I just bought a second hand Canon 2x teleconverter for my telephoto lens so that I can have a little bit more reach for my wildlife photography until I can finance a bigger lens. I have done some research on it and it seems like most of the info out there is fairly subjective so I hope that things turn out well. The most common complaints are that the lens will become slower, softer, and show less contrast. But that is true when you put a UV filter in front of your lens. This baby is optical quality glass that should perform wonderfully. I will leave a review when I learn more.