Fixation disparity and near visual acuity.
Twenty patients having associated heterophoria values of 2 delta and greater, as recorded with a Mallett unit, had their near monocular and binocular visual acuities measured. In the latter case, measurements were taken both with the associated heterophoria corrected by prisms and without prism correction. Bradford Near Vision charts, specially designed to measure visual acuity at near in seconds of arc, were used in the acuity measurement. The improvement of binocular acuity compared with monocular acuity was less than would occur in normal subjects without associated heterophorias. When the associated heterophorias were corrected with prisms the improvement in binocular over monocular acuity was similar to that found in normal subjects, with zero associated heterophoria readings.