iris diaphragm

American Heritage® Dictionary of the English Language, Fourth Edition

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

GNU Webster's 1913

1. 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.

WordNet 3.0

1. n. diaphragm consisting of thin overlapping plates that can be adjusted to change the diameter of a central opening