Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 183)

High ISO Speed Noise Reduction
You can reduce the image noise generated. This function is especially effective when
shooting at high ISO speeds. When shooting at low ISO speeds, the noise in the darker
parts of the image (shadow areas) can further be reduced.
1.
Select [ : High ISO speed NR].
2.
Set the level.
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