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iris diaphragm[′ī·rəs ′dī·ə‚fram]
a type of stop that is often used in photographic lenses and other instruments to adjust the brightness of the image and to vary the depth of field. It consists of thin, opaque, crescent-shaped vanes that overlap one another, thus forming an approximately circular opening. By moving the stop ring on the lens or a lever connected to it, all the vanes are turned simultaneously, so that the objective’s opening (aperture ratio) is varied smoothly.