Canon EOS-1DX Af Setting Manualbook page 109

Set up steps
[Switch to registered AF functions]
Assign [Switch to registered AF functions] to the
button or
button on the assign function's selection screen, and
from various AF functions, select the [AF area selection
mode], and select the mode that you want to use.
button with [Custom Controls]. Press the
109
Assign to the
depth of field preview
button
Press the
now
Select the [AF area
selection mode]
or
button