Canon EOS 5D Mark III Instruction Manual page 235

An increase of the camera's internal temperature may cause movie
shooting to stop before the maximum recording time shown in the
preceding page (p.247).
Even if [Record func.] is set to [Auto switch card], the card cannot be
switched automatically during movie shooting.
About Full HD 1080
Full HD 1080 indicates compatibility with High-Definition
featuring 1080 vertical pixels (scanning lines).
3 Setting the Movie-Recording Size
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