I have on approval to buy a 50mm lens with viewfinder. Visible through the finder are irregular "marks," like blotchy shapes tinged slightly blue. These marks are visible in most lighting conditions, usually appearing towards the center of view. They vary as I move the finder, changing in shape and location. I described this to a pro camera rental and was told that the coating has worn in places from the finder's glass, and that what I'm seeing are "chromatic aberrations." True? I've used another external viewfinderfor the 43mm and haven't seen this before. Would appreciate hearing from anyone who has, or anyone with insight as to what these "marks" may be. Many thanks in advance.