Is this lens as sharp as the nifty fifty? I'm thinking about getting the 17-55 and unloading my 18-200 and 50. I've heard nothing but good things about the 17-55mm, but its still quite expensive, so I want to know how the image quality compares to that of the 50mm f/1.8. Anyone who has experience with the 17-55 please give me your input. Thanks.