Great addition to my 70-200mm f/2.8 IS lens. Ideal for sports and outdoor photography without spending a fortune on dedicated super telephoto lenses. Provides excellent image quality with minimal focus speed slowdown. Even effectively used for astrophotography (on camera). I was able to attach it to the Celestron SCT by covering the electrical contacts with masking tape and using it on the camera in manual mode, but couldn't focus effectively on our first try - we'll try again when we have more time. and worthy heaven. As stated on Canon's website, this is only compatible with Canon fixed focal length and large-aperture L-series multi-zoom lenses. Be sure to check compatibility before taking the plunge.