Here's a comparison among the FA35/2, DA40/2.8, FA43/1.9, and the CV 40/2 Ultron. Test conditions were not ideal, as this lensman plainly states. Just go to the photos and study them carefully: there are inconsistencies in these examples, so you'll need to put on your thinking cap in order to sensibly analyze what's here. Nevertheless, I think you can draw some fair conclusions.
This is the link: information.uni-bremen.re/foto-ag/old page/Ultron/test.htrr
The obvious plusses for the Ultron include the really fine performance at f.2.0 and near optimum real world performance for this design at f.2.8. Tests elsewhere are consistent with this result. A not so typically Pentax-like performance, then, and plainly useful. Also, the performance at the borders (right to the corners) is particularly notable. The color rendition is somewhat cooler than Pentax's; contrast appears snappier in the details.
Personally, I cannot at present relate to the effective equivalent focal length/FOV offered up by the FA43mm on APS-C (not having tried this); I feel 40mm on a crop sensor body is pushing it too close to no man's land for my requirements as it is. That's why I'm not yet trading off my Nikon version of this lens for the Pentax Ultron, if and where it might be available. I think 40mm is a wonderfully versatile FL for full frame, more to my liking than a 50mm in most situations. So I guess I'll just wait to see how things shake out. BTW, note the performance of the DA40 here vs. the FA43, when it's stopped down a bit. Again, given the shakey control of variables, FWIW. But, that's another reason I'm happy for now having both the Ultron in NAI-P and a DA40 XS. Both have their place, given our present options. Also see the strong close-up performance of the Ultron on the f-stoppers website.
Last edited by Kayaker-J; 04-24-2014 at 09:38 AM.