I recently bought a 'new', old Biometar 80 lens for my Pentacon 6 camera and did some preliminary tests with an adapter on my Nikon DSLR. Everything looked great, so I went on holiday with the new lens and the Pentacon, shot quite a few rolls and was incredibly disappointed as I came home, developed the films and it turned out that most, if not all shots were out of focus.
After having made several shots of a focus test chart, it is obvious that the actual focus point lies in front of whatever I am trying to focus on, as can be seen here (I have focused on the 0 point of the scale):
My first thought was that I for some reason had gotten a problem with the Pentacon's focusing screen, but then I realized that the problem was only apparent in the pictures I had shot with the new Biometar. If I shoot the focus test chart with a different lens, here a Sonnar 180, the focus point is spot on:
I also have an older version of the Biometar, with which I don't have any problems to focus:
As I initially wrote, I had done some tests with the new lens on a Nikon DSLR without noticing the focusing issue, so I took some more test shots, now with the Biometar on a DSLR, and here there are also no issues with the focus:
So my findings are:
If I put the new Biometar on any other camera, I have no issues with the focus, the lens is just as sharp as expected and there are no other obvious problems.
If I put any other lens on the Pentacon 6, I can focus just as expected with no other problems.
If I put the new Biometar on the Pentacon 6, I have a consistent front-focus issue. On the captured picture, the actual focus point lies in front of whatever I see sharp on the focusing screen.
Focus shift is not the cause of the problem. I tested all combinations of camera and lens over a wide range of apertures and the focus plane stays at the same point. The first picture (showing the front focus issue) was also taken with the lens wide open, just as it was set when I focused.
Has anyone of you a suggestion on what may cause this problem?