No idea on the mk3s, but think its mostly corner sharpness improvements. I've got a mk2 2x if you want to try it out?
Failing that could hire onehttp://www.lensesforhire.co.uk/canon-extenders-11-c.asp
Drew
Gaz J said:Was just wondering Drew. Quite happy to buy a MkIII x 2 as I don't have one. Need it for those pesky little owls on skomer this year!!
Admirable. Thanks for the links.
You can stack the 1.4X or 2X III behind the 2X II. You cannot stack either the 1.4X II or the 2X II behind the 2X III. The opening in the back of the Mk III is just a little smaller in diameter and not quite as deep as the Mk II.
If you have a third-party teleconverter that does not have a protruding element in the front, you can stack it behind almost anything including the Mk II and many other lenses that normally don't accept a teleconverter.