Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 234)

Exposure Lock (AE Lock)
AE Lock Effects
You can lock the exposure when you want to set the focus and exposure separately or when
you will take multiple shots at the same exposure setting. Press the <
> button to lock the
exposure, then recompose and take the picture. This is called AE lock. It is effective for
shooting backlit subjects, etc.
1.
Focus on the subject.
Press the shutter button halfway.
The exposure value will be displayed.
2.
Press the < > button ( ).
A < > icon is displayed in the viewfinder, indicating that the
exposure is locked (AE lock).
Each time you press the < > button, the current exposure setting is
locked.
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