I allready upgraded to the Sigma 100 - 400 APO to give me better glass over my basic canon glass, I have 5 from 18mm to 300mm, F1.8 to F5. I found in a test yesterday that in low light I am not getting detail no matter what lens or settings. I have been having troubles that have shown up as focus issues. Go look at my Help Blurry pics thread in wildlife for some examples. I have been working on this problem for months. Unless my subject is lit right up I do not get detail. If I am near my max aperature I am going to have an issue. If I speed up ISO or decrease shutter the problem is not going away. It changes slightly but the lack of deatil problem is still there. LOL, at least that is where I am at today, my Xti has a problem with low light, any kind, even a shadow. This weekend I can compare with a coworkers 30D and see what happens. Not that the 30D is a better camera but maybe my Xti is faulty. Right now, we are not getting along.