Dee Dee,
I shoot with the 580 in Program, Av, Tv and Manual modes. Instead of trying to understand how it does it, just put it on the camera, set it to ETTL and put the camera in Program mode and give it a try. If you like what you see from that combination, try it in the other modes. Again, outside you're just using it for fill so at high-speed synch, it'll give you better shadows and a bit of catch-light for the eyes. Believe me, the flash is smarter than both of us .

Jim