Threshold Angle Research Articles

Some observations and theory on the variation of visual acuity with the orientation of the test object.

The superiority of visual acuity obtained with a grating test object oriented vertically or horizontally over values obtained with oblique orientations is demonstrated for various combinations of pupil diameter and retinal illuminance level. This superiority, which has an average value of 7 percent of the mean threshold angle of resolution for all meridians, increases with pupil diameter but is not affected systematically by illuminance level.The data are discussed with reference to the assumptions that meridional differences in acuity are a result of dioptric or of retinal factors. The systematic effect of pupil diameter points to the dioptric origin of some of the observed meridional differences. The influence of retinal or post-retinal factors as well is suggested by the persistence of the effect at a high illuminance level and with large pupil diameters, a condition where a slight decrease in retinal image contrast is shown not to lower acuity.