I happen to own a Macro Planar 120/4. This lens has about the worst MTF curves at infinity anyone has seen.
I don't know if this is proper usage, but the curves are here:
Now, keep in mind, this is a macro lens, corrected for short distances. The MTF for the macro range is here:
What I see is about the worst MTF I have ever seen at infinity, but pretty good at close up range.My guess is that the problem is field curvature at infinity. The curves are close and bend uniformly. I guess that if you can focus on a given point, it will be pretty sharp.
The odd thing is that it seems that everyone loves this lens. So according to MTF data from vendor it is good for close up but pretty bad for normal woking distances.
I have the Sonnar 150/4, and that lens is excellent both according to MTF and my experience. The Planar 80/2.8 is, well OK, about as the MTF curves. My Distagon 50/4 FLE is also consistent with MTF data. The Macro Planar is nice, but sharpness is nothing to write home about. I would say, it needs to be stopped down to f/11 or even f/16 for corners. Very much consistent with MTF data!
The MTF data were taken from this publication: http://www.hasselbladhistorical.eu/pdf/lds/CFi120.pdf
Now, I often shoot MF at f/11 - f/16 anyway, to gain DoF and also to compensate for my weak focusing technique (which I am working on), and that reduces the problem. On the other hand, small apertures cause problems, as seen here in my diffraction series (on Sonnar 150/4):
My take on the issue? Zeiss intended the Macro Planar as a macro lens. It is not intended for landscape work. If pressed into landscape work, it my perform decently at small apertures but cannot match the other lenses intended for longer working range.
Macro Planar on P45+ corner at f/8:
Sonnar 150/4 on P45+ corner at f/4 (a very good lens):