Just got the Biogon-C.
It is good companion: Summicron 35mm Version ! and the 40mm M-Rokkor.
The sharpest (honest) is the Cron, the Rokkor comes second and Biogon third, my impression so far. It is in fact the combination of resolution and micro-detail and contrast.
However, looking at highest resolution of details is different from rendering objects. The Biogon wins in the field of high contrast (of course) and trees remain relatively dark. [My Rokkor is getting some hze. I often have it in the pocket of my trouser. With all the salt sweat that is a kind of sea-coast atmosphere]
The Biogon might win in color stauration; The other two also have a great saturation and balance, more than good enough on the M240.
However, the Biogon-C has purple edges on high contrast picture material - like many modern lenses. And that I like less. It is something I do not like (and why I did not buy the Heliar 75). Old lenses have a much better control of CA. I just looked at a picture of the ZM 28mm on the M8, trees were dark but the leaves all spoilt from CA-bloom. I hope the Biogon behaves better. Some say the sensor needs to be better than the M8, smaller pixels . . to not see the fringing, but I have no understanding of that.
And I detect a slight backfocus on the lens at about 3 meters, not at 1 meter and not at infinity. I'll have to look into that.
And finally, the sharpness is retained up to 90% in the corner. The Cron is softer away from the center.
The separation of subjects wide open: the Biogon-C did not yet push me off from my chair. [Other vintage 35mm lenses have ...] In that, for now it is just one of the pack.
I have not looked at the Ultron lenses.