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Lose contact in a snowstorm for a few days and this is what gets discussed? ;-)
Someone is complaining about soft corners with a 50mm lens? Don't ALL 50mm lenses still have soft corners at f4.0? Yes, the 50MP is no different, performance drops off into the extreme corner -- but right up to that corner is is outstanding. I have both the 50MP and the 35-70 VS and at 50mm the 50MP is miles ahead of the 35-70 at f4.0 -- which also has weak corners, not to mentioned weak edges). The 50/1.4 P isn't much different in this regard, either. If you don't want weak corners, don't use a lens with a focal length less than 100mm of shoot at apertures larger than f5.6. It is a very rare and exceptional 50mm that can dismiss those rules. The ZF/ZE 35/2, which I also have, also falls off in the corners at large apertures, but is a little better than the 50mm's in this regard. I don't however see it as an alternative to the 50MP, because of the focal length difference. You should have BOTH lenses. of course.
No one should be surprised that a short focal length lens will falloff into the corners at large apertures. Further, 100mm macros are almost always better than 50 or 60mm macros in the corners (it was the same with the Contax c/y makros).
If your test is showing you that the 35-70 is better than the 50MP at f4.0, either there is something wrong with your test, or you need your eyes checked. The 50MP gives you SO much more right up to that corner than the 35-70. Looking at your corner crops, it seems pretty clear to me, however, that the 50MP is superior -- there is more detail (look at the bottom third) and better contrast overall. It really doesn't seem unexpectedly bad or unacceptable.
Unless you need the extra stop of speed at f1.4, the 50MP can replace any 50mm lens you might have and you'll never regret it. It is exceptional at infinity as well as close up.
If all you are concerned about is a sharpness into the extreme last millimeter of two of the corner, than you should look at lenses with medium format size image circles and only use the center half of it (like you do with the 24 TSE II). Of course, these won't be as good in the center or even over 95% of the image area. There are always trade-offs.