At 70mm and above we begin to note some issues with sharpness. In particular, corner softness is fairly high when used at ƒ/4 from 70-120mm; a small patch of central sharpness, surrounded by 3-4 blur units of corner softness. It's worst at 85mm, where the corner softness is in the range of 5-6 blur units. However, stopping down to by just one stop to ƒ/5.6 almost completely cures this problem, and we note the same excellent performance of the wide- and mid-range angles. Fully zoomed-in performance (120mm) is actually a bit better than that found at 70 and 85mm.
With the lens mounted on the full-frame D3x, corner issues are much more problematic; what was excellent on subframe becomes merely good on full, and what was mediocre becomes quite poor indeed.
Specifically, wide-angle and mid-range performance is still fairly good, but in this case optimal performance is no longer found at the lens' widest aperture settings, as corner softness is now a factor. Stopping down reins in the corner softness, and so the recommendation here for optimal sharpness is to stick to ƒ/5.6, where we see between 1-2 blur units across the frame.