Canon EOS Rebel T3 18-55mm IS II Kit Instruction Manual page 219

C.Fn II: Image
C.Fn-3
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.
With settings 1 and 2, the continuous shooting speed will be slower even
with shutter speeds faster than 1 sec. (with the image-recording quality
set to JPEG). The maximum burst during continuous shooting will also
decrease (with the image-recording quality set to JPEG). Settings 1 and
2 are recommended only when shooting long exposures.
With settings 1 and 2, 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 with setting 2
than with setting 0 or 1.
With setting 1 or 2, if a long exposure is shot with the Live View image
displayed, "BUSY" will be displayed during the noise reduction process.
The Live View display will not appear until the noise reduction is
completed. (You cannot take another picture.)
C.Fn-4
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
Long exposure noise reduction
High ISO speed noise reduction
3 Custom Function SettingsN
2: Strong
3: Disable
219