Visual acuity for a grating test object was determined as a function of artificial pupil diameter at five luminance levels. In order to eliminate the effect of variation of retinal illuminance with pupil size changes as a factor in acuity, the luminance of the acuity test field as viewed through each artificial pupil was previously adjusted to match a reference field of constant luminance viewed by the other eye through a fixed pupil at five luminance values of the reference field. Selection of maximum artificial pupil size was made on the basis of photographs of the natural pupil taken under conditions similar to those which obtained in the main experiment. As pupil size is increased, acuity improves rapidly at first and then more slowly before reaching a maximum value of acuity. Further increase in pupil size, when possible, results in lowered acuity. The rise in acuity up to the maximum is steeper the higher the luminance level, whereas the drop in acuity which occurs when the pupil is made even larger is steeper the lower the luminance level. The results are interpreted as reflecting the increased importance of aberrations as a factor in acuity as pupil size is increased and the luminance level is lowered. The data are plotted to show the effect of pupil size as a parameter of the acuity-reference luminance function.
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