The other day a friend of mine gave me a black Olympus OM-1 with Zuiko 50mm 1.8. Checking it out today I noticed that the lens does not stop down either when hitting the dof preview button, or when pressing the lever on the rear of the lens. Sometimes it will stop down but slip back open while I am holding the dof preview button in or flicking the lever triggered by the body. Anyone know if this is something I may be able to fix myself, or should I just look for another lens. The body on the other hand is excellant and the shutter speeds sound good.