Canon EOS-1 Settings Manual page 41

The appearance of high ISO speeds on the
EOS-1D Mark IV are the result of an advanced
image sensor and image processing technology.
The Custom Function C.Fn II-2 High ISO speed
noise reduction setting is set to a default setting
(option: 0) of Standard. In order to get an idea
of the noise reduction function effects, you
should compare photos taken with ISO speeds
Standard
Standard
of ISO1600 or higher by changing Noise
Reduction from Disable to Strong.
You will be able to see that approaching high
ISO speeds of ISO12800, when set to Disable,
n o i s e w i l l b e c o m e m o re v i s i b l e . E v e n a t
extremely high ISO speeds, if Standard and Low
noise reduction are working, noise will be
considerably reduced.
ISO6400
Disable
Standard
ISO12800
Disable
Standard
Low
Strong
Low
Strong 41