iris diaphragm

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A circular device with a variable diameter, commonly used on cameras to regulate the amount of light admitted to a lens.

from the GNU version of the Collaborative International Dictionary of English.

• An adjustable diaphragm, reminiscent of the iris of the eye in its action, by which the diameter of an approximately circular opening may be controlled, as for regulating the aperture of a lens; it consists of a number of movable thin curved plates fastened at regular intervals around the inside of a ring, the positions of which are simultaneously adjusted by a single knob on the outside of the lens. It is used in cameras and microscopes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun diaphragm consisting of thin overlapping plates that can be adjusted to change the diameter of a central opening