The origin of the pinhole camera is camera obscura which is a dark room one of whose wall has a tiny hole, and goes back to the great artist and scientist, Leonard da Vinch. He drew drawings by tracing images on the wall projected through the hole of the other side of it.
The pinhole camera doesn't have a lens, which means that there is no focal length and you don't have to focus the camera.
The below is a formula of focal length of the pinhole camera.
Focal length =
length from a pinhole to the surface of a film / 0.3mm
Although the picture taken with a pinhole lens is not as sharp as the one done with a lens, it makes a unique soft-focus image, especially when you use a black-and-white film.
MJ pinhole lens accepts 39mm filters.
It includes a metal lens cap which also can be used as a body cap for a Leica mount or Praktica mount camera.