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Meaning of iris diaphragm

i'ris di"aphragm

Pronunciation: [key]
— Optics, Photog. Optics,Photog.
  1. a composite diaphragm with a central aperture readily adjustable for size, used to regulate the amount of light admitted to a lens or optical system. Also called
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.