Canon Rebel T2i EOS 550D Instruction Manual page 193

When 1 or 2 is set, high-speed sync cannot be used with an external Speedlite.
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.
With setting 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 2, if a long exposure is shot during Live View display, "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-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 greatly
decrease.
Long exposure noise reduction
High ISO speed noise reduction
3 Custom Function SettingsN
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