The Iscomorphot 8/1.5x doesn't seem to have a really good taking lens.
I thought I'd take a stab at finding one.
I recently picked up a Nikkor-O 35mm f/2 lens that I heard worked wonders with the Baby Bolex on a GH2. I assumed that it would have similar vignetteing characteristics as they have the same screw size on the back. Turns out that serious vignetting kicks in below f/4 on a GH2 (probably won't set in on a GH3 until 5.6) but this combo has the least smearing I have seen with the Iscomorphot. This combo smears a hell of a lot less than when it's on the Helios 44-2.
Here's the comparison I did between the Nikkor-O and Helios with a Voigtlander Nokton 25/0.95
(all Voigtlander shots are the same picture, put next to the others for easier reference).
https://benwabbott.minus.com/lFF8XQjURfUTx
Tomorrow I'll do a better comparison by rolling through the apertures on each in video form.