Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 400)

Select [ Adjust by lens ], then press < >.
5.
Check the result of the adjustment.
Take a picture and view it ( ) to check the results of adjustment.
Adjust toward “+: ” to correct focusing in front of your targeted
position, and adjust toward “–: ” to correct focusing behind it.
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.
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