Stereoscopic Base Length

the distance between two points at which observation is carried out (with the right and left eyes, right and left television camera tubes) or motion-picture filming or photographic surveying of an object is done; as a result, two images forming a stereopair are obtained. With respect

Figure 1. Separate right and left images viewed with a stereoscope are represented by P₁ and P₂, with O₁ and O₂ representing the anterior nodal points of the observer’s eyes

to human vision, the stereoscopic base length is the distance between the anterior nodal points of the eyes. The magnitude ranges from 58 to 72 mm, with an average value of 65 mm.

To improve the acuity of binocular vision of, for example, distant objects or stereopairs, optical devices (prism- or mirrortype) that increase the ocular base length are used. Among these devices are prism binoculars, stereotelescopes, and stereoscopes. For example, the base length C_s (Figure 1) of a stereoscope (in contrast to the ocular base length C_e) is equal to the distance between the centers of the mirrors (2). With an increase in base length, there is a falling off of depth of focus but an increase in visual acuity. The base length is therefore selected with the optimal combination of these criteria.