okay so i was messing around with my 580EX and a friends 420EX. i had mine set to master and his 420EX on slave in group A chanel 1. i really dont understand, sometimes it would work sometimes it wouldnt. is there a particular range on it. or some certain direction they have to be facing. I couldnt make a pattern about it. sometimes it wouldnt even work three feet from the camera. sometimes it would work from 50.