Canon MARK IV User Manual (en)

Movie Shooting Cautions
Recording and Image Quality
When you shoot movies at high ISO speeds, noise (horizontal banding, 
dots of light, etc.) or irregular colors may appear.
If the attached lens has an Image Stabilizer, the Image Stabilizer will 
operate at all times even if you do not press the shutter button halfway. 
The Image Stabilizer may cause the total movie shooting time or the 
number of possible shots to decrease. If you use a tripod or if the Image 
Stabilizer is not necessary, you should set the IS switch to <2>.
The camera’s built-in microphone will also pick up camera operation 
noise. If you use a commercially-available external microphone, you can 
prevent (or reduce) these noises from being recorded.
To autofocus again during movie shooting, press the <p> button. 
(AF will not work by pressing the shutter button halfway.) However, 
autofocusing during movie shooting is not recommended since it can 
momentarily throw the focus way off or change the exposure.
If the card’s remaining capacity is not enough for movie shooting, the 
movie shooting remaining time (p.148) will be displayed in red.
If you use a card having a slow writing speed, a five-level 
indicator might 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 
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.
During movie shooting, certain Custom Function settings will be disabled