Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 808)

HDMI Resolution
Set the image output resolution used when the camera is connected to a television or
external recording device with an HDMI cable.
1.
Select [ : HDMI resolution].
2.
Select an item.
Auto
The images will automatically be displayed at the optimum resolution
matching the connected television.
1080p
Output at 1080p resolution. Select if you prefer to avoid display or
delay issues when the camera switches resolution.
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