Beseler 23c focus slipping
Hi. I've noticed a focus problem with my 23c II. I've done a search and I found one thread discussing it the problem and my situation is likely similar.
This is for 35mm. I start by focusing at f/2.8. I stop down to f/8 and check the focus. The grain is sharp. After a minute or two if I recheck the focus at f/8, it's slightly out of focus. My 23c II has the heat absorbing glass in it.
The possible causes I think are
1) heat reaching the negative
2) the mechanics are slipping causing this problem
Any other ideas? If it is a mechanical problem, is there a way to fix this on my own?