I have a Tamron AF 17-50mm Wide Angle lens for my EOS 50D. It came with a lens hood. When I take a lot of pictures with the lens hood on, I have a black section in the corner. Why is that? Have a look at the linked picture for an example.
http://img684.imageshack.us/img684/9276/img3186h.jpg
http://img684.imageshack.us/img684/9276/img3186h.jpg