[Brightcolours]

Taken at f/1.8 - https://www.dpreview.com/sample-gallerie...0556364263

looks plenty sharp in the corners.

Eheh, the same photo I keep mentioning over and over but people seem to ignore it somehow ;-)

Maybe that is because the link you post does not actually work. It gives an XML file from the server.

Somehow you ignore that DPReview posts post processed images, and Lenstip does not? ;-)

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Clearly, if in the photo you mention, the focus had been on the corners, they would be sharp enough.
The results you observe are related to where it was focused.

I'm still waiting for your explanation as to how the borders here (link) can be sharp (especially the bottom right).
You seem to conveniently ignore the photo I mention.

The photo is focussed on the building, which is not in the corners. Subjects on the borders (left/right) are in the same distance ballpark as the building, and not as sharp as the MTFs promise.