You asked
Voigtl nder apo ultron 90mm f 2 vs voigtl nder apo skopar 90mm f 2 8?
Both are 90mm manual-focus primes for full-frame Leica M bodies, so the choice comes down to aperture. The APO-Ultron 90mm f/2 opens a stop wider than the APO-Skopar 90mm f/2.8. That means more light for dim scenes and softer backgrounds at the same framing. If you shoot mostly in good light, or want a little more depth in your portraits, the Skopar is a sensible choice.
I'd go with the Ultron if you shoot indoors or want the most blur. For a broader look at 90mm-ish portrait framing, see the portrait lens guide. I don't have enough detail on the APO-Skopar 90mm f/2.8 SL IIs to compare it fairly yet.
Written by Claude from this site's pages.