Canon EOS Rebel T6 Instruction Manual page 291

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

automatically becomes slow (slow-sync shooting) so that both the
subject and background are properly exposed. To prevent a slow
shutter speed, under [53: Custom Functions (C.Fn)], set [3: Flash
sync. speed in Av mode] to [1: 1/200-1/60sec. auto] or [2: 1/200sec.
(fixed)] (p.261).
The built-in flash rises by itself.
In shooting modes (<A> <C> <2> <4> <6>) whose default

setting is <a> (built-in flash auto firing), the built-in flash will be raised
automatically when necessary.
The built-in flash does not fire.
If you use the built-in flash too often in too short a period of time, the

flash may stop firing for a while to protect the light-emitting unit.
The external flash always fires at full output.
If you use a flash unit other than an EX-series Speedlite, the flash will

always be fired at full output (p.273).
Under [z1: Flash control], if [Flash metering mode] in [External

flash C.Fn setting] is set to [TTL], the flash will always fire at full
output (p.201).
Flash exposure compensation cannot be set for
the external Speedlite.
If flash exposure compensation is set with the external Speedlite, flash

exposure compensation cannot be set with the camera. When the
external Speedlite's flash exposure compensation is canceled (set to
0), flash exposure compensation can be set with the camera.
Troubleshooting Guide
291