Quote Originally Posted by Steve Large View Post
Some have commented that using a UV filter can cause something like this but I have not done any testing with my UV filter off (your thoughts?).
I was going to suggest that too, especially if it's an el-cheapo filter. Doesn't cost you anything to try some tests without it!;) I use the 100-400 on a 40D with more than satisfactory results, and I used it on a Rebel XT for 3 years before that with the same great results.