[mst]

If I need sharp borders. I'll stop dwon anyway, but I really want to know a single case or subjectwhen you need high res corners wide open? In my book there's none of this subjects.Not even in theory.

"Theory?" - that's my keyword, right?

Seriously: if you compose your images following the rule of thirds or golden rule, then your main subject is close to or (if you stretch the rule a little) almost at where we measure border performance. In any case closer to border than to the center.

Edit: Oh and by the very way: The Nikon got 4 stars at it's time. At 14 mm and f/2.8, it is in no respect better than the Sigma - not on 20 and especially not on 45 MP!

Besides the fact that we keep preaching "thou shalt not compare results across systems": are we looking at the same reviews? The Nikkor has excellent borders at f/2.8 already (on the D3x... on the D850, we will hopefully see in not too distant future).

It got "only" 4 stars because of the flare issue, otherwise the rating would have been a little higher.