For example, in a telescope, the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. In general, these structures are called stops, and the aperture stop is the stop that primarily determines the ray cone angle and brightness at the image point. These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system.

More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.Īn optical system typically has many openings or structures that limit the ray bundles (ray bundles are also known as pencils of light). In optics, an aperture is a hole or an opening through which light travels.

Definitions of Aperture in the 1707 Glossographia Anglicana Nova

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