I made a recent trip into Manhattan with my son (we Brooklynites call it “going into the city”) and brought my Yashica-Mat. It had a bit of bright yellow spike tape on it that said “ORTHO 25,” so assuming that to be the case, I finished the roll in it metering for EI 25. The film had suffered, presumably from being in the camera for so long, and probably having been expired before that. Who knows? You can see spots on the film that look like black rain.
Martin Luther King, Jr. Memorial Sculpture at Martin Luther King, Jr. Educational Campus in New York City
But here on the very last photo on the roll, look at the center of the frame, near the top. A perfect circle of no “black rain.” What is that?
Detail of “the perfect circle.”
Recipe:
Yashica-Mat
Rollei Ortho 25 shot at 25
Developed in Rodinal 1:25 for 4 minutes – 1 minute agitation followed by 3 inversions per minute
Water stop – 1 minute
Fixed in TF-5 for 4 minutes
Rinse
LFN wetting agent
Hang to dry for two hours
Scan
David Shoots Film 
Humidity caused the dark spots. As to the mystery circle, I got nuttin’.
Maybe something that signifies the end of a larger roll? QA mistake?
Actually, this may have been my mistake, then. I was running a humidifier in the same room that the film was drying to prevent curling. Hmmm…
New York is pretty humid to begin with, especially right by the river. As far as the circle, the first thing that comes to mind is the circular red window on a lot of medium format cameras, but I don’t know if yours has one…
Joe – good idea on the red dot, but that Yashica-Mat doesn’t have one. I had left the film in the camera for far too long. Just as I had neglected this blog for far too long!