See reply 23
It's an extreme sample, but sort like stories have been appearing on several sites.
I tested a 1DsIII a while ago and also found that f11 was noticable softer that f8 and this was with a 70-200 f2.8 L IS.
This behavior is no different than the 1Ds2.
It varies by lens:
I have used the 16-35 Mark I and the 16-35 Mark II, both on the 1DsM3. At 16mm and at 24mm, the 16-35 Mark I is noticeably sharper at F8 than at F11. At 16mm and 24mm, the 16-35 Mark II is a touch sharper at F11 than at F8.
The 50/1.2 is very sharp at f/11.
The 24/tse lens is sharper at f/11 than f/5.6 but it's very soft at f/22. Similarly, the 14/2.8 II lens is sharper at f11 than f/5.6 or f/22. And same goes for the 24/1.4 lens.
Also, much depends upon what you are after. I have some architecture shots made last year, on the 1Ds2 and the 90/tse lens. In one series, f/5.6 is sharper at the focus point than f/16 but f/16 still has better depth of field.