I'm normally shooting at f/11-f/16, to gain enough DoF. On the other hand, when I test a lens I am looking for weaknesses and trying to learn how to work around them.
I don't agree with your comments fully.
1) I also arrived at the conclusion that the 120/4 is best used stopped down to f/11 (or even f/16)
2) The Sonnar 150/4 I have is an older lens than the Macro Planar and it is perfectly sharp across the field at f/4, so I don't think that this is a generation issue
3) The simple explanation for the behaviour of the Macro Planar is that it is designed close up work. Field is nearly flat at close distance and curved at infinity. Check Zeiss MTF curves. Ugly at infinity, very nice at 5:1!
4) The curvature of field cause a gradual defocus when you move away from the focused point. What you focus on is sharp. I think I have a pretty nifty demo of that here:http://echophoto.dnsalias.net/ekr/index.php/photoarticles/74-zeiss-macro-planar-120-on-sony-alpha-99-with-an-arax-tilt-adapter
5) So if you shoot at close distance or repro, the lens would work perfectly fine at any aperture. If you can focus accurately on subject and use selective focus the lens would work fine at any aperture.
6) You can check the MTF data from Zeiss.
7) With selective focus bokeh also plays a role.
The tangential and sagittal curves are quite close which indicates that both astigmatism and lateral chromatic aberration is well corrected. The curves at infinity are very bad. The probable cause is that the focal plane is curved. But if you look at the second set of curves at close up range the curves are very good.
Regarding the Distagon 40, it seems that there are at least three generations. The original 40/4, the 40/4 CF (FLE) and the latest 40/4 FLE IF. The FLE adds a floating group which corrects for field curvature. The FLE control is implemented as an extra focusing ring. The FLE IF is a newer design with internal focus.
The DPReview article you refer to tests then Planar on a 24x36 sensor, but I am shooting on a P45+ which covers a much larger area causing the field curvature being a much more significant problem. When I shot my 120/4 on the Sony Alpha 99 I have seen less issue with field curvature and I could focus wherever I wanted, exactly, using LV manual focus.
To sum up, I use the Planar 120/4 macro extensively, but I need to stop down more than the other lenses I have.
I have shot a diffraction series on Sonnar 150/4 (see below), that lens performed best at f/5.6 and there was a significant drop of at f/16. But I still think that shooting at f/16 is OK and it is often needed for DOF.
Sorry for responding so long, but I guess that this discussion may be of some interest to potential buyers.
yes you need to stop down the 120 Macro, but at best aperture (f11) it is pretty good.
See here: http://www.dpreview.com/lensreviews/hartblei_120_4_pc_n10/4
The good part of being a bit soft wide open is: itīs perfect for portraits too.
These older lenses were never designed to be used wide open. Totally different philosophy than todays digital lenses.