Journal of the Society of Motion Picture and Television Engineers (1950-1954)

separate effect on depth perception presents problems in experimentation because of the reciprocal influence of one on the other.6 To the extent that dissociation is possible, however, Swenson has found that the spectator senses the image at a point somewhere between the accommodation and convergence distances.10 Generally the image appears about three-fourths of the way between the accommodation and convergence distances and closer to the latter. This result should not be directly applied to the peculiar conditions of stereography, but Swenson's results seem to indicate that, in the absence of other cues, convergence (or retinal disparity gradient) is a stronger cue to depth perception than is accommodation (or retinal blur gradient). However, the fact that the accommodation distance has some effect on depth perception might indicate that a transmission geometry which uses only the projection theory of vision to determine the calculated location of the composite image will result in a picture which is compressed toward the screen. The comparative effects of accommodation and convergence on depth perception are of less importance than a comparison of the relative influence of either accommodation or convergence on depth perception as against that of other cues, cues such as binocular parallax, size of retinal image, interposition, aerial perspective, linear perspective, detail perspective, motion parallax, brightness, and light and shadow. The first one is a binocular cue, the remainder monocular. When binocular cues are in conflict with monocular, it is generally the latter which influence the determination of depth, particularly for distant images. For instance, the cue of interposition, or overlapping, is stronger than stereopsis.12 The reason for the superiority of monocular cues may lie in the fact that binocular vision requires some outside factor to give it scale,13 and this outside factor is usually one or more monocular cues.2 All the monocular cues, with the exception of motion parallax, are present in 2-D and 3-D motion pictures. Therefore, it is concluded that accommodation at the screen or convergence at the image has relatively little influence on the depth perception of the composite image. In regard to the second possible effect of the separation of accommodation and convergence, namely the effect of restricting the location of the image, it is known that this separation or dissociation is not unlimited. Furthermore, it is not necessarily constant for any one spectator, being a function partly of the psychological condition of the spectator.11 For the average case in the clinic, the ability to diverge the eyes is about three prism diopters (about 1.5°), while adduction, the ability to converge the eyes, is about nine diopters (about 5°) for each eye. Since in normal stereography the spectator's eyes are accommodated essentially for optical infinity, the spectator is theoretically able to diverge from infinity to minus 7 ft and to converge on a point only 1^ ft in front of his eyes. The dissociation of accommodation and convergence seems theoretically not to restrict the location of the image in the theater. However, comfortable viewing is impossible unless the extreme planes are kept within certain limits, and DukeElder states that as a good general rule people are able to exercise only the middle third of their relative convergence without visual fatigue.11 The approximate application of this rule to motion pictures means that visual comfort is maintained as long as the geometrical location of the image is restricted to the region from 5 ft in front of the spectator to infinity. The restricting case for all spectators is, of course, determined by the distance from the screen to the closest spectator. Duke-Elder's rule would indicate a maximum separation of point pairs equal Levonian: Stereography and Physiology 203