As the other person said, the AF assist beam works in all shooting modes, but does not fire in AI Servo AF. I had mine set to AI servo once (I though I had set it to one shot), and when the beam wouldn't fire I thought I had broken my brand new 580 II[].


I would say however that Av is not the best for this situation. ETTL II autoflash on Av uses a slow shutter speed in low light conditions. I personally use M most of the time. You might want to look at the 430 manual, in the basicoperationssection at the part regarding autoflash behavior on various shooting modes for more info.