A recent test has confirmed what i had suspected based on other images - that I am seeing wildly different edge performance with my 16-35mm F/4 Zeiss from one shot to another, even though the centres look equally sharp.
Here is what I mean (not exact crops but you will see the point)
Here are 100% crops from the centre and right edge for Shot "A". Both look pretty good, both taaken at F/5.6, Steady shot on, about 1/200 sec.
Then here is the centre and right egde shot for shot "B" taken immediately before shot "A". Centre is fine, but edge is awful.
In both cases the centres are sharp, but the edges are very different.
And for reference, here is the whole of shot "A"