Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 187)

Mirror Lockup
Mirror lockup keeps the mirror up in viewfinder shooting to reduce blur from mirror
movement. Useful when shooting close-ups (macro photography), using a super telephoto
lens, shooting at slow shutter speeds, etc.
1.
Select [ : Mirror lockup].
2.
Select an item.
Select [ Enable ] or [ Enable:Mirror down w/ ], then press < >.
3.
Focus on the subject, then press the shutter button completely.
The mirror will swing up.
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