Canon EOS-1D X Mark III Advanced User's Manual page 158

Distortion Correction
Distortion (image warping) can be corrected.
Caution
To correct distortion, the camera captures a narrower image area than the area
seen in viewfinder shooting, which crops the image a little and slightly lowers the
apparent resolution.
For movie recording, [Distortion correction] will not appear. (Correction is not
possible.)
In Live View shooting, applying distortion correction may change the angle of view
slightly.
Areas outside the recording area may be displayed if you magnify edges of the
screen in Live View shooting.
Images with distortion correction applied will not have the Dust Delete Data ( )
appended. Also, the AF point may be displayed out of position, relative to the time
of shooting.
Magnifying the periphery of the image may display parts of the image that will not
be recorded.
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