Ok, this is going to be a long post, after which you should know what kind of tea I usually drink
. I re-tested both lenses at 70mm. I attached the focus target - a tea bag - with some tape so that it would remain upright during the test. Also, to confirm correct focus, I placed a focus test chart directly underneath the tea bag.
First up was the Pentax. I took the following three shots at f/4, 5.6 and 8 respectively:
Narrowing the aperture clearly improves things. Next up was the Tamron at f/2.8, f/4 and f/5.6 respectively:
A clear overall improvement over the Pentax. The grey pattern is already visible at f/2.8, with f/4 and f/5.6 adding additional sharpness.
At this point, I noticed the focus test chart in the Pentax pictures. It was off, indicating backfocus. So I did as follows: starting out from f/8, I retook all the Pentax's shots. But this time, I moved the camera slightly forward, focused, then pulled it back for about 4cm before releasing the shutter. I think the results - from f/8 to f/4 respectively - speak for themselves:
The softness in the f/4 photo probably is due to the narrow DOF and me over-compensating for the focus offset. When that result is disregarded, the DA 17-70 also shows the grey pattern, both at f/5.6 and f/8. So this lens is capable of very sharp results, at least where center sharpness is concerned.