If you're a Leica M shooter looking for a budget wide angle lens, the Voigtlander Ultron is a great lens. However, there are now two versions widely available in the used market, the current M-mount Ultron f2 (below left) and the discontinued screw-mount Ultron f1.9 (below right).
The current Ultron f2 is slightly smaller, just a little lighter and narrower, has a focusing tab and a screw-on hood.
The Ultron f1.9 is screw mount but easily adapts to M mount, which could increase its versatility if you have an older rangefinder. It has a knurled focus ring and a larger press-on hood. (Cosmetically it looks suspiciously like the 1970s Summilux although it is bother taller and wider than its 50mm doppelganger.)
Various forums on the internet have complained about backfocus issues with the Ultron f2, but this lens practically lived on my M8 for more than a year, and I never had any backfocus issues whatsoever.
So is there really a difference between f2 and f1.9? What was improved that caused Voigtlander to discontinue the f1.9?
Test shot Ultron f1.9 at f1.9
Test shot Ultron f2 at f2
After this first test shot, I realized this wasn't going to be a very exciting shoot out. When viewed at 100 percent, the f1.9 photo might have slightly less depth of field but at the expense of some sharpness and color smearing along the edges of the letters on the bottles -- especially on the Kraken label.
Test shot Ultron f1.9 at f5.6
Test shot Ultron f2 at f5.6
At f5.6 the smearing of the word Kraken has disappeared in the Ultron f1.9 photo. However, the Ultron f2 photo is still crisper and the text on the soda can is noticeably sharper.
Test shot Ultron f1.9 at f8
Test shot Ultron f2 at f8
At f8 lens performance seems identical. I could not tell the difference between the two when viewing 100 percent crops.
So what about the out-of-focus areas? How is the bokeh?
Bokeh test Ultron f1.9
Bokeh test Ultron f2
Once again, not a very exciting test. The f1.9 has slightly softer circles wide open, but both lenses have perfectly circular bokeh at all apertures -- not that you're going to get much bokeh with a wide-angle lens.
In real world shooting, I don't think you'd see any difference between these two lenses. If I had to pick a winner, I'd go with the newer Ultron f2 for its superior technical sharpness, reduced flare and focus tab. However, with only the subtle differences, if I was in the market now, I think I'd go with whichever is cheaper. Either one is a winner on your M and comes in at a quarter of the price of a Summicron.