Canon EOS Rebel T5i 18-55mm IS STM Lens Kit - Manual (Page 309)

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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, refer to 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 compatible with flash function settings
(p.220), only [Flash exposure compensation] can be set for [External
flash func. setting]. (With certain EX-series Speedlites, [Shutter
synchronization] is also settable.)
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 0.
If the Speedlite’s Custom Function has the flash metering mode set to
TTL autoflash, the flash will fire at full output only.
COP
Y