Reverse ring for macro photography with two lenses with M49 filter threads.

With a reverse ring, you can mount a camera lens in front of another lens in reverse (filter thread to filter thread) to create a macro lens. The close-up photo of the printed ring was taken using this technique, with a Helios 44-2 lens mounted in reverse on a Jupiter 9 lens.

The threads are designed to M49 x 0.75 mm specification. You will probably need to scale X and Y axis down a bit for it to fit, depending on your printer, material, layer height and slicer. I am using 99.93% scaling factor for PLA with 0.05 mm layers on Prusa MINI+. With lower layer height, the threads seem to work smoother, although it works even with 0.2 mm layers. I recommend printing this with no infill, perimeters only (just set the number of perimeters high enough, 10 will definitely work).

If you tighten the ring too much and cannot easily unscrew it, use the slots on the sides with a spanner wrench or (carefully) just one of them with a small screwdriver.

I will publish a parametric file for creating different filter thread sizes when I get to it. Meanwhile, if you need a different size of filter thread, send me a message and I will create and upload it.