I bought one of these second hand some time ago and I recently noticed some kind of defect on the inside or the rear element. I have no idea what it is but it looks like I might be able to clean it off if I removed the rear element.
How do I do this? It appears to be mounted in a ring with a pair or notches on opposite sides. I tried using a wide, this piece of metal as a screwdriver to try and unscrew this ring but it wouldn't budge.
