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Viewing equipment The process of air photograph interpretation usually involves the use of stereoscopic viewing, which provides a three-dimensional view of the terrain. Stereo viewing is possible because we have binocular vision. We have two eyes that are slightly separated and continually view the world from two slightly different perspectives. Whenever objects lie at different distances each eye sees a slightly different view of the objects. The differences between the two views is synthesized by the mind and provides depth perception. Therefore the two views provided by our separated eyes enable us to see in three dimensions. Stereoscopes vary in complexity, the simplest being a pocket lens stereoscope, which consists of plastic lenses with a magnification of about 2 placed on 12 cm long retractable legs. This instrument is ideal for fieldwork as it is light and collapsible although it has the disadvantage of only viewing a small area at one time. A mirror stereoscope overcomes this problem. Light rays reflected from the photographs do not take a direct route to the eyes as in a lens stereoscope, but reach the eyes after being reflected from angled mirrors and a prism. This widens the view by a factor of 3 thereby allowing a larger area to be examined stereoscopically. In overlapping two photographs two views are provided taken from separated positions. In viewing the left photograph of a pair with the left eye and right with the right eye, we obtain a three-dimensional view of the terrain surface. A stereoscope allows this stereo viewing process and many textbooks contains stereopairs, which can be viewed in three-dimensions using a lens stereoscope. View a Stereopair and geomorphological map for the area of Butteremere, Cumbria.
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