Mirror lock-up

Mirror lock-up (often abbreviated to MLU) is a feature employed in many Single Lens Reflex (SLR) cameras.

Combined with a remote or cable release, this greatly reduces the potential for vibration of the camera.

Some lenses are designed such that they extend into the mirror box when properly mounted on an SLR.

Mirror lock-up must be activated prior to mounting such lenses, and kept in place in order to prevent damage to the lens or camera.

Prior to the development of lenses utilizing the Angénieux retrofocus concept, mirror lock-up was essential to wide-angle SLR photography.