A pinhole camera, also known as camera obscura, or "dark chamber", is a simple optical imaging device in the shape of a closed box or chamber. In one of its sides is a small hole which, via the rectilinear propagation of light, creates an image of the outside space on the opposite side of the box.
The image in the pinhole camera is created on the basis of the rectilinear propagation of light. Each point on the surface of an illuminated object reflects rays of light in all directions. The hole lets through a certain number of these rays which continue on their course until they meet the projection plane where they produce a reverse image of the object. Thus the point is not reproduced as a point, but as a small disc, resulting in an image which is slightly out of focus. This description would suggest that the smaller the hole, the sharper the image. However, light is essentially a wave phenomenon and so, as soon as the dimensions of the opening are commensurable with the dimensions of the light wavelength, diffraction occurs. In other words, if the hole is too small, the image will also be out of focus. The calculations for the optimal diameter of the hole in order to achieve the sharpest possible image were first proposed by Josef Petzval and later perfected by British Nobel prizewinner Lord Rayleigh (see Making the pinhole). He published the formula in his book Nature in 1891, and it is still valid today.
The image created by a pinhole camera has certain characteristics which we won't find in classical lens photography. Since the process entails a central projection, the images in the pinhole camera are rendered in ideal perspective.
Another special characteristic is the infinite depth of field which, in a single photograph, allows objects to be captured with equal sharpness whether they are very close up or far away.
The pinhole camera takes in an extremely wide angle. The rays of light, however, take much longer to reach the edges of the negative than the centre, thus the picture is less exposed along the edges and therefore darkens.
A certain disadvantage of the pinhole camera is the amount of light allowed through (small aperture), which complicates and sometimes prevents entirely the photographing of moving subjects. Exposure time is normally counted in seconds or minutes but, in bad lighting conditions, this could be hours or even days (see Determining exposure times for pinhole cameras).