Canon EOS Rebel T1i Instruction Manual page 185

C.Fn II: Image
C.Fn-4
0: Off
1: Auto
For 1 sec. or longer exposures, noise reduction is performed
automatically if noise typical of long exposures is detected. This [Auto]
setting is effective in most cases.
2: On
Noise reduction is performed for all exposures of 1 sec. or longer. The
[On] setting may be effective for noise that cannot be detected or
reduced with the [Auto] setting.
After the picture is taken, the noise reduction process may take the same
amount of time as the exposure. You cannot take another picture until
the noise reduction process is completed.
At ISO 1600 and higher, noise might be more pronounced at the 2
setting than at the 0 or 1 setting.
With setting 2, if a long exposure is used during Live View shooting,
"BUSY" will be displayed and the Live View display will not appear untill
the noise reduction process is completed. (You cannot take another
picture.)
C.Fn-5
Reduces the noise generated in the image. Although noise reduction is
applied at all ISO speeds, it is particularly effective at high ISO speeds. At
low ISO speeds, the noise in the shadow areas is further reduced. Change
the setting to suit the noise level.
0: Standard
1: Low
2: Strong
3: Disable
With setting 2, the maximum burst for continuous shooting will decrease.
Long exposure noise reduction
High ISO speed noise reduction
3 Custom Function SettingsN
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