Originally posted by 5r82 This photo looks fantastic for such a "vintage" glass. I was looking for some nice glass like that one but couldn't find it at good prices. So, I was unexpectedly gifted Carl Zeiss Jena Flektogon MC 20mm f2.8. Unfortunately despite what its online price may suggest regarding its quality it can't match at all Pentax SMC coatings. It's also full of aberrations even stopped down. I was shooting mostly Pentax old lenses and now realize how Pentax excelled in quality in the days of thr past.
I believe there is something wrong with your Flektogon.
A certain number of late MC versions in black mount left the factory misaligned. The elements are tiny, correctly assembling a lens like that takes some time, and skilled workmanship. A quick check should follow. Those lenses were assembled manually, with no automation. Operator errors became quite common, approaching the fall of the Berlin wall.
In theory late MC versions should be better. In practice, most old lenses are impeccable, and give everything their optical design allows, while newer ones had a relevant number of defected lenses that should have been checked and rejected/reassembled.
On top of that, vintage lenses, even examples that look impeccable, could have been subject to vibrations and/or shocks that affected the correct positioning of the various groups... so in reality when you buy one you never know what kind of performance to expect.
I suggest to have the lens checked. It should perform AT LEAST at the level of the Pentax-M.
I had both, and now I have the Pentax-A 2.8/20mm. I think a good Flektogon would be very close to the A in performance (leaving alone ergonomics).