Industrial inspection performance depends on both viewing distance and oculomotor characteristics

The influence of viewing distance and individual differences in the oculomotor resting states (dark vergence and dark focus) on both inspection performance and visual fatigue was investigated by asking 24 college students to perform a visual inspection task for 40 min twice, once with a 20 cm viewing distance and once with a 60 cm viewing distance. During the task participants visually searched enlarged images of contact lenses for six different types of defects. Lens images were presented at a constant visual angle (17.5 degrees) on a CRT. Inspection performance varied significantly across the two viewing distances: participants spent on average more than twice as much time inspecting individual lenses in the near condition (mean = 18.32 s lens(-1)) than in the far condition (mean = 7.25 s lens(-1)). Participants also reported greater visual fatigue in the near condition than in the far condition. At both viewing distances, participants who experienced relatively large inward shifts in dark vergence also tended to be slow inspectors. Neither inspection performance nor measures of fatigue were related to dark focus. These results support existing evidence against the use of near viewing distances and suggest that an oculomotor mechanism links inspection performance and visual fatigue to viewing distance.