4x4 Lensboard and Screw Thread adapter for Large-Format Enlarger Lens

One of my nicest 4x5 format lenses is a 150mm f/4.5 Rodenstock Omegaron, so when I saw a chance to buy a 210mm version very cheaply, I jumped on it. It's a beautiful lens intended for use on an enlarger. However, when it arrived, I discovered that it is MUCH larger than the 150mm, and doesn't actually have a screw thread mount. Apparently, several of the Rodenstock enlarger lenses targeting 5x7" or larger have a rather odd mount with no obvious screw thread (see the right side of the last image above).

After some head scratching, I realized that I could 3D-print a screw thread ring with a split, so the ring could be pulled apart to mount on the lens and then would hold firmly together with a mating screw thread in the 4x4" lensboard. It works great. I also have a 60mm extension tube design here, although I haven't yet needed to print it myself.

Odds are good that other lenses have similar odd mounting arrangements, but probably with different details about the sizes, etc. So, unless you have an Omegaron 210mm f/4.5 lens, most likely this design will not work, but the same idea would still apply.

Printing

You need to print at least the first two of these three parts:

• 4x4_20220626omega0.stl is the split 72x2mm thread ring. Print this with 100% fill. I printed this black, but any color is fine -- it doesn't even need to block light.
• 4x4_20220626omega1.stl is the 4x4" lensboard with 72x2mm thread. It doesn't need more than 25% fill, but you need it to be opaque. I used silver silk PLA for this, but then painted the back with Black 2.0 to ensure light tightness. If you prefer that the 4x4" lensboard not say "Omegaron 210mm f/4.5" on it, a blank board is also available as 4x4_20220626blank1.stl .
• 4x4_20220626omega2.stl is the 60mm 72x2mm threaded extension tube. I'd suggest printing it in opaque material with 25% fill, however, I didn't actually need it yet so this is untested.

All parts print without supports. Note that, in order to make the threads print more reliably, these are with a 2mm pitch but 45 instead of the standard 30 degree metric angle.

Post-Printing

Pretty straightforward:

1. Pull the split screw thread ring apart enough to slip it over the rear of the lens and lock under the metal flange so that it is sitting on the back of the black ring with the aparture reference mark on it. You might want to use a touch of loctite or a similar glue to hold it closed and in place.
2. Screw the now threaded lens into the 4x4" plate.
3. If your lens plate material was not completely opaque or was shiny, paint the back of the 4x4" lens plate with black paint (e.g., Black 2.0).

The plate should mount normally on typical 4x5" camera bodies, such as my B&J shown. (I didn't bother with adjusting the rangefinder coupling.) Keep in mind that these are enlarger lenses, so they have a nice aperture iris built in, but do not have a shutter.