Canon EOS 100D Instruction Manual page 148

Mirror Lockup to Reduce Camera ShakeN
In very bright light, such as at the beach or a ski slope on a sunny day,
take the picture promptly after mirror lockup.
Do not point the camera toward an intense light source, such as the sun
on a sunny day or an intense artificial light source. Doing so may
damage the image sensor or the camera's internal components.
If you use the self-timer and bulb exposure in combination with a mirror
lockup, keep pressing the shutter button completely (self-timer delay
time + bulb exposure time). If you let go of the shutter button during the
self-timer countdown, there will be a shutter-release sound, but no
picture will be taken.
Even if you set Drive/self-timer to <i>, <M>, or <q>, the camera
will still shoot in single shooting mode.
When [z3: High ISO speed NR] is set to [Multi Shot Noise
Reduction], four continuous shots will be taken for the single picture
regardless of the [5: Mirror lockup] setting.
If 30 seconds elapse after the mirror has locked up, it will go back down
automatically. Pressing the shutter button completely again locks up the
mirror again.
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