Canon EOS Rebel T6 Instruction Manual page 261

C.Fn-3
You can set the flash-sync speed for flash photography in the
aperture-priority AE <f> mode.
0: Auto
The flash sync speed is set automatically within a range of 1/200
sec. to 30 sec. to suit the scene's brightness. With an external
Speedlite, high-speed sync will also be possible.
1: 1/200-1/60 sec. auto
Prevents a slow shutter speed from being set in low-light
conditions. It is effective for preventing subject blur and camera
shake. However, while the subject will be properly exposed with
the flash, the background may come out dark.
2: 1/200 sec. (fixed)
The flash-sync speed is fixed at 1/200 sec. This more effectively
prevents subject blur and camera shake than with [1: 1/200-1/
60sec. auto]. However, in low light, the subject's background will
come out darker than with [1: 1/200-1/60sec. auto].
When 1 or 2 is set, high-speed sync cannot be used with an external
Speedlite.
Flash Synchronization Speed in Av Mode
3 Custom Function SettingsN
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