Andy Rouse wrote:
"To be truthful, a better method here is to take the 2x converter, place it gently on the ground so that it is bathed in lovely soft, evening light. Then paint your teeth red and smash it violently to pieces with a large baseball bat, cackling maniacally all the time at passers by. Go on do it, it will feel so good. Now before you think I have an issue against Canon 2x converters I don't! I hate ALL 2x teleconverters equally!!! I consider them a complete waste of space and money, it is always better to use a 1.4x teleconverter and crop. Of course that is just my view!"
That last sentence is opinionated twaddle from the Ken Rockwell school of creative writing. The MTFs for the 2xTC III on the 300mm f/2.8 II are, as seen in the collage I posted, very good and the contrast etc excellent. None of the photos he has posted, as good as they are, show very fine detail as you would see on bird plumage at high resolution. If he took such photos and compared a rezzed up 1.4 with a 2x he would see the advantage of the 2x - my 1.4x TC spends most of its time in its pouch.