Mirror lock-up

From Nikonians Wiki - FAQs, Photo Glossary, Good Photo Locations, Help

Mirror lock-up is a setting on most recent Nikon SLR cameras which locks the mirror up and out of the way prior to shooting. Typically, in mirror lock-up mode, the shutter release is triggered once for the mirror to be locked up, and a second time for the actual exposure. Once the exposure is complete, the mirror returns to its normal position.

Mirror lock-up is principally a means of defeating shutter shake.

Generally speaking, mirror lock-up should be used in conjunction with a remote shutter release, since the action of depressing the shutter release button is also a cause of shake at low shutter speeds.