[mst]

Strange in which way?

The borders take a deep dive wide open, center and corners are much better.

Yes, the lens needs to be stopped down to achieve best resolution, but on the two units I used for review center and corner were very good wide open (and borders just fair). For portraiture, that's of course slightly unfortunate behaviour, because you may want to place your subject's eyes where the lens shows the weakest performance. But then, on the other hand, stopping down a bit is common with these kind of portraits anyway (unless you want a blurry nose with the sharp eyes).