I am using two 580s.
The one on camera is set as master. Both same channel.
I set the apparture high so I would think sync speed should not be a problem but then let me rechech and will post more info with this in consideration.
My main reason to use on AV is to have a better DOF as set on Auto, Flash TTL everthing works well but very shallow deepth due to mostly chosen highest F stop.
should work - we shoot AV often - high f should cause lower shutter speed - no high speed sync needed unless you are in blazing sun.
One thing to realize is that in most all shooting modes (AV, TV, etc) Canon wants to do fill flash - i.e., the camera tries to set the exposure so that the background is correct and uses flash to fill-in. Thus, problems can occur in dark conditions where setting a high f would not be possible (shutter speed cannot go low enough to compensate for the narrow aperture) for correct background exposure. You can over-ride this with manual settings, but will probably end up with a really dark background - that or you have to use to high ISO settings to allow for a correct aperture/shutter speed combo.