An observer’s just discriminable difference in depth is a function of his interocular distance, his threshold difference in binocular parallax, and the observation distance. Magnification devices consisting of lenses and/or prisms will change the threshold of depth resolution because the latter now depends on the optical properties of the device as well as on the observer’s characteristics. An equation has been derived expressing this change in linear threshold of depth resolution as a function of the lens and prism power of the device, the separation of the components of the device from the eye, the observer’s interocular distance, and the object distance. Free choice of the optical properties of such magnifiers is, however, restricted since these properties must permit a clear retinal image of the object to be formed simultaneously on the foveas of both of the observer’s eyes. In general, plus lenses and base-in prisms will enhance the depth resolution and this points to the use of such devices in tasks demanding high depth resolution at close observation distances, as, for example, in surgery or inspection of surface defects.
An experimental determination of the change in linear depth resolution produced by a binocular magnifier showed close agreement with the theoretical prediction.
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