Effects of Viewing Distance with the Howard-Dolman Apparatus

Abstract

Studies of depth acuity under commonplace viewing conditions have not found the effects of viewing distance to be in accord with stereoscopic theory of depth perception. To account for the results, it was proposed previously that vernier acuity may be the major basis for the quantitative discrimination of real depth; that binocular parallex, although important to the impression of depth and critical for stereoscopic vision, may provide at best only a weak quantitative aid to real depth perception. Such an explanation might have been erroneously forced from the “commonplace” studies performed since in those studies all target edges were always clearly defined for the subject and this might be expected to enhance coincidence-type depth clues without necessarily enhancing the stereoscopic clue. On the other hand, in the standard Howard-Dolman two-rod test upon which the “commonplace” studies were modeled, only the vertical edges of the rods are visible, and this might be expected to enhance the relative utility of stereopsis as a depth clue by reducing the effectiveness of vernier-type depth clues. This experiment was designed primarily to investigate this matter by studying the effects of distance with the Howard-Dolman apparatus.It was found that a distance function derived earlier from the average acuities obtained in a “commonplace” study provided a very good fit to the average acuities obtained with the Howard-Dolman test. It was concluded, therefore, that the possible relative disadvantage of stereopsis as a clue in the “commonplace” studies did not affect the results obtained or vernier hypothesis proposed.