Instant-return mirror

Glossary Terms

An instant-return mirror is a mirror in the reflex viewfinder of an SLR that automatically returns to the viewing position after an exposure, and so restoring the view through the viewfinder. The moving mirror is the cause of mirror slap movements.

An SLR mirror sits in the light path, in front of the film plane; to take a shot it must move out of the way, which blacks out the viewfinder. Early SLRs had to have the mirror returned - and the viewfinder restored - manually.