Canon eos rebel t1i - Manual (Page 196)

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Basically operates like a built-in flash for easy operation.
When an EX-series Speedlite (sold separately) is attached to the
camera, almost all the autoflash control is done by the camera. In other
words, it is like a high-output flash attached externally in place of the
built-in flash.
For detailed instructions, see the EX-series Speedlite’s instruction
manual. This camera is a Type-A camera that can use all the features
of EX-series Speedlites.
External Speedlites
EOS-dedicated, EX-series Speedlites
Shoe-mount Speedlites Macro Lites
With an EX-series Speedlite not controllable by the camera, only [Flash
exp. comp] and [E-TTL II] can be settable for [External flash func.
setting] (p.139).
(Certain EX-series Speedlites also enable [Shutter sync.] to be set.)
If flash exposure compensation is set with the external Speedlite, the
flash exposure compensation icon displayed on the camera’s LCD
monitor will change from y to C.
When the flash metering mode is set to TTL autoflash with the flash’s
Custom Function, the flash will fire at full output only.