For such a wide-angle lens, the 8-16mm provides impressive results for sharpness, especially where it's likely to be used most: wide-open (ƒ/4.5) at its widest angle (8mm). At this setting, there is a generous sweet spot of tack-sharp (1 blur unit) clarity in the center of the frame; in the corners, it's around 1.5 blur units. There isn't much point to stopping down - there is only a slightly marginal improvement at ƒ/5.6 - and after that there is slight degradation, though it isn't really noteworthy until about ƒ/11 where we note results just shy of 2 blur units across the frame. At ƒ/16 it's just over 2 blur units, and fully stopped-down at ƒ/22, it's around 3.5 blur units. Performance is similarly good at 14-16mm, though the maximum aperture gets even smaller at ƒ/29, and the results there are mediocre (around 5 blur uneven blur units).
There's a very slight drop in performance between 10-13mm - that is to say, it's excellent, but not quite as excellent as either the wide end (8-9mm) or the other wide end (14-16mm). Corner softness is just slightly higher at this middle setting, hitting 2 blur units wide open at ƒ/5. But the differences are very slight indeed.