I cancelled my ZM35/1.4 order in favor of the Voigtlander 35/1.7 Ultron (the new version). In respect to sharpness I find nothing lacking with the Ultron, even wide open. I briefly shot with the Distagon and it too is excellent right from wide open. The Ultron is maybe 95% that of the ZM, perhaps just slightly lower in contrast.
In respect to distortion caused by the lens, such as barrel or pincushion, I cannot see any in the Ultron. It's also a fair amount smaller than the Distagon.
Though the Ultron is f/1.7, IMO, it's effectively more an f/2 lens. At f/1.7 you gain a very, very slight exposure increase in the very center of the frame, but the outer 2/3 remains the same at both aperture settings.
If you're shooting digitally, is it with Leica M cameras, or with Sony or some other mirrorless system? Just a warning that the Ultron and Distagon perform best on digital Leica M and suffer edge smearing on Sony, unless the sensor is modified with a thinner cover glass (i.e. Kolari Vision modification).
FWIW, I have the ZM35 C-Biogon too. It's a very nice, very small lens. Great contrast, rendering and flare resistance. However, it's not as sharp in the outer 1/3 of the frame compared to the Ultron or Distagon at wider apertures. It's not terrible, just that the Ultron and Distagon are better. The ZM35/2 is a great lens for across-frame sharpness by f/4, with negligible distortion. But if you shoot wide open a lot, at f/2 it has fairly noticeable lower contrast with a somewhat glowy quality that clears up around f/2.8.
There's also the Voigtlander 35/1.2 II.... I'd classify it as a more medium contrast lens vs. the higher contrast of the Ultron and Distagon. It also has some barrel type distortion, though not terrible and fairly easy to fix in post. It's pretty good wide open, though I think the Distagon is better. Where I had the most trouble with it wide open was holding focus across the frame. It appears to have some field curvature, which doesn't seem to be as noticeable a problem with the Distagon or Ultron. I found it needed stopping down to f/4-5.6 for good across-frame sharpness in technically challenging scenes. At this point, one might as well use a smaller, less expensive lens instead, unless the wide open speed was a significant benefit. While it's not all that much bigger than the Distagon, it certainly feels a lot heavier, even if at 470g it's only 89g heavier than the Distagon. I think the size of the Distagon makes you expect it's going to be very heavy, but then you pick it up and it feels lighter than you thought. At least that was my initial reaction. That said, the black version of the Ultron (aluminum rather than the chrome version's brass construction) is only about 240g...
