I was not entirely convinced that the Canon 200mm/2.8L II works well enough with a 2x extender to be better than using the same lens with the 1.4x and then simply cropping. So I had a quick shoot-out between the 200mm+2x and a 400mm/5.6L at our local Canon dealer, handheld inside a shopping mall. The 400mm was visibly sharper. So I bought one. I then thought I'd better test them a bit more diligently; tripod, autofocus, mirror lock, etc. Cropped results with both at an effective 400mm focal length and f5.6 (i.e. both opened wide) are shown above. Inset shows the full 1Ds frame. To see the crops at 1:1 click "original" below right. The results of the quickie test at the shop confirmed? Rather obvious, ain't it? Well, look again, now at the mountains in the background. Sharper with the extender! I have never noticed this back-focusing on the 200mm lens previously and I almost always use it wide open. Now I'll have to retest using manual focusing and then perhaps have the lens/extender/camera bodies recalibrated by Canon :-( They'll probably say that the lens focuses perfectly well without an extender! Follow-up on next slide...