Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 235)

3.
Recompose and take the picture.
The AE lock exposure value and current exposure value are shown in
real time at right in the viewfinder.
When you are to take more pictures while maintaining the AE lock,
keep holding down the <
> button and press the shutter button to
take another picture.
Note
AE lock is not possible with bulb exposures.
AE Lock Effects
Metering Mode
Selection
Selecting the AF Area and AF Point
Automatic Selection Manual Selection
Exposure value centered on the AF
point in focus is locked. Exposure value centered on the
selected AF point is locked.
AE lock is applied at the center AF point.
* When the < > is set and the lens's focus mode switch is set to < >, AE lock is applied with
the exposure value weighted on the center AF point.
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