I have a Bolex lens (filter) question for you Bolex experts: On many of the Kern D-mount lenses (for the 8mm cameras) the thread size is 21mm . This takes a retaining ring and lens hood (Bolex catalog code: “ADUNI” ) . You screw in the retaining ring, then drop in a Series 4.5 filter , then screw on the lens hood to hold the filter in place.
I have found the selection of Series 4.5 filters to be limited and hard to come by. I want to attach a step-up ring to be able to use more commonly found 37mm or 43mm filters. I have found a 21mm to 37mm step-up ring on eBay which I hope will fit the thread pitch on the Kern lenses … OR my other thought was whether it would be better to leave the Bolex retaining ring on the lens and attach the step-up ring to the retaining ring ? However, I don’t know exactly what the thread size is on the upper part of the retaining ring … it’s slightly wider than a Series 4.5 filter (25.4mm) … according to this website: https://www.apotelyt.com/photo-accessory/series-filters Series 4.5 filter retaining ring size is 28.6mm … I have seen Step-Up rings listed for 28mm to 37mm or 28mm to 43mm . Would a 28mm step-up ring work or do I need a 29mm step-up ring ? (I’m not seeing such a thing as a 29mm step-up ring available , so I’m hoping 28mm will work ) . See the photos.