I absolutely concur with Daniel, the 17-55mm is a much better in terms of sharpness and contrast. It is called the "hidden L" for 1.6 bodies. Some people such as yourself prefer the range of the 24-105mm on a crop body and I absolutely respect that, I just think people should be informed in there choice.
John.


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I must say that weather-sealing for me proved to be more important than I'd ever imagined when I bought the lens. I've been out in a few rainy days, but more important I've had quite the amount of blowing sand and dust to my gear. In those occasions I felt a lot more secure with a weather-sealed lens on my camera. I have used the Sigma 50mm 1.4 in a sandy environment one time and there was sand all over the focus ring and it needed to be cleaned. 

