As you know, I have developed a new fondness for folding cameras from the 50s. I have three: a Minolta Semi-P, a Minolta Semi III, and a Konica Pearl II. The top shutter speeds range from 1/200-1/500 of a second. These relatively slow speeds make using larger apertures a problem on sunny days with fast film. Lomography Color 400 film is one of my favorite medium format emulsions, which can mean trouble on a sunny day. Neutral density filters are a simple way to prevent gross overexposure.
I asked around and was told that the Pearl II required 30.5mm filters. Unfortunately, all the 30.5mm filters I could find were screw-in, which would not work. Next, I went looking for push-on 30.5mm filters—no luck. I gave up on filters for the Pearl II.
The Semi P has no rangefinder, so I never intended to use it other than for snapshots. Also, with a max shutter speed of 1/200, I never expected to use it with anything other than 100 or 50 ISO film. Then, after shooting a few rolls, I thought it might be good to use it more. So, I was back to looking for filters again. I looked everywhere and couldn’t find any filter info for the Semi P. After measuring manually, I bought 30.5mm screw-in filters (they were inexpensive), but they were slightly too small to fit. I gave up again.
Then, one day searching on eBay for filters, I came across a listing for a 32mm slip-on filter holder, which seemed like a possible solution. The filter holder has an inner thread for screw-in filters, which perfectly fits the 30.5mm ND filters. I tried the ND filter/filter holder combo on the Pearl II, and it was a perfect fit. On the Semi P and Semi III, the combo is a little loose but serviceable. As best I can tell, a 31mm slip-on would work for the Semis. However, what I have is close enough. Now to load that Lomo 400!