Canon EOS-1D X Mark III Advanced User's Manual page 299

Canon Log Image Quality
Noise may increase slightly.
Caution
General Canon Log shooting precautions
With Canon Log, autofocusing may be more difficult for subjects under low light, or
for low-contrast subjects.
Canon Log 2 and Canon Log 3 are not supported.
Note
Playing movies recorded with Canon Log
View Assist is not used during movie playback.
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