grain elevator
Member
Where did you assume the film plane to be? Many folders and some of the simpler TLRs have a rather wide film channel and it's up for debate (or rather testing) where the film rests. My guess is that right after winding, it should rest back against the pressure plate due to curl. But after a while, or through suction when opening a folder, the film could move forward. Correct infinity focus very much depends on this issue.Maybe there were lots of bad repairmen in the past. My statement is based on some observations I took with a recently acquired autocollimator from a forum member. After reading this thread I have been checking my cameras for focus accuracy at infinity, using the reflection of a collimated target off real loaded film (following these instructions: https://learncamerarepair.com/downloads/pdf/NatCam-Auto-Collimators.pdf) and have been blown away by just how bad most of them are. They were so bad that it made me wonder if the autocollimator's calibration was off, but I tested it with a mirror and by testing the infinity focus of a lens in my DSLR (with a known good infinity, tested by taking a picture of a building a mile away right before the test), and that combination tested near perfectly. In comparison, my TLRs and folding cameras have been stupendously bad, returning focus error at infinity of an actual focal point around 30-50m. Why haven't I noticed this before? Well, infinity is well within the hyperfocal range at 50meters/f11 and I am guilty of racking the focus all the way to infinity rather than thinking about hyperfocal range most of the time. One of the more interesting tests was that several of the lenses I sing the most praise of on this forum (Hasselblad 60mm, Mamiya TLR 80mm) "just so happen" to have the best infinity calibration, and the lenses I have often said I "got a bad copy of" (Mamiya TLR 65mm, Perkeo II color skopar) are the most out of calibration...