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| p.1 #2 · New to light studio equipment with a problem... |
As long as the slave flash is firing with the on camera flash when the shutter is open, and not due to pre-flash, you should be OK. If you can see the effect of all the flashes in your image you are fine. Otherwise you need to set your camera to not generate pre-flash. I am a Nikon shooter so don't know how that is done with Canon, but I am pretty sure you can do it.
Hope that helps,