I have a new Canon 70-300mm f/4-5.6 IS USM lens for my aging 350D (Rebel XT). I have sent my Tamron 70-300mm in for a repair because, among other things, it wouldn't stop down. I had to shoot wide open or the camera would show an error 99 and refuse to shoot. Plus, the Canon is a much better lens, with image stabilization. It's nice to be able to zoom in and use f/8 if I want to. I took a few dozen shots today, trying it out. I'm still using manual focus, so sharpness may be lacking.