Canon 16-35mm Wide angle lens Canon 17-40mm Wide angle lens I'll be upgrading my lens collection soon, and I'm really looking forward to a nice ultra-wide angle lens. I'm wondering if you guys can provide some insight into these two lenses. How is the 16-35mm worth $700 more than the 17-40mm? If I go the cheaper route and get the 17-40, what am I giving up? For mostly cityscape photos, would the f/2.8 make much of a difference from the f4.0?