Extender works great. I use it with an IS II 2.8 70-200mm lens and even at 400mm/5.6 wide open I get very clean and sharp images. If you're using it with a modern Canon body, you should still get autofocus at maximum focal length. Using this combo on my 7d instead of my 5dMIV gives me 640mm. Some YouTube reviewers say they don't produce sharp images. Nonsense! The bottom line is that you shouldn't see any noticeable degradation in image quality when used with a quality lens. However, I will say that in my opinion I get cleaner images on my 30MP full frame sensor (when cropped to the same zoom) than on the crop sensor with its 1.6x "zoom". Nevertheless, both combinations deliver great images even with 100% screen magnification.