Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 304)

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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 < > recording mode. Applies when the frame rate of the
movie recording size is or .
• 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.
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