I have a Canon T3i and will be purchasing a 430EXii soon. What actually triggers the off-camera flash (430EXii)? The light from the internal flash or is there a transmitter in the camera (T3i) that only works when the internal flash is open? Reason I ask is I have an old Vivitar 285 that I could use as the primary flash, but it must use the shoe, either on-camera of off by means of a cable, leaving the internal flash unusable as it will not open if the shoe is occupied. Will the light from the 285 trigger the off-camera 430 to fire or does the internal flash have to do that? Thanks.