Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 181)

Long Exposure Noise Reduction
Noise that tends to occur in long exposures at shutter speeds of one sec. or slower can be
reduced.
1.
Select [ : Long exp. noise reduction].
2.
Set a reduction option.
Auto
For images exposed for 1 sec. or longer, noise reduction is performed
automatically if noise typical of long exposures is detected. This setting
is effective enough in most cases.
Enable
Noise reduction is performed for all images exposed for 1 sec. or
longer. The [
Enable ] setting may reduce noise that cannot be detected
with the [ Auto ] setting.
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