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In photography, the diameter of the aperture expressed in terms of the focal length of the camera lens. The greater the f-number, the less light reaches the film and the more in focus the objects in the foreground and the background will be. For example, with an f-number = 16 the objects in the far background will be in focus. Whereas, with an f-number = 2 the background will be blurry (while foreground objects are in focus).
In photography, the camera lens opening and its relative diameter. Measured in f-stops, such as f/8, etc. As the number increases, the size of the aperture decreases, thereby reducing the amount of light passing through the lens and striking the film. (source: Ackland Art Museum)
something which restricts the diameter of the light path through one plane in an optical system
diameter of the aperture (in the sense above)
An opening; an open space; a gap, cleft, or chasm; a passage perforated; a hole; as, an aperture in a wall.
(optics) Something which restricts the diameter of the light path through one plane in an optical system.
(astronomy, photography) The diameter of the aperture (in the sense above) which restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens. e.g. a telescope may have a 100 cm aperture.