Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 257)

257
2 Mirror Lockup
Do not point the camera toward an intense light source, such as the sun
or an intense artificial light source. Doing so may damage the image
sensor or the camera’s internal components.
In very bright light, such as at the beach or a ski slope on a sunny day,
take the picture promptly after mirror lockup is stabilized.
During mirror lockup, shooting function settings and menu operation are
disabled.
When [ Enable ] is set, single shooting will take effect even if the drive
mode is set to continuous. When [ Enable: Mirror down w/ s ] is set,
the current drive mode will take effect for shooting.
You can also use the self-timer with mirror lockup.
If approx. 30 sec. elapse after the mirror has locked up, it will go back
down automatically. Pressing the shutter button completely locks up the
mirror again.
When shooting with mirror lockup, using a tripod and Remote Switch RS-
80N3 (sold separately) or Timer Remote Controller TC-80N3 (sold
separately) is recommended (p.258).