Canon EOS 5Ds Instruction Manual page 288

k Shooting Movies
If [Minimum] is set to [L(50)] in [ISO speed range], and you switch from
still photo shooting to movie shooting, the minimum setting for manual
ISO range for movie shooting will be ISO 100. It cannot be expanded to
ISO 50.
Changing the shutter speed or aperture during movie shooting is not
recommended since the changes in the exposure will be recorded.
When shooting a movie of a moving subject, a shutter speed of 1/30 sec.
to 1/125 sec. is recommended. The faster the shutter speed, the less
smooth the subject's movement will look.
If you change the shutter speed while shooting under fluorescent or LED
lighting, image flicker may be recorded.
Under [83: Custom Controls], if [s: Expo comp (hold btn, turn
)] is set (p.423), you can set exposure compensation while Auto ISO
is set.
When Auto ISO is set, you can press the <A> button to lock the ISO
speed.
If you press the <A> button and recompose the shot, you can see the
exposure level difference on the exposure level indicator (p.289)
compared to when the <A> button was pressed.
By pressing the <B> button, you can display the histogram.
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