Canon EOS-1 DX Mark III Advanced User's Manual page 401

Clearing All AF Microadjustments
When [ Clear all] appears at the bottom of the screen, pressing the <
all the adjustments made for [All by same amount] and [Adjust by lens].
Caution
Precautions for AF Microadjustment
The AF's point of focus will vary slightly depending on the subject conditions,
brightness, zoom position, and other shooting conditions. Therefore, even if you
perform AF Microadjustment, focus may still not be achieved at the suitable
position.
The adjustment amount of one step varies depending on the maximum aperture
value of the lens. Keep adjusting, shooting, and checking the results until you are
satisfied.
The adjustment will not be applied to AF in Live View shooting or movie recording.
When checking the results of adjustment, set to One-Shot AF and set [
Shot AF release prior.] to [Focus] before focusing.
Note
Notes for AF Microadjustment
It is best to make the adjustment at the actual location where you will shoot. This
will make the adjustment more precise.
Using a tripod when making the adjustment is recommended.
When you will check the results of adjustment, consider shooting with image size
set to JPEG (Large) and JPEG/HEIF quality (compression) set to 8 or higher.
401
> button will clear
: One-