The ability to see in three dimensions or stereo is common to all mammals with their eyes positioned to look forward. Each eye perceives an object from a slightly different position which is translated by the brain to 3-D image. A quick way to appreciate this is to hold up a finger and look at it with one eye then the other; you will find it moves against its background this is parallax.
Stereoscopic Imaging is a technique that uses parallax to create or enhance the illusion that an image has depth by showing two slightly offset images separately to each eye of the viewer. Both images are of the same scene or object but from a slightly different angle or perspective. This is meant to trick your brain into synthesizing that the small lateral displacements between the positions of the images are implying spatial depth. It provides a more familiar view and a relative level of detail between objects on the ground greatly aiding the interpretation of known and unknown objects. Special equipment is usually required in order for the brain to make sense of the picture. During WW1 various stereoscopic viewers were used.
British Intelligence Officer uses a stereoscopic viewer to interpret aerial reconnaissance photographs © IWM (Q 26946)
The RFC’s first stereo pictures were collected using the A Type hand held camera and necessitated a rapid plate changing process. Experiments were carried out using what was virtually two Type A bodies joined together with the lenses spaced apart the required distance to obtain stereo images. This exposed two plates at the same time producing the necessary parallax.
Pair of aerial cameras on the Mackenzie stereo fitting mounted on a scarff ring. © IWM (MH 33749)
Stereo Mount for P1 Type Cameras. © IWM (Q 12287)
Stereo photography significantly enhanced the interpretation of aerial photographs. The 3-D effect was particularly useful at revealing camouflage attempts which to the naked eye, on a single mono photograph, would remain hidden. The photograph below is a stereo pair taken over Mount Kemmel, South West of Ypres on 22 July 1918.
Stereoscopic aerial photograph showing area south of Mount Kemmel taken by No 10 Squadron Royal Air Force 22 July 1918. © IWM (Q 115374)