Canon EOS-1D X Mark III Advanced User's Manual page 922

Caution
AF is not available with Extender EF 2x (I/II/III) attached to the EF 180mm f/3.5 L
Macro USM lens.
Precise autofocusing may not be achieved if you use a lens and Extender EF1.4x
III/EF2x III combination marked with an asterisk or a lens and extender in a
combination marked with two asterisks. In this case, refer to the Instruction Manual
of the lens or extender used.
Note
If you use a TS-E lens, manual focusing is required. TS-E lens group designations
apply only when tilt or shift is not used.
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