When my Jupiter 8 for Leica arrived from Ukraine and I shot my first roll with it, I was shocked by the horrid quality of the images it formed. Close examination of the lens showed that it had a rattle -- the rear retaining ring and rear element were loose. I tightened up the retaining ring; and the lens now performs quite well.
I'd be interested in hearing reactions from Sonnar and Jupiter-8 users to the resolution test, a series of shots of the brick buildings facing mine 20m away at f/2, 2.8, 4, and 5.6, with sun slanting obliquely across them.
What I find most curious is that in the center, f/2 is sharpest, slightly falling off as the lens is stopped down; while corner sharpness, already very good wide open, steadily improves as the lens is stopped down.
The performance of this Jupiter-8 is very similar in resolution and contrast to my 1950s collapsible Summicron