Canon EOS-1D X Mark III Advanced User's Manual page 400

Select [Adjust by lens], then press <
5.
Check the result of the adjustment.
Take a picture and view it (
Adjust toward "+:
position, and adjust toward "–:
Repeat the adjustment as necessary.
Note
When shooting with the intermediate range (focal length) of a
zoom lens, the AF's point of focus is corrected automatically
relative to the adjustments made for the wide-angle and telephoto
ends. Even if only the wide-angle or telephoto end is adjusted, a
correction will be made automatically for the intermediate range.
) to check the results of adjustment.
" to correct focusing in front of your targeted
" to correct focusing behind it.
400
>.