Mirror lockup.

SLRs contain mirrors which allow you to look through the lens while metering and focussing. When you take a photo this reflex motor swings up out of the way, allowing a clear path from the lens to the film surface. (except in the case of SLRs with fixed pellicle mirrors)

This mirror motion, however, inevitably causes some vibration - mirror slap - within the camera as it moves and strikes the bumpers at the top of the mirror box. This vibration can induce some motion blur in a final image. For that reason some cameras let you lock up the mirror manually prior to taking the photograph. This lockup can also be useful for cleaning.

Some cameras also support mirror prefire, which is a similar thing - the mirror flips up a second or two before the shutter opens. This mirror prefire gives enough time for any mirror movement induced vibrations to die down.

Note that it’s a bad idea to point the camera at the sun if the mirror is locked up. Direct sunlight, focussed and concentrated by a camera lens, can easily destroy a camera shutter.

cf. mirror slap, pellicle mirror, reflex mirror.

Entry last updated 2002-04-20. Term 788 of 1487.

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