CAMERA OBSCURA - a darkened enclosure (room) in which images of outside objects are projected through an aperture (hole) onto a facing surface.
PINHOLE - a somewhat misleading term which has nothing whatsoever to do with sewing pins. Can refer to minute holes in photographic emulsions or equipment (e.g. bellows). In this context refers to small round holes deliberately made in thin sheet metal - usually with a hand sewing needle.
Precision, laser ‘drilled’ pinhole apertures are available commercially from Lenox Laser - The Leader in Small Hole Technology (USA)
FOCAL LENGTH (F/L) – strictly speaking, applies only to lenses and is used in pinhole photography to describe the distance between the pinhole aperture and the ‘focal plane’ where light-sensitive materials are positioned (back of the camera).
DIFFRACTION – is the name given to the phenomena which occur when light passes through a very narrow aperture or close to the edge of an opaque obstacle, and arise from light deviating from the rectilinear path – light rays tend to bend around the edges of the hole. Such diffraction occurs in a lens when it is stopped down to a small aperture and resolution is impaired - in practical terms f22 or f16 is not as sharp as f8 or f5.6. Diffraction is explained by the wave theory of light.
INVERSE SQUARE LAW - when a small light source illuminates a surface squarely, the illumination of the surface is inversely proportional to the square of the distance between the source and the surface - in practical terms when the distance from light source to subject is doubled the intensity of the light is quartered (one quarter the level of illumination).
RECIPROCITY FAILURE – the law of reciprocity states that the effect of exposure on a light-sensitive emulsion is independent of time and intensity but is a function of the product of the two. Photographic materials are designed for a wide range of applications and provide various aperture and shutter speed combinations within a specific range e.g. 1/1000 to 1 second. When used outside the designated aperture/shutter speed range photosensitive emulsions are subject to reciprocity law failure and in practice additional time will be required to achieve the required density/exposure.