DIY: Opto Sigma Plano-Convex filter frame

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 uknown 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 previosly 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 glassperfectly centers glass and holds it firm.


I’ve made that rubber ring slightly wider than glass borders, it should more evently 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.


6 thoughts on “DIY: Opto Sigma Plano-Convex filter frame

  1. Hi, I just ended reading about this. Would like to ask you if you know of any filter than they came already ready for this?
    I mean, I really don’t have the patient to do what you did 🙂
    I google a bit on some stores that everyone knows and I found this,
    This is it, or is nothing to do with the solution of the field curvature?
    I have the voigtlander 35mm vm 1.7 on a7iii+techart pro.

    Many thanks.


    1. Thanks for your question. I’ve checked the optical theory and more details on Plano-Convex glass from OptoSigma. Unfortunately that CloseUp filter will not work for correction of Voigtlander 35mm F1.7.
      Focal lenght (mm) = 1000 / diopter
      diopter = 1000 / focal length (mm)

      OptoSigma SLB-50-5000PM has 50mm diameter and 5000mm focal length. So diopter value is 1000/5000 = 0.2

      Heliopan 46mm Close-Up +1 is the thinnest CloseUp filter, it has diopter +1. So its focal length is 1000/1 = 1000mm, it’s 5 times shorter focal length than required to correct field curvature of Voigtlander 35mm F1.7. If you put it on lens it will just allow you to focus more close in macro mode, but curvature will be “overcorrected” to the other side, and maximum focusign distance will be limited as well.


  2. Hi, thank you for your additional thoughts on the PCX filter frame. I have just finished my OptoSigma SLB-50-5000PM filter assembly but can’t reach infinity with the Fotodiox adapter recommended by BastianK.
    Which adapter do you use?


    1. Some inexpensive adapters are thinner than needed, and overpassing infinity focus of adapted lens. People either pay 10x more for perfectly sized adapter, or buying few cheap samples to find most precise sizing. OptoSigma 5000PM requires about 1mm thinner than normal adapter. I used CV 35mm f1.7 on Fuji X camera and had to disassemble adapter and use metal file and caliper to grind off 1mm of adapter thickness. It took few hours of work. Looks like your adapter sample is perfectly made, so you need to look for worsely sized sample (1mm thinner) or do some metal grinding of your adapter sample.


        1. The adapters I have consist of 2 rings: camera side mount thick ring made of aluminum, and leica lens mount ring made of brass or plastic. I disassembled adapter and grinded thick aluminum ring surface facing lens from 10mm to about 9mm. Adapter screws still secure very well. I’ll post some pictures later.

          Liked by 1 person

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