Mirror lockup
Mirror lockup was originally a feature available on some SLRs, designed to be use in conjunction with wide angle lenses where the lens otherwise intruded sufficiently into the camera body such that the rear element and mirror could come into contact.
It has since been adopted as a feature that allows the reflex mirror to be locked in the 'up' position before a shot, and so eliminate the camera vibration caused by mirror slap, helping the photographer produce sharper photos.
Many manufacturers have never included a mirror lockup on their models, and it is a matter of opinion as to whether this feature has any real benefits.
An article entitled Mirror lock-up is a conspiracy draws the conclusion - Mirror Lockup makes absolutely zero visible difference in the sharpness of the photo IF you shoot from a rock solid tripod and ball-head.
