Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 318)

3 Setting the Movie Recording Quality
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At Full HD quality, you can shoot movies at a high frame rate of 119.9
fps or 100.0 fps. Good for shooting movies to be played back in slow
motion. The maximum recording time of one movie clip is 7 min. 29 sec.
Images are recorded in L 2 W
D or L 3 W D quality.
High Frame Rate movies do not record
sound.
If the time code is displayed during
movie shooting, it will count up 4 sec. for
each sec. in real time.
Since the High Frame Rate movie will be recorded as a 29.97 fps/25.00
fps movie file, it will be played back in slow motion at 1/4 speed.
High Frame Rate
Cautions for [High Frame Rate: Enable]
Under [ z 5: Time code ], if [ Count up ] is set to [ Free run ] (p.326), the
time code will not be recorded.
[ MOV/MP4 ], [ Movie rec. size ], and [ 24.00p ] cannot be set.
Even if you set it back to [ Disable ], the movie recording size will not
revert to the original setting. Set the movie recording size again.
If you shoot a High Frame Rate movie under fluorescent or LED lighting,
the movie image may flicker.
During the High Frame Rate movie shooting, the frame rate of the movie
image displayed on the LCD monitor is different from the frame rate of
the movie image being recorded.
Headphones cannot be used. (You cannot listen to the sound.)