MOD - filter frame for Opto Sigma Plano-Convex lens

 First of all I’d like to thank HaruhikoT and BastianK for sharing extremely useful approach and great article about Rangefinder wide angle lenses on A7 cameras: problems and solutions. It significantly improves IQ and performance of many lens, and I’m happy to see that it also works for Voigtlander Ultron 35mm F1.7 lens.

In this short article I’ll show how to build up simple and durable filter frame for Opto Sigma Plano-Convex lens.

I recently purchased CV 35mm F1.7 with already mounted Opto Sigma filter. I appreciate the effort of the unknown person that assembled initial filter frame case, though it required some minor improvements. I’ve disassembled whole frame construction and cleaned Plano-Convex glass from the black clay previously used to center it.

After that I’ve used black paint to darken lens edges, which should eliminate unnecessary light reflections. After that I’ve cut thin rubber ring from bicycle tire tube and attached it to the glass edges.

Previous owner selected great parts for the frame:

  • 46-55mm step-UP ring
  • 55-52mm step-DOWN ring
  • 52mm filter inner securing ring

Opto Sigma SLB-50-5000PM glass has 50mm diameter, so about 1.6mm layer required to center it inside 52mm filter thread of 55-52mm ring. The rubber ring around glass perfectly centers glass and holds it firm.

I made that rubber ring slightly wider than glass borders, it should more evenly distribute pressure of securing ring. Black paint on lens border does great job eliminating side reflections.

Rubber ring also additionally secures frame rings together.

It’s great and compact field curvature correcting filter. I like the idea of combining 46mm-55mm and 55mm-52mm rings – it allows to use another 52mm filters at front. Previous owner designed it perfect, I only had to add minor paint and rubber ring modification to increase filter mount quality.