I am a newbie starting on to dslr. My interests are landscape and low light photography. After reading through tons of web resources I think I need a fast lens with a good range. Do you think that the above lenses are suitable for me? Is 17-50 f2.8 much better over the other? If I use a lens at 50mm zoom for landscape at f/2.8 will the depth of field be shallow? At what circumstances I need f/2.8 at 50 mm? If large aperture is not very useful at long focal lengths I could go for 17-70 which is cheaper?