Please allow me to write my first post and to share my small experience:
In order to match a 7D and a Canon 70-200 f4 IS, I purchased following extenders in June 2012:
- Kenko Teleplus MC PRO 300 DGX 2.0x, then...
- Canon Canon Extender EF 2x III
The reason I sent the Kenko back doesn't show on samples:
I found I was unable to achieve reliable focus:
I made small tests on contrasty building fronts, 200m distance, center point AF, under cloudy skies.
In 50% of the cases, focus was achieved after 4 seconds.
In 25% of the cases, the 7D reported no focus.
I found it not acceptable for my use.
I never faced such issues with the Canon 1.4x II, I owned before.
I am aware that handling f4 and 2x might be beyond the 7D's capacity.
Sadly, sharpness and color with the Kenko seemed ok to me.
(I do have samples using the Kenko, but I now know that I didn't spend enough time AFMA'ing my 70-200)
I now use the Canon 2x III on a Canon 70-200 f2.8 IS II, and I am very pleased with the match.
The Canon 2x III doesn't show the ugly OOF-artifacts I observed on the 1.4x II.
I hope this is useful to some extent,
Looking forward to having feedback from other users.