Quote Originally Posted by thekingb View Post
BTW, I'm not fully following your conclusion that the 600II with a 2x has worse IQ than the 500II with a 2x. To my eyes the center actually looks better on the 600II with 2x; but the corners are much better on the 500II with a 2x.
I noticed this to. The corners and mid frame are very much better on the 500mm II. It appears the corners and mid frame will be unusable on the 600mm by my standards. It will probably be 10 years before we see a 500mm III, and if Canon keeps heading the direction it is going there will be only full frame high end cameras, the corners and mid frame will really matter.