I bought this strobe without a manual. I have a stupid question about the different modes. There is A1, A2 and M. Can someone tell me the difference between the A1 and A2 settings? By looking at the matrix on the rear of the strobe it appears that A2 might be more powerful than A1? I am using this on a GA645 camera. Also, in manual and slave modes can I adjust the flash output by, say, one or two stops? Thanks!