An iris diaphragm on a camera lens.
iris diaphragm definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a device consisting of thin, overlapping metal blades that can be adjusted to form an aperture of varying size for camera lenses, etc.
iris diaphragm definition by American Heritage Dictionary
A circular device with a variable diameter, commonly used on cameras to regulate the amount of light admitted to a lens.