Well, with some tinkering, I managed to significantly reduce the magnitude of the problem on the M2, but I can’t get it fixed:
While I momentarily forgot my laws of optics (The top left edge of the printed picture is the bottom right corner of the film in the camera…) with the help of Youxin Ye, I managed to zero in on the problem. This is done with the help of an LED light source through the camera (with the back open) repeatedly firing the shutter at 1/1000s, to determine if it seems to be opening fully. It is very nuanced –there is nothing more than the slight suggestion of a dark line along the right side while the rest of the frame is blinding you.
The phenomenon above is called “Capping”. Essentially, the shutter does not open correctly or completely at high speeds (or the second curtain overrides the first curtain at the beginning of the frame…) so that the exposure is incomplete or uneven. Tensioning the first curtain (turning the opening curtain take-up reel COUNTER-clockwise 3 or four clicks at a time…) should fix the problem;
Here is a diagram of the bottom of an M2 with the guard off:
In my case, I’ve tensioned the reel as much as I’m comfortable doing (about a full turn–you can break the spring) but the problem still persists intermittently to varying degrees.
As such, I really do think it needs a complete disassembly and lubrication.
(Fine, fine…I’ll send it in).