aperture diaphragm

Definition: (1) Also known as the iris diaphragm in a camera optic. * It is placed to determine the diameter of the bundle of light transmitted through the lens. * Its effective diameter, if not circular e.g pentagonal, is the diameter of a circle with the same cross-sectional area. (2) A second-stage adjustable iris diaphragm in an optical system, standard in microscopy. * Usually located beneath the light condenser of a transmitted light microscope and used to adjust the size (i.e. conic angle) of the cone of light entering the objective, within the limits of the condenser's maximum aperture.

Related Terms: f/number

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