I've moved to the 5Dll from my 50D, and am trying to decide which UWA to go with. I know the 17-40 will work fine for landscapes, since I'll be stopped down to f/8 or smaller. But what about shooting the milky way? Or (if I'm lucky) the Aurora Borealis? For long exposures at higher ISO will I need the extra stop to 2.8? I have stacking software... I'm just trying to decide if the extra cost of the 16-35ll is worth it... Can I get away with f/4?

