I must remember that next time I fit my 1.4 or 2x converter (Canon) to my 70/200 F4 IS L, especially when I compare the results to my 400 5.6L (no IS), despite being "not recommended" (by whom) they seem to work rather well, they certainly represent good value for money in terms of AF and IQ. Maybe I'm just lucky, either that or the 5Dmk3 reacts better than some other cameras or manufacturers.