Canon EOS Rebel T3i EOS REBEL T3i / EOS 600D Instruction Manual - Page 253

3 Custom Function SettingsN 2: 1/200 sec. (fixed) The flash-sync speed is fixed to 1/200 sec. This more effectively prevents subject blur and camera shake than with setting 1. However, the background may come out darker than with setting 1. When 1 or 2 is set, high-speed sync cannot be used with an external Speedlite. 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 [On] setting may be effective for noise that cannot be detected or reduced with the [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 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.) 253