Vision
A stereoscope is a device that is used to view pairs of photographs as a 3-D image. To understand how a stereoscope works requires a basic understanding of human vision. Humans see in what is called binocular vision, in which both eyes are used to produce a single image. On average, human eyes are about 60mm apart. When we look at something, such as a person walking toward us or a tree in the distance, the retina of each eye receives a flat, two-dimensional image of the same thing at slightly different angles. The eyes transmit the dual images to the brain, which merge the two images into one 3-D representation of the image. The goal of a stereoscope is to artificially mimic this process.
The Photographs
Taking photographs to be used with a stereoscope is a deceptively complex task. It's not as easy as taking a photograph with your camera, moving slightly left or right and taking a second photograph. The reality is that there is little margin for error. The photographs must be taken on exactly the same plane. The focal length, shutter speed and lenses used must be identical for both photographs. There are several approaches for achieving this uniformity. There are cameras with multiple lenses that can perform the task, but they can be prohibitively expensive. The most straightforward method is to use two cameras that are mounted on a bar designed to help photographers achieve 3-D pictures, and then take the pictures simultaneously. One of the cameras is attached to a slide mechanism that allows the distance to be adjusted to accommodate subjects at variable distances. There is no universal rule regarding exactly how far apart the cameras should be, but Bob Manekshaw's PhotoStuff website provides a reference chart developed by Dennis Brown. Poorly coordinated photographs generally will not render a proper image.
The Stereoscope
Actual stereoscopes have been in use since the 1840s and have gone through a number of different designs, but the base principles that govern their functionality are the same. The photographs are mounted in pairs opposite one another. The mass produced View-Master stereoscopes achieve this same effect by mounting images on circular card stock that rotates pairs of images. Light, either natural or artificial, is placed behind the images. There are two viewing apertures, one for each eye. The left eye is shown only one of the pair of photographs and the right eye is shown only the other. The brain then interprets the two images in the same manner that is does live images and constructs a 3-D representation out of the two images.
Tags: shown only
