Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 263)

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Mirror Lockup
Mirror lockup keeps the mirror up in viewfi nder 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 [ z : Mirror lockup].
2 Select [Enable].
3 Focus on the subject, then press
the shutter button completely.

The mirror will swing up.
4 Press the shutter button completely
again.
 The picture is taken and the mirror goes
back down.