Need a BIG favor!! Dual D800 and 5DmkIII owners chime in!

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I haven't played with a D800 myself (have a 5D and 5DIII), but shooting at f/11 I think it's not surprising that you don't see much difference in sharpness. On the 5DII diffraction starts being visible at f/10, and on the D800E before f/8.

To see the full difference (which is not huge anyway), you may need to shoot at f/5.6 or even f/4, and make sure you're using a lens that's sharp corner-to-corner at that aperture and with enough resolution for that sensor - there are not many of those, unfortunately.