Canon EOS-1 DX Mark III Advanced User's Manual page 356

AF Shooting Tips
Even when focus is achieved, pressing the shutter button halfway will focus again.
Image brightness may change during autofocusing.
Depending on the subject and shooting conditions, it may take longer to focus, or the
continuous shooting speed may decrease.
If the light source changes as you shoot, the screen may flicker, and focusing may be
difficult. In this case, restart the camera and resume shooting with AF under the light
source you will use.
If focusing is not possible with AF, focus manually ( ).
For subjects at the edge of the screen that are slightly out of focus, try centering the
subject (or AF point, or Zone AF frame) to bring them into focus, then recompose the
shot before shooting.
With certain lenses, it may take more time to achieve focus with autofocus, or accurate
focusing may not be achieved.
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