I did a brief search and did not find an answer to my question. I currently own two bodies, the first is a 60D (that I rarely use any more) and a 5DIII that I use regularly. I own a 70-200isII and a 400 5.6. I use my 1.4II extender often with the 5dIII and the 400 5.6. While I am happy with the results, I know they could be better. Some day I hope to purchase a 500 or 600mm lens for wildlife and birding, but for now they are just not in the budget.
I read a lot about the series III extenders when they were introduced a few years ago. I am almost certain I read that with the 60D and the 400 5.6 lens it made no sense to 'upgrade' from the 1.4II I own to the 1.4 version III extender, am I correct in my assumption? Does the same hold true with my 5dIII, will I benefit with the newer version of the 1.4 (or 2.0)extender? I seem to recall that the version III extenders were geared towards the version II lenses that were introduced in the last few years (300 2.8II, 400 2.8II, 500 4.0II and the 6004.0 II).
Btw, the 400 5.6 combined with the 1.4 extender does a pretty good job of focusing on the 5dIII. Yes it's f8 wide open, but with good lighting its quite usable.