The interrelationship of visual acuity at different distances.

Abstract

In this study, a multiple-choice checker test was used to test visual acuity of 400 subjects in an age range of 17-36 yrs. The subjects were required to locate a checkered test square by indicating its position relative to 4 other control squares. The apparatus was set up so that 8 levels of visual acuity could be obtained at distances of .20, .25, .33, .40, .50, 1.00, 5.00, and 10.00 meters. Illumination in the vicinity of the apparatus was under 1 foot candle, and on the target it was 8 foot candles. Half the subjects started at the near distance and worked to the far, the other half worked in the reverse direction. The subject''s visual acuity at each of the prescribed distance stations was the smallest target on which he could make 1 original and 4, later, successive, correct judgments. Since there were always 4 alternative positions to select in each of these 5 judgments, a subject''s chance was only 1 in 1,024 of making 5 correct responses when he could not see the location of the checkers. Eighty-nine additional subjects were tested twice to check the reliability of the test at the various distances. Reliability as judged by retest was greater at 5 and 10 meters than at the shorter distances. The interrelationship between visual acuity at various distances became steadily lower as the difference in diopters of focal power between the tests was increased. The spread of individual differences was more normally distributed for the near point tests than the far point tests. This low relationship between visual acuity at a given distance and acuity at any other distance, plus the steady decrease in this relationship as the 2 distances move farther and farther apart, would indicate the importance to industrial vision testing of measuring acuity at a distance at least approx. equivalent to that at which the employee must work. Basing measurement of an individual''s visual acuity at 20 ft. results in frequent error when the job requires acuity at much closer range.