Like I said, it's ONLY based on my experience. There are 2 events took place before I decided to sell my 17-40L. 1st is on one of my classmates' birthday. I took my 17-40, his father also equipped with Canon, actually using older model: 350D but 17-55 f2.8 lens. Days after, when we compare the results I found 17-55 results is better. That time I only think, this is because f2.8. Another event involving another brand of DSLR (which I don't want to specify details here) which I also despised my results, event in 'AUTO' mode.