Canon REBEL T3I EOS 600D User Manual page 282

Troubleshooting Guide
When I use the <f> mode with flash, the shutter speed
becomes slow.
If you shoot at night when the background is dark, the shutter speed
becomes slow automatically (slow-sync shooting) so that both the
subject and background are properly exposed. If you do not want a
slow shutter speed to be set, set [3: Flash sync. speed in Av mode]
in [7 Custom Functions (C.Fn)] to 1 or 2 (p.252).
The built-in flash pops up by itself.
In the A, 2, 4, and 6 modes, the built-in flash will pop-up
automatically when flash is necessary.
The built-in flash does not fire.
If you shoot continuously with the built-in flash at short intervals, the
flash might stop operating to protect the flash unit.
I cannot set flash exposure compensation with [External
flash func. setting].
If flash exposure compensation has been set on an external Speedlite,
[Flash exp. comp] (p.182) cannot be set in the [External flash func.
setting] screen. Also, if you set flash exposure compensation with the
camera and then set flash exposure compensation with the external
Speedlite, the Speedlite's flash exposure compensation setting will
override the camera's. When the Speedlite's flash exposure
compensation is canceled (set to 0), flash exposure compensation can
be set with the camera.
High-speed sync cannot be set in the <f> mode.
Under [7 Custom Functions (C.Fn)], set [3: Flash sync. speed in
Av mode] to [0: Auto] (p.252).
The camera makes a noise when it is shaken.
The built-in flash's pop-up mechanism moves slightly. This is normal.
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