Viewing distance and the perceived size of simulator imagery

The apparent size of a simple stimulus was measured in order to quantify the effect of viewing distance on the perception of real (i.e., uncollimated) CRT imagery. The viewing distances tested were those used in several operational, real-image flight simulators. All identifiable distance cues to object distance except for ocular vergence and accommodation were eliminated. For six observers, it was found that perceived size changed significantly with changes in stimulus distance between 800 cm and either 61 cm or 94 cm, or vice versa. The data are consistent with the changes in perceived size reported for other stimuli and viewing conditions, and suggest that simulator imagery presented within about 1 m of the observer must be magnified by 15-25% in order to provide size cues consistent with those provided by objects located at optical infinity. For two of the six observers, measured vergence levels were consistent with the viewing distances tested, whereas measured accommodation levels were less than expected. Thus, at least for these two observers, it appears that differences in the level of ocular vergence were primarily responsible for the observed changes in perceived size.