I have a problem with a 50mm 4 element El Nikkor lens. I recently noticed what I can only describe as a circle of frosting that has a clear area within it. When I hold the lens up to a light and examine it at maximum aperture through a loupe, the frosting appears to be on an internal element. I normally work at two stops down (F8) which uses the clearer area of the lens, but Im now wondering how long this has been there, and does anyone know what might have caused it. The lens still performs well, and is about 35yrs old.