Canon EOS rebel xs - Manual (Page 167)

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External Speedlites
With an EZ/E/EG/ML/TL-series Speedlite set in the TTL or A-TTL
autoflash mode, the flash can be fired at full output only. Set the
camera’s shooting mode to <a> (manual exposure) or <f>
(aperture-priority AE) and shoot.
When using a Speedlite which has manual flash mode, shoot in the
manual flash mode.
With an EX-series Speedlite set to TTL autoflash with the flash’s
Custom Function, the flash will fire at full output only.
Sync Speed
The camera can synchronize with compact, non-Canon flash units at
1/200 sec. or slower. Be sure to test the flash unit beforehand to make
sure it synchronizes properly with the camera.
Cautions for Live View shooting
A non-Canon flash will not fire during Live View shooting.
Canon Speedlites other than the EX-series
Using Non-Canon Flash Units
If the camera is used with a flash unit or flash accessory dedicated to
another camera brand, the camera may not operate properly and
malfunction may result.
Do not attach a high-voltage flash unit on the camera’s hot shoe. It might
not work.