I used to think it was in the nature of leaf shutters to expose the center more than the corners... Considering the time it takes for the blades to move open and then move closed, the center of the shutter is open longer than the periphery. And if so the effect should be strongest at the fastest shutter speeds because there the proportional difference in time is greatest (the blades are then always moving, not held open).
If this were true, then a dragging sluggish shutter, or one that sticks open slightly, would have the same effect, of exposing the center of the image more.
But I've come to think my idea above is not correct, optically. The counter argument points out that the aperture blades are adjacent to the leaf shutter, or in some cases are one and the same. But we don't see f/22 imaging just a small circle of light in the center of the film frame, do we. It doesn't expose the center more than the corners... Indeed, if anthing, the effect is just the opposite, as lenses usually "cover" a larger area of illumination stopped down than wide open.
If this present situation were a matter of additonal exposure in the center, then the extra light must have come in through the lens, and is coherent, focused, and imaging the scene... or alternatively it could be flare. But these samples appear to have light mainly from one long side, with a rather distinct edge near that side, and fanning out across the frame from there... as from a back-door light leak. And light leaks for some reason seem to be reddish in color. I'm not saying that's absolutely it, just that seems to fit the evidence. 🙂
