I had bought this extender to get with my EF 200L II 2.8 to 400mm. Unfortunately, the focus was very poor over the entire image field. The image at 400mm F / 5.6 did not show more details, rather less than a corresponding portion of the image without converter at 200mm. Only when stopping down the quality of the converter at F / 8 or more was useful. I have used the combination of a Canon 6D (mm format). I have returned to the converter at the end. Maybe I had just caught a bad copy, but I have not let me come to try another.
