I recently bought a 5×4 pinhole camera. Its a box, and instead of a lens there is a very small hole. Smaller than a needle. And on the back is a holder to load a sheet-film holder. I was out to do a shoot in Argyll so I packed the camera just incase I had some spare time. On the way back I stopped off in Arrochar and shot a few sheets of 5×4 inch film. The pinhole has an aperture of f233. So it takes a bit of calculating to estimate the exposure. Actually, my exposures were not that far off.
The pinhole camera. Snapped with my phone.
5×4 Pinhole Camera. HP5. D75. Exposure about 2 minutes.