True. I was thinking it might help narrow the issue down to the shutter and eliminate light leaks at the back as the cause. But I guess you'd have to set up a blind so that the lens sees a dark room but you have a very bright light shining onto the back. It does look like a shutter issue.
Another test might be to swaddle the camera camera in a black cloth with the lens poking out, make a few shots outside.
Another test would be to load film in the dark at night, go out and make some long exposures (like one minute), but cover the lens with a lens cap just before the shot and just after. That way, any light coming through the lens and a malfunctioning shutter would be minimized in relation to the much stronger exposure on the film from the exposure proper.
How does the shutter behave when you operate the camera with the back open? Shutter bounce? Sticky curtain? Curtains shift as the film advances?
Had this problem on a nikon em. The light leak came in from door hinge area. Foam was breaking down. A thick piece of black tape on the hinge and a test roll proved it. Refoamed and it's working fine again.
This lines change place because as exposed film is rolled to the right, a different section is exposed based on the changing diameter or the spooling.
If you take pictures at night, or consecutive images with your hand on the side hinge these leaks won't be there. The second you let you camera down into the light like by hanging around the neck, the previous frame will have that streak. Check your previous shots
A shutter fault ought to be in roughly the same place each frame. What about a faulty light seal on the cassette letting light in during loading/unloading? That would explain the fogging being in different places. Are you using a reloadable or shop-bought cassette? Are the less affected frames earlier in the roll?
If this is the case, the longer you wait before advancing the film, the denser the light leak will be on the frame. Using that premise, you can test this on a tripod. Do some shots where you advance immediately. Do others where you wait different periods of time before advancing. Do others where you cover the lens immediately after taking a shot, and then advance the film with the lens covered. But don't change the position of the camera between shots. The amount of light coming in will not be constant shot to shot if you are moving the camera. Make sure to keep track of what you did on each frame.
The light seal at the door hinge gives the same effect described above.
The easiest test would be to tape the hinge with an opaque tape.
Just refoam the entire camera.
I had a similar problem but the opposite.
In my case the shutter curtains were out of sync...one traveled faster and one too slow than spec.
I believe the issue I had is known as shutter fade. What happens is the first curtain catches up to the second curtain that is traveling too slow so the one part of the neg is underexposed.
The first curtain travels across first, the second curtain follows. what you are describing is the first curtain slowing.
This usually shows in an uneven exposure from side to side, not a vertical band.
It is sometimes just a distinct white line and sometimes a wide band. It also varies it's location across The frame. Upon further inspection of both bodies, the foam is completely degraded along the hinge of the back. I will try tape - run a test roll - and if it works, will re foam both bodies. Is this a DIY job?
Yes, it's DIY. John Goodman likely has specific kits for your cameras and they come with instructions even I can handle.
I use bambo skewers for the grooves. Couple of drops of alcojhol or lighter fluid as a solvent works very well too.
Replacing seals is a little time consuming, if you sit down and get to it maybe 1/2 hour per camera. I tend to start something, wander off and eventually remember what I was doing in the first place. It may take me days.
You will need to be very careful in the mirror box, you don't want to get degraded foam on the focusing screen. It's very sticky and a pita.