Originally posted by Arjay Bee I had a couple of cameras do that. There is a part of the pentaprism housing that touches the prism silver and that is where the blemish comes from over time. Doesn't affect image but its a pain to view through.
VERY common in old Zenits and Prakticas, but I've seen it in almost every make at one time or another. In Zenits it's usually a faint line of brown down the middle of the prism image, two sides of the prism meet there and the protective coating over the silvering was often poor at the angle. But I've got an old Minolta SR with a very thick line of brown horizontally across the middle of the image - it's almost impossible to see past it, but fortunately I'm just using the body to check that lenses stop down etc., not as a working camera, so it's no big deal.