Canon EOS-1D C Instruction Manual page 35

Image Area
When H, I, A, or B is set, the respective image sensor area
below is used to record the movie.
Still photos
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.
3 Setting the Movie-recording Size
H 4K
I Super 35mm Crop
35