Mamiya 645 80mm f/2.8 N: Loose Focus Ring and Slow Diaphragm
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
Servicing an 80mm f/2.8 N Mamiya 645 lens. This lens came into the shop with the complaint of a loose focus ring. A straightforward repair where some disassembly is done and the screws put back in place (one or more are usually loose in the barrel) and then refocus of the lens.
But while examining the lens I found that the diaphragm blades, although working, where not quite as snappy in opening and closing as they should be. While maybe not causing an issue immediately, in cold weather there would likely be a problem and this would progress at some point to having the diaphragm not stopping down consistently.
Upon disassembly on the rear of the lens (not necessary for a diaphragm service but I like to get a better idea of the lens condition) I noticed that all three of the rear lens mount screws had been damaged from use of an incorrect screwdriver at some point in its life. Photo two shows what can happen when you use the wrong fitting screwdriver on a screw (1.7mm x 8mm) that has been installed with sealer from the factory. It takes a little more work to get these out and I replaced them with new screws. I do use a small amount of sealer on installation but it’s of low strength.
Photo four shows the culprit in causing the slow diaphragm. This one blade had oil residue on it and was slowing up the operation. It takes very little oil to cause a problem with diaphragm blades that are operating under light spring tension.
Photo five shows all of the cleaned blades happily in place and ready to have the control ring installed on top of them.
Photo six is near the end of the operation with focus being set on the lens before the focus ring is installed.
