Canon EOS-1D C - Manual (Page 35)

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3 Setting the Movie-recording Size
When H, I, A, or B is set, the respective image sensor area
below is used to record the movie.
Image Area
Still photos H 4K
I Super 35mm Crop
C SD
A Full HD/B HD
When I or A is set, the movie will be recorded in 1920x1080 resolution.
When B is set, the movie will be recorded in 1280x720 resolution. When
C
is set, the movie will be recorded in 640x480 resolution.