Canon EOS-1D X Mark III Advanced User's Manual page 304

[ 3]
Auto slow shutter
You can choose whether to record movies that are brighter and less affected by image
noise than when set to [Disable] by automatically slowing the shutter speed under low
light.
Available in < > or <
movie recording size is
• Disable
Enables you to record movies with smoother, more natural movement, less affected
by subject shake than when set to [Enable]. Note that under low light, movies may
be darker than when set to [Enable].
• Enable
Enables you to record brighter movies than when set to [Disable] by automatically
reducing the shutter speed to 1/30 sec. (NTSC) or 1/25 sec. (PAL) under low light.
Note
Setting to [Disable] is recommended when recording moving subjects under
low light, or when afterimages such as trails may occur.
> recording mode. Applies when the frame rate of the
or .
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