Originally posted by FozzFoster If your m42 lens doesn't stay at the chosen aperture when moving the aperture ring (only stops down when you press the pin) then you need to modify the lens by either super gluing down the pin or pushing it so far in it won't come back out.
Generally this is only required with M42 lenses that don't have an auto/manual (A/M) clutch located near the aperture ring. Unfortunately I think this includes the aforementioned Helios types (or some of them at least).
The A/M clutch was a thoughtful little device designed to ensure backwards compatibility for cameras that didn't have a flapper plate to link stop-down to shutter actuation, or when the user wanted to mount them on bellows, or extension tubes without feed-through connectors for the pin. These days, of course, it ensures
forwards compatibility with essentially all DSLRs (if there is an M42 mount DSLR with a flapper plate actuator out there, I haven't heard of it but I reckon fans of the old glass would be all over it, as
this old thread proves).