If your m42 lens has an auto iris then this adapter is not for you. There is a protrusion on the inside that touches the height of the bezel, preventing you from adjusting the bezel while screwing it in. You literally have to unscrew the lens to readjust the screw. I have 4 of these and only use them on my manual lenses, but for the few that have automatics I've found an all metal lens (oh yes they are plastic) that has no burr and works smoothly with automatic lenses. The lip inside is not needed and is cheap plastic (price reflects).
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