Good testing, James.
2xTC takes its toll on every lens Canon makes. However, in my experience with f/2.8 lenses, particularly the MkII ones, they fare better than f/4 and f/5.6 lenses.
When using 2xTC mkIII on 400 f/2.8 IS MkII, I was very pleasantly surprised.
When using 2xTC MkIII on 300 f/2.8 IS MkI, the IQ was slightly better than expected, although AF responsiveness took a serious hit.
I also thought 70-200 f/2.8 IS MkII did OK with 2xTC MkIII, although not to the point of clearly rendering 100-400 obsolete.
Other slower lenses with 2xTC MkIII are unsatisfactory to me, and I think such combos are fit for a kennel.
Oct 04, 2012 at 04:11 PM
Previous versions of PetKal's message #11016452 « 400mm: 100-400L vs 70-200II + 2xIII »
