A few months ago I was asking for repair places for my Mamiya 645 500mm/f5.6 manual focus lens. I never did get around to sending it out and now I'm glad because I just fixed it. Hopefully this will save someone else the cost of a repair. Probably applies to all of the MF lenses that have a spinning tripod mount attached (APO come to mind). The problem was that the back half of the lens barrel was loose which allowed you to move the lens elements in relation to the lens mount and tripod mount. At first I tried going in through the lens mount, but didn't find any way to tighten the barrel up. Today I finally worked the screws loose that hold the lens mount end of the barrel on. Pulled the lower barrel out and still didn't see anything except for the threaded end of three small screws sticking through the barrel. Looking around I could not see any method to get to those screws to tighten them until now. I was looking at the tripod mount, and just in back of the 1/4 and 3/8 threaded holes was a small screw. So I pulled the screw and what do you know, it lines up with the 3 screws that hold the lower barrel together. Symptoms of one of these screws being really loose would be that the tripod mount will not rotate smoothly - one of the screws may have backed out far enough to hit the tripod mount. Thankfully mine were just slightly loose (all 3 of them) so I caught it before grinding metal. Now if only I hadn't broken one of the lens mount screws way back when... Still need to pull the rest of the screw and find a long replacement, but that shouldn't be too hard. Just need a little loctite solvent to free the rest of the screw and a pair of pliers to grab it (just the head broke off).