I'm not an ETTL expert, but I think the answer to your question is this--the pop-up may be contributing to the exposure, and maybe not. It depends on how you have it set up. However, I will say that the initial burst of flash you see coming from the pop-up isn't contributing to the exposure. The initial burst is used to communicate information from the master flash (the pop-up) to the slave (580EX, in this case). However, once the inital burst is finished, the pop-up may fireat the same time as the 580EX thereby contributing to the exposure ifthe pop-up isdesignated to do so through the menu system. For testing purposes this weekend, I had the pop-up set to contribute to exposure in a ratio (designated by me) while using the 580EX as a slave flash.