Canon EOS Rebel T5 18-55 IS II Kit - Manual (Page 260)

3 Custom Function SettingsN
260
C.Fn II: Image
C.Fn-4 Long exposure noise reduction
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 [2: On] setting may be effective for noise that cannot be
detected or reduced with the [1: Auto] setting.
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 may 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.)
COP
Y