If you missed the first part, you can see more here: www.mattparvin.com/adapted-lenses/firin-20mm
I had some time to kill yesterday so spent a while longer with the Firin kicking around town and thought I'd share a few more of my findings. First, let's talk flare. I noticed it when trying to shoot this door way. I has a pretty narrow angle that affects it, the hood or a blank filter or two may well fix it, but it's there. I might even like it for this image, but maybe not for all. Lens was probably hitting the front element at about a 65 degree angle or so. Next, shooting into the sun, this is no Loxia. It's not terrible, and it's fixable with a Dehaze adjustment brush, but had to be careful playing with sunstars. Too much light and it fell apart. Shots below are an average with the Firin and a Sony 16-35 for comparison (at f/14, Firin was at f/16). More direct testing is needed, but happened to take that shot earlier in the day for a client so thought I'd include for comparison's sake.
Since we're on the topic, let's look at Sunstars. Once again it's not a Loxia, but I personally like it. This is better performance with a bright sun in a clear sky than I've been able to get in the past. At f/16 it's nice enough for me, but I'm not a junky like some of you are. The lens does lose some contrast shooting into the sun, but in general it's within editing tolerances. I can live with it for the price difference, others may not.
Alright, one last thought... Distortion. I thought I saw it in some landscapes so went to the old reliable brick wall. Sure enough, the lens doesn't have significant amounts of distortion, but it is a complex wavy distortion. As there is no lens profile yet, it remains to be seen how well it will correct. If they get the pattern right it's small enough that it should correct with very minimal loss of resolution. I probably wouldn't really hesitate to use it for architecture, but it could limit 1 Point Perspective compositions if you have lines near the edge of the frame. I'll withhold final judgement until there's an Adobe profile.
I'll leave you with a few random shots around Southport. The lens is a fun walk about. With a wide aperture it handled well in dark shops and outside on a nice spring day. None of the above dampened my enthusiasm for the lens, like any its best just to know how it performs and work with it.