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Light enters an optical system through an opening called the aperture. The aperture is the hole that light passes through, and its size controls how much light gets in as well as the depth of field in an image—the larger the opening, the more light and shallower the depth of field; the smaller the opening, the less light and greater depth of field. The term also appears in popular culture as Aperture Science, the fictional AI-controlled corporation in the Portal video game series.

The other terms refer to related parts: a lens is the set of glass elements that focuses light; the iris is the diaphragm that adjusts the size of the opening; and the shutter is the mechanism that opens and closes to expose the sensor or film. So the hole through which light enters is called the aperture.