This is a Leica Thread Mount (LTM, or M39) pinhole body cap for Leica, Russian, and Japanese rangefinders, among others. This is a great way to experiment with pinhole photography using a vintage Russian rangefinder like Zorki, FED, or Zenit. The film transport system in a mass-produced camera is vastly superior to the usual method of overwinding or guessing with homemade cameras.
These cameras can often be found on eBay for less than US$20, in fine shape. Even if the shutter durations are incorrect, provided the BULB setting ("B") works, you're in business. Use a standard locking cable release to minimize jostling the camera during an exposure. Another concern with older fabric shutter curtain cameras is small pinholes in the shutter cloth. A flashlight can disclose this. if you can take the back off the camera (Leicas and some older Russian rangefinder do not let you view the back of the curtain directly. If you are shopping for a rangefinder for this purpose, I like the FED2 model.
This lens system has a Body Flange Distance of 28.8 mm, the pinhole cap puts the aperture at that distance from the film, for a so-called focal length of 28.8 mm. The viewfinder in the camera is designed for a 50mm lens, so keep in mind that the viewfinder will only show you what is in the center of the frame.
I use a 0.23 pinhole, and shoot at a nominal f/135 f-number. If you use a more commonly sourced 0.20 mm pinhole, try f/155 for starters, or add a half stop to the durations on the exposure chart. The threads were accurately modeled in OpenSCAD, but remember they're plastic, so use a light touch when threading for the first time.
As always, test your filament for opacity before 3D printing cameras or parts. Ask me how I know this.
Here's a link to all the pictures I have and will continue to take with this camera
The body spacers allow for thread adapters if your camera has a 3/8" tripod mounting point instead of the modern 1/4-20 thread.
Interestingly, I called this the "мутант", Russian for mutant, but I had problems with STL corruption when named with Cyrillic letters. Not sure if it was TinkerCAD, or Slic3r, but I quit doing it.
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