I have a Nikon F2 made in 1973 which has developed a problem. The aperture lever (facing the front side of the body, left of the mirror) has simply stopped functioning. the black DOF button has no resistance giving the impression that the camera is constantly in 'stop-down mode'. I can remove a lens and then move the lever itself with a finger (its not stuck), but with a lens attached, it's constantly stopped-down - the lever fails to retract. I'm thinking there might be a lubrication problem...hopefully nothing more complex.