Today I built two pinhole lenses out of two old, damaged M42-lenses.
Yes, "pinhole lenses" is a contradiction, but you know what I mean.
First I cored the two lenses (the glasses must go!), then I put on each lens a lens cap and fixed them. After this I measured the focal lengths and used the pinhole size calculator to determine the diameter of the holes. Later I pricked with a pin the holes in the lens caps.
Now I have two Pinhole-M42-lenses for my M42-slr's!
Focal length: 75mm
Image circle: 144mm
Original: Revuenon Special 2.8/35mm
Focal length: 55mm
Image circle: 106mm
Original: Meyer-Optik Görlitz Domiplan 2.8/50mm