I recently bought a Canon 17-40mm f/4.0L lens for my Rebel XTi. After shooting some outdoor shots with and without a circular polarizer filter, I noticed that the sky was very noisy. I was curious if this is me being too paranoid, if there is something wrong with my camera/lens or that I need to change about the way I take pictures. The following two urls show outdoor pictures taken with two different lenses. The noise that I am wondering about can be seen in the first link (besides the firework shot, which was taken with the kit lens). Canon 17-40mm f/4.0L Canon IS 28-135mm *It is worth noting that this lens had focusing issues and was returned. Also, all shots were taken with a cpf as well. Thanks!