I purchased this lens from KEH. Their description: "87-90 of original condition. Cosmetically, this lens is in very good shape with very limited wear on the lens barrel. The focusing is very smooth. The aperture is in great condition. The glass is very clean." When I turn the aperture ring, nothing happens. Aperture is as it is in the photo whether the ring is set to 2.8 or 22. Since KEH is a reputable dealer and their description described the aperture as in great condition, I'm assuming my newbish self is just doing it wrong. Suggestions?