The relation of visual-acuity to convergence and accommodation
Journal of Experimental Psychology
Measured visual acuity is usually expressed as the reciprocal of the angle that the minimally visible spatial extent or gap subtends at the observer's eye. This practice implies that the actual size of the acuity target and its distance from 0 are immaterial and that what determines the resolution threshold is the size of the retinal image of the object. Several experiments have, however, been reported showing that there are exceptions to this rule. The evidence appears to show that visual acuity is independent of observation distance wheen the distances are larger than one or two meters, but that it decreases with decreasing distances below this limit (1, 5, 7, 8, 9).
Various explanations have been offered to account for this phenomenon. The most prominent among these has been the suggestion that, since convergence and accommodation necessarily covary with the distance of the test object, either or both of them may be related to this phenomenon (3, 8, 10). So far no direct evidence has been available to evaluate this hypothesis. The experiment reported here was designed to provide such information.