Automatic diaphragm
From The Glossary of Digital Photography
Also called automatic iris. A diaphragm or iris that stays open while viewing through the lens and then closes down to the desired aperture before the exposure occurs. Cameras without an automatic diaphragm must have the diaphragm manually closed down after focusing and viewing wide open. There is generally a ring on the lens for this function. When the diaphragm is open, the viewfinder is brighter, making composition and focusing easier. The camera will automatically close the diaphragm when the shutter is tripped if the lens and camera are both automatic.
