Canon EOS R3 Advanced User's Manual page 111

Button Backlighting
< >, < >, < >, < >, and < / > button backlighting is activated during
LCD panel illumination, 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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