Mirror lock-up

With mirror lock-up the mirror (2) flips up towards (5) well before the shutter (3) opens. As a result, light no longer reaches the eyepiece (8).

Mirror lock-up (often abbreviated to MLU) is a feature employed in many Single Lens Reflex (SLR) cameras. It allows the operator to reduce vibration-induced motion blur during exposure. It also allows the mounting of lenses which extend into the SLR's mirror box when mounted.