I have same camera and same lens, DG version the last one. Unfortunatly i suppose it is a sigma problem. With 1.4tc aperture become f6.3 so af should work without any problem. But with canon tc af doesnt work on 5dmk3 and on 1dmk3 too. The main problem is that also sigma tc cannot allow af.... The best thing to do is to buy a kenko dgx pro1.4. Buy it and af will work very well. With this tc you will have af also on 7d and all the 1d. On 5dmk3 kenko will allow all the af points. This is another important point.as example on 100/400 with new fw you can have single af point with canon and all the af points with kenko. I tried canon tcIII and it is slower than kenko... Try it!!