You can make considerable correction to sharpness with DPP or Photoshop. It wont be the same as without the extender but it can help. You also need to make sure that it is focusing properly with the extender on. There are some variations between cameras lenses and even the extenders themselves and the error can compound if all three are just off by a small amount. Try manual focus using live view with 10X magnification and see how it compares with using just the center AF point in normal AF mode.
Also, the sweep spot for that combo will be around F8 for the best possible image quality.
quote author=Bruno97 link=topic=14164.msg272906#msg272906 date=1369403989]
I recently bought a 70-200 2.8 L II and then sold my 70-300 L. Tooking advantage of this sell and buying a 2x III extender was a big wait for me.
I recently had the opportunity to give the 2x III a try on my 70-200 2.8 II during a Canon show... and I was damn so disappointed !
I compared many shots @200mm f/2.8 vs @100mm+extender f/5.6 and I found the difference was more than noticeable in terms of sharpness in favor of the shots made @200mm f/2.8.
The Canon salesman couldn't agree more... he admitted the 1.4x & 2x extenders were not made to be combined with any 70-200 lens ! Their purpose is to be mounted with the long prime lens such over 300mm. Especially the newer versions III.
Honestly, I'm getting lost when I see some people posting sharp pictures with the 2x III - 70-200 2.8 L II combo.
Please don't tell me Canon gave me a bad copy of their extender...