Canon MARK IV User Manual (en)

Although using the self-timer or Remote Switch can prevent camera 
shake, using mirror lockup to prevent camera vibrations (mirror shock) 
can also help when you use a super telephoto lens or shoot close ups 
(macro photography).
When [8C.Fn III -17: Mirror lockup] is set to [1: Enable] or [2: 
Enable: Down with SET] 
(p.226), shooting with mirror lockup is 
Focus the subject, press the shutter button completely 
and let go of it.
The mirror will lockup and <2> will blink on the top LCD panel.
Press the shutter button completely again.
The picture will be taken.
With [1] set, the mirror will go back down when the picture is taken.
With [2] set, the mirror lockup will remain even after the picture is 
taken. To cancel the mirror lockup, press <0>.
 Mirror Lockup
In very bright light such as at the beach or a ski slope on a sunny day, 
take the picture promptly after mirror lockup.
During mirror lockup, do not point the camera lens at the sun. The sun’s 
heat can scorch and damage the shutter curtains.
If you use bulb exposures, the self-timer, and mirror lockup in 
combination, 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 is actually taken.
When [1: Enable] is set, single shooting will take effect even if the drive 
mode is set to continuous. When [2: Enable: Down with SET] is set, the 
current drive mode will take effect for the shooting.
When the self-timer is set to <k> or <l>, the picture will be taken 
after 10 sec. or 2 sec. respectively.
The mirror locks up, and after 30 seconds, it will go back down automatically. 
Pressing the shutter button completely again locks up the mirror again.
For mirror lockup shots, using the Remote Switch RS-80N3 or Timer 
Remote Controller TC-80N3 (both sold separately) is recommended.