I have a lens that has the manual/automatic aperture for my mechanical camera. When I placed it in automatic mode, it does not open to full aperture but instead seems to be stuck in manual mode untill I step down to f5.6 aperture. By then it will be a bit dim to focus. What is the problem? Can the lens be repaired/lubricated? I have another focal length lens that works fine so I don't think its the camera body.