there is noticeably less CA with the 1.4 Mk III which makes a difference for some of the big prints I make and in the harsh lighting that I often encounter here in Florida.
I think that's what the TDP review said. Is it possible to correct some of the CA in post? I wouldn't make big prints, but the lighting here in Houston is pretty harsh.
Thanks for the reply and I'm sure the lighting is every bit as harsh there. The CA with the Mk II isn't bad at all and yes, it can be corrected in post, but sometimes it's not as easy as it would appear. This is especially true with osprey wings against a gray sky and things like that. I do a fair bit of macro with my 180L and that is actually where I've seen the biggest difference between the 1.4x II and III.
Also, the other thing that I've noticed (and meant to mention) is that the flare resistance with the Mk III seems better. In bright light, it seems to hold contrast better than the Mk II. This isn't something most reviewers would test, but it's what I've seen.
They are subtle differences and probably not worth the extra money considering how much extra money it is. The only thing that is probably worth the money are the extra screws and better construction. With a $7k camera on one side and a $7k lens on the other, more screws holding it all together is definitely better!
For the 1.4x, unless you have a Mk II supertele (and money to burn), it's not worth the huge difference in cost. The 2x extenders are much different, however and worth it, even with the 70-200s, 135, 180, 100-400 and others.