C-mount extension ring
thingiverse
These are useful extension rings / tubes when one needs to reduce the minimal focal distance of a c-mount lens. E.g. I was trying to get a 25mm/1.4 lens focusing at a distance of 30-60cm but it wouldn't let me. The stock c-mount extension rings (or cs to c adaptors) commonly found in the market have a 5mm extension at minimal and will make the lens only suitable for macro shots. These rings serve as a good complementary tools in this situation.
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