iris diaphragm

noun, Optics, Photography
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 iris.
Origin
1885-90
British Dictionary definitions for iris diaphragm

iris diaphragm

noun
1.
an adjustable diaphragm that regulates the amount of light entering an optical instrument, esp a camera. It usually consists of a number of thin metal leaves arranged so that they open out into an approximately circular aperture Sometimes shortened to iris