Canon EOS-1D X Mark III Advanced User's Manual page 88

Button Backlighting
< >, < >, < >, < >, < >, < >, and < / > button backlighting is
activated when the LCD panels are illuminated, or during menu or Quick Control screen
display or image playback. This makes it easier to see the button layout in low-light
locations. Button backlighting is deactivated when the metering timer starts, which is
triggered by pressing the shutter button halfway, for example.
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