Quote Originally Posted by Peltigera View Post
Surely a light leak would cause a white band? Is it possible that there is some frayed material or a lump of fluff stuck to the leaning or trailing edge of one of the blinds?
I had exactly this problem with my M4-P. Although the effect on the negative was a little different which may be because I've darkened the attached image to make the problem more pronounced. It only showed up with faster shutter speeds where the gap between the curtains was narrow and being almost spanned by the fluff.

Holding the camera at an angle with the back removed and the shutter held open on B, I could just about see some tiny strands of loose felt attached to the metal strip (cap?) on the edge of the curtain towards the bottom of the camera. A combination of tweezers and a bit of huffing and puffing removed them.

Hope this helps.