The proximity-fixation-disparity curve and the preferred viewing distance at a visual display as an indicator of near vision fatigue.

PURPOSE This laboratory study investigates the relation between measures of fixation disparity (FD) (and other optometric measures) and near vision fatigue at a computer workstation. METHODS Young adult subjects with normal binocular vision performed three blocks of a visual task of 30 min each. In Block A, the viewing distance was 100 cm, as a reference without near vision. In Block B, the viewing distance of 50 cm induced a defined near vision load. In Block C, subjects were free to choose a comfortable viewing distance. This preferred viewing distance was used as an indicator of near vision fatigue because subjects adopting longer viewing distances in Block C had more near vision fatigue at 50 cm in Block B. RESULTS Subjects with preferred viewing distances longer than average (63 cm) had steeper slopes of FD as a function of viewing distance (100-30 cm), as shown by discriminant analyses. CONCLUSIONS Thus, this steep proximity-FD curve indicates a weak disparity vergence system that may cause near vision fatigue. This may explain why some young adults prefer longer viewing distances at the computer workstation.