My Sonnar 90mm head has M42 base, so it is easier to use on a7.
(M42->E helicoid adapter at the lens' infinity position in photo)
This helicoid extends to 58mm (from 27mm), so MFD is 0.62m.
When you use 36-90mm M42 helicoid, the MFD becomes 0.40m.
At this MFD, the magnification is a little bigger than half size.
In this case you may get a white/hot spot when you close down.
I don't know the reason, but guess the internal reflection from
the helicoid black tube which is very shiny even they are black.
(Recently I used this lens with Nikon bellows for very close shots
there was no white spot appeared for high-mag and large F-stops.)
See > http://www.pbase.com/kkawakami/image/162209049
I also tested with Minolta Portable bellows, the images do not have
white spot. But this bellows is a bit too thick to focus to infinity. :(