OK, maybe this will help: The 50D isn't fully weather sealed, so the sealing on the 17-40L isn't a decider. Unless you are pretty rough with your lenses, you won't need the rugged L build. With the 17-55 the extra 15mm reach at the long end and theextra stop(f2.8 v f4) will probably be very useful to you. If you're not planning a FF upgrade and you treat your lenses with care then its the 17-55.


Hmmmm.... the more I think about this, the more I'm considering trading in my 17-40L for a 17-55 []