Photography is currently undergoing a revolution. Digital cameras have already begun to replace conventional film images, and are increasingly finding use on archaeological sites. The wide use of computing has also led to the combining of images with graphics software to produce `virtual reality' images of buildings and landscapes. Such techniques will surely establish themselves in archaeology, but for the moment they do have limitations, and there are useful alternatives.
One of these alternatives is stereoscopic photography, also known as stereography or 3D photography. This technique has enjoyed sporadic use in archaeology in the past, but as conventional photography has developed it has tended to be overlooked. The rise and -- more especially -- the fall of 3D moving pictures fostered the belief that stereo photography was little more than a gimmick. The purpose of this paper is to review the use of this technique in archaeology and to suggest ways in which it can be further developed as archaeological photography enters the 21st century.
Stereo photography may fairly be described as `Victorian virtual reality'. The technique was `conceived in the infant years of photography and was perfected between 1850 and 1854' (Darrah 1977: 1). The concept built on the still earlier development of stereoscopic drawing which, like stereo photography, has now become a rarity.
The principle is a simple one and, given that we see in three dimensions, one which might have been expected to hold an important position in both modern photography and archaeology. The technique relies on two photographs taken with an appropriate distance between them, such that when they are viewed with a stereoscope the effect produced appears to be three-dimensional. The distance between the camera-lens positions can be as little as 65 mm, approximately the distance between the human eyes, and many regard this distance as the optimum for `true' stereo photography. In practice however, greater distances are often used to produce what is more correctly known as `hyper stereo'. This has the effect of enhancing the three-dimensional effect seen by the viewer.
Images may be produced in a number of ways. A single camera can be used to produce two images at a distance apart, the method used in the earliest stereo photographs. This is only suitable for stationary subjects, but is a technique that can readily be used for recording sites etc. today. Secondly, a purpose-built stereoscopic camera may be used, which records a pair of images each time it is operated. Such cameras are today both relatively rare and often expensive, since few manufacturers produce them, and the market in second-hand equipment is limited. Third, a pair of cameras may be mounted together on a bar and synchronized, providing a cheap and versatile system (for currently available options see Stereoscopic Society 1999: 23-5). `Beam splitter' devices are also available for standard SLR cameras, but the format of the images and overall quality are often regarded as unsatisfactory.
Although, with practice, it is possible to view stereoscopic images with the naked eye (a technique known as `free viewing'), it is usual to view them through some kind of viewing device. The simplest of these is the stereoscope. This comprises a pair of lenses mounted in a frame, often with some kind of divider between the lenses to ensure that the viewer sees only the left image with the left eye, and the right with the right. Many archaeologists will be familiar with such devices from work with aerial photographs.
It should be stressed that, although the earliest stereographs were black-and-white prints, it is possible to view colour transparencies in the same way. However, colour transparencies can also be projected in stereo, and this may be of particular value in archaeology where enlarged images are required. Projection is most commonly achieved using two projectors, set either one above the other or side by side. …