I have been very happy with the focus on my two 5d3 bodies. I've shot two weddings with them now.
This shot was hand held using 24-105 and a 12mm extension tube.
Chris - are you glad with the quality of pics with the use of extension tube?
I'm not Chris, but extension tubes are just hollow tubes with no lens or glass elements in the tube, so IQ will not suffer like it does with teleconverters, which do have lots of extra lens elements. Extension tube will only change the focal length closer for more magnification but now one can't focus near infinity.
Thanks for explanation though I know the difference and the way ETs work. Since I don't own my ET and never experimented with one, I'm just curious in impressions of people who did. I don't own true macro lenses and am aware of IQ degradation without it. All I did was stacking lenses 24-105 and 70-200 with 17-40 (front to front with 17-40 dettached from body) to achieve macro effect and was not amazed with IQ at all, although magnification was really big