I received this from HB in New Jersey today. It looks like Photomultiplier was on the right track. It’s definitely the best explanation I’ve heard. Since it happens with an SWC and a 501, decaying black palpus isn’t the issue. I’m going to use a lens shade religiously from now on.
“A technician in Sweden had a similar response. He said it was light admission into the film back. Are you using a lens shade or shooting towards a light source when it happens? It’s possible the black palpus in the cameras are cracking and deteriorating. If that’s the case , it won’t absorb light. This can’t be repaired but we can still check the cameras to see if that is the case. Also, we can evaluate the film back to see if it’s because of an issue with it.”
“A technician in Sweden had a similar response. He said it was light admission into the film back. Are you using a lens shade or shooting towards a light source when it happens? It’s possible the black palpus in the cameras are cracking and deteriorating. If that’s the case , it won’t absorb light. This can’t be repaired but we can still check the cameras to see if that is the case. Also, we can evaluate the film back to see if it’s because of an issue with it.”