I just want to ask about image quality between the two lenses. Which one is sharper? According to the charts, it seems that the 24-105 is a bit sharper than the 24-70 but how about on real life images? f4 is f4 and f2.8 can be stopped down to f4, is the 24-70 sharp wide open or do you need to stop down to f4 atleast to get a sharp image?