On the preferred viewing distances to screen and document at VDU workplaces

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Abstract
Previous research has shown that during work at visual display units (VDUb) most subjects prefer viewing distances greater than 50 cm. The present study investigates whether this is also true in those VDU tasks that require the subjects to shift their gaze frequently between the VDU screen and a paper document. In a laboratory VDU task two conditions were compared: one in which both screen and document were located at the same viewing distance of 50 cm (as recommended in most ergonomic guidelines) and another in which the distance to the document was 50 cm and the distance to the screen was 70 cm. Gaze shifts between screen and document were made every 2 s. Visual strain of 20 subjects aged 19-51 years was estimated with a questionnaire. Contrary to expectation, visual strain was not greater when the viewing distances to screen and document differed. Thus, the dynamic load on accommodation and convergence due to the different viewing distances (50 and 70 cm) did not produce stronger visual strain than the static load at identical viewing distances of 50 cm. When subjects were free to shift the screen to the most comfortable position, they preferred screen distances of 50-8 lcm (mean 65 cm) despite gaze shifts to the document at 50 cm every 2s, These results are evidence against the widespread ergonomic concept that viewing distances to both screen and document should generally be about 50 cm. It is suggested that VDU users may select viewing distances that they find comfortable in the range of more than about 50 cm.