Don't laugh, please! I have just loaded a fresh roll of 220 film into my Contax 645. I just realized that I have forgotten to change the plate from 120 size film to 220 size film? The camera has already loaded the film and ready to shoot. What can I do?
find a darkroom. (bathroom may work if you can seal the door cracks)
unload the film and wrap the leader tightly and tape the spool.
shield the roll if possible just in case it is not completely tight.
now flick on the lights and turn the plate and reload.
If you have a changing bag/darkroom I would just open the camera, unwind the take-up spool enough to change the plate setting, then rewind the film until taught. Once I had the camera closed again, I would advance the film a couple or three frames before shooting anything of any real value. Could probably do this in any dark environment and not loose too many shots.
I'm not familiar with the Contax 645. Perhaps it's possible to unload the camera in complete darkness and correct the situation. It certainly would be with many MF cameras.
Just shooting the film with the wrong pressure plate in place probably wouldn't result in problems with images. If the pressure place controls the exposures available, however, you might find shooting after frame 15 a problem.
If your camera has powered film advance, wind the film back out by hand. If you open the back and close it again, it will assume you just loaded a roll and advance it to position 1.
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I know with my Mamiya 645AF, I can't just "rewind" the film, even if I take out the magazine, the spools will only turn one way..than you've got the paper to deal with.