I read threads like this and I wonder if I have made a mistake on ordering the 21. It should be here some time this week, so I can see. I was thinking about the selection earlier this morning. I guess I will go out this weekend and select an appropriate landscape, setup the tripod and use 20mm @ f8 across the 21ltd, 12-24, 16-45 and 18-55 and just see what the results are.
It needs to be at least to be at least as good as the 16-45 and hopefully better than the 12-24. Right now, I have no idea as to what to expect.
I selected the 21 since I tend to be in the 20 area for a lot of the stuff, and the lens is a bit faster (since I have f4 for most of the rest) - but really across the entire range of 12-24. So pay your money and take your chances......