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A flanged or plain ring with a restricted aperture, located in an optical system at any of several points, that cuts off marginal light rays not essential to image formation. Diaphragms are used as field stops, to limit the field of view to that portion which is fully illuminated; aperture stops, to limit the light-gathering power of the instrument; and antiglare diaphragms, to eliminate reflections from the sides of the tube and consequent glare in the field of view. Lens cells or the sides of the tube may act as diaphragms. The rays eliminated by diaphragms are those that would cause aberrations or glare at the final image.
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