As we know the 3GS camera picks a spot and focuses on it. We also know that touching a spot on the screen overrides the camera choice and in addition adjusts the exposure accordingly. I was taking some outdoor shots today and it seems that, even if the two focus spots are the same, that camera autofocus and touchfocus on the same spot will yield different exposure results. Touchfocus will essentially establish a spot meter in conjunction with the smaller focus area. I'm going to continue testing further to confirm.