Canon EOS 5D Mark III Instruction Manual (en)

82: Custom Controls
Pressing <
0> will display the menu on the LCD monitor.
Pressing <
0> will play back images.
Press <
0> to magnify the images recorded on the card. See p.257 for 
the operation procedure. You can also magnify the Live View image 
during Live View shooting and movie shooting when focusing with Live 
Mode, Quick Mode, and manual focus (p.212, 216).
Use this setting when you do not want to assign any function to the 
: Menu display
: Image playback
: Magnify/Reduce (press SET, turn 
: No function (disabled)