Canon EOS 5D MARK III Instruction Manual page 27

._,2: Custom Controls
Pressing <8> wiii display the menu on the LCD monitor.
Pressing <8> wiii play back images.
Press <8> to magnify the images recorded on the card. See p.251 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.210,214).
Use this setting when you do not want to assign any function to the
button.
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