Canon EOS 5D Mark III Instruction Manual (en)

Movie Shooting Cautions
Recording and Image Quality
If you use a card having a slow writing speed, a five-level 
indicator may appear on the right of the screen during movie 
shooting. It indicates how much data has not yet been 
written to the card (remaining capacity of the internal buffer 
memory). The slower the card, the faster the indicator will 
climb upward. If the indicator becomes full, movie shooting 
will stop automatically. 
If the card has a fast writing speed, the indicator will either 
not appear or the level (if displayed) will hardly go upward. 
First, shoot a few test movies to see if the card can write fast enough.
Still Photo Shooting During Movie Shooting
Regarding the image quality of still photos, see “Image Quality” on page 
Playback and TV connection
In autoexposure shooting, shutter-priority AE, or aperture-priority AE 
modes, if the brightness changes during movie shooting, the movie may 
freeze temporarily. In such cases, shoot movies with manual exposure.
If you connect the camera to a TV set (p.274, 277) and shoot a movie, 
the TV will not output any sound during the shooting. However, the 
sound will be properly recorded.