Photography

Aperture

In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. The aperture defines a set of rays from each point on an object that will come to a focus in the image plane, called a bundle of rays or cone of rays, mathematically a generalized cone (or a bundle of half-lines). An optical system typically has many structures that limit ray bundles. 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. These structures are called stops, and the aperture stop is the stop that primarily determines the base of the cone of rays that an optical system accepts (see entrance pupil). As a result, it also determines the ray cone angle and brightness at the image point (see exit pupil). Optical systems are typically designed for a particular stop to be the aperture stop, but it is possible for different stops to serve as the aperture stop for objects at different distances. Some rays from object points away from the optical axis may clip on surfaces other than the aperture stop, creating a vignetting effect. The aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system. In some contexts, especially in photography and astronomy, aperture refers to the opening diameter of the aperture stop through which light can pass. 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). One then speaks of a telescope as having, for example, a 100-centimetre (39 in) aperture. In astrophotography, the aperture may be given as a linear measure (for example, in inches or millimetres) or as the dimensionless ratio between that measure and the focal length. In other photography, it is usually given as a ratio. A usual expectation is that the term aperture refers to the opening of the aperture stop, but in reality, the term aperture and the aperture stop are mixed in use. Sometimes even stops that are not the aperture stop of an optical system are also called apertures.

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See also

• ▸Film noir
• ▸Architecture of the Cucuteni–Trypillia culture
• ▸35 mm movie film

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