Aperture (optics)

An aperture is the opening through which an optical instrument admits radiation. Its size and shape help determine the instrument’s brightness, resolution, and depth of field.

Aperture stop: The optical element that limits the cone of rays passing through an instrument. It is the system element that directly sets the effective aperture.
Depth of field: The range of object distances that appear acceptably sharp in an image. Stopping down usually increases the range that appears in focus.
Focal length: The distance characterizing how strongly an optical system converges or diverges parallel rays. It combines with entrance-pupil diameter to define the f-number.
Field stop: An optical element that limits the extent of the scene imaged by a system. Unlike the aperture stop, it limits field of view rather than the accepted ray cone.
Exit pupil: The image of an optical system’s aperture stop as seen from image space. Its position and size govern how light reaches the detector or eye.
Exposure value: A logarithmic measure used to represent combinations of camera exposure settings. Changing aperture alters image exposure and can be balanced against shutter time.
Geometric optics: The approximation that models light as rays and neglects its wave behavior. Ray geometry explains pupil images and aperture stops before diffraction is considered.
Iris diaphragm: An adjustable assembly of overlapping blades that forms a variable circular opening. It is a common physical mechanism for changing a camera lens’s aperture.
F-number: The ratio of a lens’s focal length to the diameter of its entrance pupil. It expresses aperture size relative to focal length and predicts image irradiance.
Bokeh: The visual character of out-of-focus regions in a photograph. Aperture shape and size influence the appearance of defocused highlights.

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