A pair of stereoscopic images must simulate as closely as possible the two images perceived by a pair of normal human eyes. Eyes are separated by a distance that varies from person to person but which is normally in the range 58mm to 68mm. This distance, known as the pupillary distance, determines the extent of the small separation in viewpoint recorded in the perceived images. The separation of the eyes is also along the horizontal plane. The eyes are designed to cope with this lateral separation and are capable of independent horizontal movement of their axes. However, they cannot move independently in the vertical plane and cannot cope with the perception of two vertically shifted images. This is simply outside the boundaries of what they are equipped to do. The alignment of a pair of stereoscopic images must therefore be arranged in a similar fashion. The two images must be perfectly aligned in the vertical plane and separated horizontally by an appropriate distance.
The horizontal separation of a pair of stereoscopic images viewed in a stereoscope determines the distance at which the merged spatial image is formed. A larger separation between the individual images moves the spatial image back and a smaller separation moves it forward. The plane occupied by the slide frames or the print borders of the individual images is generally know as the stereo window, but the spatial image can be formed either in front or behind this plane. In most cases, it is desirable for the spatial image to be formed behind the stereo window. The size of the spatial image also varies as its position relative to the stereo window changes.