The upper right corner of my Olympus M.Zuiko 12mm F/2.0 lens seems to be the weakest of the four corners.
The comparison between first two crops demonstrates that the lens has field curvature. When focusing in the center at F/2.0, the corner is pretty soft, but then when you focus using the focus point closest to the upper-right corner (the second crop), the corner gets quite a bit sharper.
Stopping down the lens to f/5.6 only results in a slight increase in corner sharpness. That’s as sharp as the corner gets. It never gets tack sharp.
Now one could say that normally there’s nothing important in the extreme corners of the photo, or that on a 12×18” print you wouldn’t notice the issue at all unless you examine the print with a magnifying glass.
But I think that when a lens sells for $799, and on top of that only has a modest (for a prime lens) f/2.0 aperture (which is only equivalent, approximately, to an f/4.0 full-frame lens), there’s an expectation of extreme image quality, and this lens falls short of being a $799 lens, especially when compared to the 12-40mm f/2.8 “PRO” zoom which is only two hundred dollars more expensive and has sharper corners.
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Read my previous 12mm F/2.0 post: Olympus 12mm f/2.0 lens, first impressions review