ATMX observed this his high-res photo taken with the E-M5 II has soft corners (although exactly how soft is not specified).
There is no lens I have used that has corners as perfectly good as in the center of the photo, but if there is one lens I’ve used that comes close to perfection, it’s the lens on the Ricoh GR. Of course the Ricoh GR is a rather limited camera. You have only a single focal length (equivalent to 28mm), there’s no viewfinder, there are “only” 16 megapixels, and I also find it kind of hard get the perfect exposure (although you can use exposure bracketing).
As far as I can tell, the sharpest Micro Four Thirds lens for center-to-corner image quality is, believe it or not, the Olympus 12-40mm f/2.8. I say believe it or not because most people would assume prime lenses would outperform a zoom lens, but that’s not the case here. The 12-40 is definitely sharper than the 12mm f/2.0 and the 14mm f/2.5, it’s a close call with the 17mm. I have not carefully compared the corner quality with other prime lenses that I have (25mm and 45mm), so it’s possible that one of those lenses might outperform the 12-40mm.
Even with the sharp 12-40mm, I can tell that the corners don’t quite have the same quality as the center, and I am sure that would be even more obvious if I used the high-res shot mode on the Pen-F which creates an 80MP raw file, although I have not experimented with the high-res shot mode.
It would certainly be very interesting to compare the 12-40mm with images from various full-frame systems like Canon, Nikon, Sony and even Leica, but I don’t have any of those other systems.
Even though Henri Cartier-Bresson said that sharpness is a bourgeois concept, if you are going to be doing bourgeois stuff like making really large landscape prints, then you need the sharpest lens if you want your really large bourgeois prints to be “superior” to other people’s really large bourgeois prints.
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When comparing lenses, it should always be noted that there’s a lot of sample variation, and my 12-40mm might be better or worse than someone else’s 12-40mm. I think that the 14mm f/2.5 and the 17mm f/1.8 are especially prone to sample variation and that I have a good copy of the 17mm and a bad copy of the 14mm. My opinion of my 14mm is that the quality is unacceptable for landscape photos.