I use it recently with a canon 7D and Canon 70-200 f2.8 giving a 140-400 x 1.6 or: 224-640! In use the opening falls to 5.6, autofocus works perfectly, I'm testing on a pro tennis match under the following conditions: zoom up to 600 opening 5.6 is 1/500 and 1000 iso and monopod to stabilize, impeccable results and nickel autofocus, so that's good for me! with this configuration there is no equivalent to this price and this opening. I posted 3 pictures of this trial in the product gallery. before you buy check compatibility on the canon website (I put the link in the comments): we read on the site: Compatible EF 70-200mm f / 2.8L USM, EF 70-200mm f / 2.8L IS USM, EF 70-200mm f / 4L EF 100-400mm f / 4.5-5.6L USM EF 400mm f / 4 DO IS USM L-series and objectives fixed length of 135 mm or more. we must also carefully consider that we lose two diaphragms thus opening book has goals beyond 2.8 has the autofocus may not work (except a 5DIII updated). and obviously only works on Canon lenses ....
