Canon EOS 750D Instruction Manual page 375

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.172).
Under [z1: Flash control], if [Flash metering mode] in [External
flash C.Fn setting] is set to [TTL flash metering] (autoflash), the
flash will always fire at full output (p.178).
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.
High-speed sync cannot be set in the <f> mode.
Under [z1: Flash control], set [Flash sync. speed in Av mode] to
[Auto] (p.174).
The camera makes a small noise when it is shaken.
A small noise may be heard when the camera's internal mechanism
moves slightly.
The shutter makes two shooting sounds during
Live View shooting.
If you use flash, the shutter will make two sounds each time you shoot
(p.193).
Troubleshooting Guide
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