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Author Message   Glenn Tinkerer Username: Glenn Post Number: 838 Registered: 07-2006 Rating: N/A Votes: 0 Posted on Saturday, August 28, 2010 - 02:08 pm:    I have just looked at the Canon parts and repair microfiches - The bayonet ring or 'Tightening Ring' as Canon calls it, part nos CF2-0330-000 is not unique to the 400mm f2.8. It is fitted to the following lenses:- nFD 300mm/f4 held by 3 radial screws nFD 300mm/f4L held by 3 radial screws nFD 400mm/f2.8L held by 6 radial screws nFD 400mm/f4.5 held by 6 radial screws nFD 500mm/f4.5L held by 6 radial screws Canon's logic here seemingly is that the heavier lenses needed more than the standard 3 tiny screws, found on the lesser focal length nFD lenses because the people using these lenses always tended to use motordrives also. This obviously makes the camera body much heavier, so the CF2-0330 ring is drilled for six screws, where as the normal CF2-0111-01 ring is only drilled for 3 screws. It is also possible that a better spec material was used for the ring fitted to the heavier lenses. Now I have no idea if the 0330 and 0111 rings are interchangeable fit wise, certainly the screw holes drilled at 60 deg spacing will allow 3 screw fixing rings to be retained in the 400mm lens if all else is equal. Unfortunately I do not have a 400mm f2.8L to experiment on, mine was ditched years ago as soon as autofocus became reliable, but I cannot see why a ordinary ring from a shorter length lens should not work temporarily if its dimensions are the same and you refrain from hanging a motordrive on the body. As anyone who has used one of these lenses will state, you do not hang a lens weighing nearly 6kg from the camera body - The lens is supported on a STRONG/SECURE tripod and the camera 'hung' from the lens unit. Thus if you take care a 3 screw fixing should make a good temporary repair, assuming a 3 hole ring will fit in your lens's rear mount. I suggest that you look very carefully at one of your standard lenses and compare the rings and if they look similar buy a gash 50mm/f1.8 and try fitting the ring in your lens. If it fits properly you could even drill and tap out the extra screw holes.