HDNitehawk: Good question. I don't know how the third party could structurally damage the lens, but I guess it is possible. Unlike the Canon, the Kenko/Promaster/Tamron (all look identical) don't have an element that enters the back portion of the lens. I've played around with a lot of teleconverters on a lot of lenses and the Kenko and Promaster work on every lens I own. My first Kenko did not make a great union with the lens (wobbled a bit), so I sent it back and the second one fit just as snug as the Canon teleconverter with the lens and camera.