Jon,


I think the 17-55 2.8 is able toget the same amount of light onto the cropped sensoras the 24-105 is on the larger one. They both use a 77mm filter so I am guessing they allow about the same in. The 17-55 just directs it into a smaller image circle. The light on that smaller circle will be brighter per unit area than the 24-105, hence 2.8.I think the ISO vs actual noise argument holds some water, I need to think about that one a bit. I am really not trying to argue and apologize if it seems that way.


Tom