Canon EOS-1D X Mark III Advanced User's Manual page 834

AE lock meter. mode after focus
For each metering mode, you can specify whether to lock the exposure (AE lock) once
subjects are in focus with One-Shot AF. The exposure will be locked while you keep
pressing the shutter button halfway. Select metering modes for AE lock and add a
checkmark [ ]. Select [OK] to register the setting.
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