Hi all!
I have a DRebel xt and I also have the kit lens. Yesterday UPS was kind enough to drop me a package with an ef 17-40 USm L lens. 
I took "identical" pictures with both lenses; all (pop-up) flash, indoors, in fully automatic mode, at the same focal length (=35mm).
The results showed significant difference in color. While the pictures taken with the kit lens were "cold", making people looking pale, the pictures taken with the 17-40 look "warm" and natural. It's a huge difference.
Of course, the 17-40 is a superior lens! But should the color differ significantly? It almost looks as if different white balance is used. But since they are shot in fully automatic with flash, this can't be. Right?
Any insight is welcome. Thanks for a great forum!
Enigma


