(1) The ability of the eye to see sharp images of objects that are at different distances.

(2) The ability of the eyes to superpose two separate images.

(3) The limits within which the projectors in a stereoscopic instrument can be rotated for proper orientation. For example, the Multiplex can accommodate small tilts (in the projectors) ranging from about 10^o about the x-axis to 20^o about the y-axis.

(4) The limits within which a stereoscopic potting instrument is capable of operating.

(5) The ability of any optical instrument to adjust to differences in distance or brightness of the scene.