I can only speak from the Nikon perspective...

I have some of the older YN-560 (I). (I don't know what the differences are between the first and II versions apart from the lcd on the II model). The YN-560 (I) can be triggered optically from another flash, although I almost went nuts trying to get them to sync. I use the camera popup flash to fire an off-camera SB-800 (using Nikon TLS - infrared), and the flash from the SB fires the YN-560's with the optical slave. The flash setting on the camera needs to be rear-curtain sync for them all to fire at the same time - I don't know if this also applies to Canon.

If no one chimes in with a definite answer in the meantime, I'll run a quick test tomorrow just triggering from the popup straight to the YN-560 (if I can find them )...



Cheers.

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You are probably already aware, but one of the potential problems with optical triggering is that it might not work so well if you are trying to trigger in bright sunlight.