aperture of a lens.

Lens diaphragms are usually iris-shaped and consist of a number of thin metal leaves (typically 5, 7 or 8) which can be adjusted to alter the size of the hole - lens aperture - through which light passes. The aperture expands or contracts much like the pupil of a human eye.

Many lenses have mechanical diaphragms operated by levers. These levers are operated either manually by the photographer rotating a ring on the lens barrel or by the camera body physically moving the lever. Other lenses, most notably EF lenses for Canon EOS cameras, have electromechanical diaphragms operated by small electric motors or actuators.

cf. aperture, iris.

Entry last updated 2002-05-03. Term 335 of 1487.

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