I was in a second-hand bookshop a while ago and a book called Photographic Techniques from 1972 caught my eye. In it Jaroslav Šimek describes over a hundred photographic techniques which allow the photographer to create unusual photographic effects. Today, in the computer age, it seems incredible when you realise what kind of results one can get using ordinary photographic methods.
The greatest surprise was waiting for me once I got home and started reading the individual instructions. There I found a chapter entitled "Pinhole Enlarger"! It describes how one can adopt an unusual and creative approach in applying the well-known principle of the pinhole camera for enlarging the negative.
Here I include the text from this chapter and a reproduction of a photograph illustrating the use of this technique. I left out the fairly complex description concerning the optimal diameter for the pinhole, which isn't so important for this technique.
The pinhole enlarger is analogous to the pinhole camera as far as treatment of the negative is concerned. We know that we can take pictures on light-sensitive photographic material without a lens, in the same way that the camera obscura was used many years ago, when photography was in its initial stages. This camera has a piece of thin material instead of a lens on its front side, most frequently a piece of metal foil containing a tiny hole. [...]
The pinhole enlarger can be used to enlarge a negative created by an ordinary camera. Unscrew the lens from the enlarger and insert a cardboard disc into the opening instead. The cardboard disc must fit in the hole exactly, so that no light can get past. Then make a small oblong opening in the cardboard and place a piece of metal foil (such as uncreased kitchen foil) with pinpricks over it. The number of times the negative image is projected corresponds to the number of pinpricks. The example here shows an enlargement made from a 6 x 6 cm negative. Since the image is multiplied, we can't really use a negative with a picture covering the entire area, such as shots of the countryside etc. The most suitable for our purposes is a negative showing just one smaller object against a neutral background. The metal foil in this case had three holes of diameter 0,3 mm set in a single line, 2 mm apart. The distance between the foil and the negative was 10 cm, the distance between the photographic paper and the foil was 32,5 cm. Turning the plate with the holes changes the direction of the image diffusion.
Jaroslav Šimek: Fotografické postupy, Práce, Praha 1972