The thing that makes this lens special is the 14 aperture blades. Way more than anything I've ever seen. That and how the aperture works as well. First, you pre-set the f-stop you want to stop down to at your minimum, then you rotate the aperture open/close ring to where you want to open up to. This is useful for stills when you are focusing, then stopping down to open the shutter. No guessing where you're stopping, because that's set using the f-stop limiter ring. It's useful for video or cine use because you can set where you want to stop down to, and then as you pan and stop down, you'll use the smooth open/close aperture ring. It's the ring in photos has two tabs to hold with.
This lens is sharp, but not as sharp as my Pentax SMC 50 1.4. It is, however, very "poppy" like you expect from a Zeiss or other high quality German lens. This copy is excellent. There is not a flaw in the glass whatsoever. There are a few microdusts inside, but only a few and not nearly as bad as other lenses I've used with no effect. They have no effect on image quality. (see this for more about dust inside lenses: http://www.lensrentals.com/blog/2015/06/i-dont-know-why-it-swallowed-a-fly-weather-sealed-lens-with-a-fly-inside) Mechanically this lens is very smooth. There is very light oiliness on the iris edges of the aperture blades but actually I think this was how they came new. They're not "oily" like I have seen and used in other lenses.
I decided that I am going through my 50 mm's and might be willing to part with this lens for the right trade. I've listed it on eBay as a "Buy it Now" for the moment (at $342) and whatever happens first I'll do. I thought I would offer it here because it's an interesting lens and a rare gem that I am hesitating on parting with but I am.
I shot the sample photo just last week.
I want something interesting in exchange. Manual focus only and not interested in junk collections. One item for one item is best. I like the Pentax 20 2.8 in particular, or a Nikon MF 20 maybe. I'm not interested in Canon lenses (focus wrong direction and f-stop backwards