Phase changes induced by optical aberrations degrade letter and face acuity.

Optical aberrations of the eye reduce image contrast and induce spatial phase shifts in the retinal image. The resulting degradation of retinal image quality hampers recognition of complex objects such as letters and faces. To study the effects of spatial phase shifts on object recognition, we simulated image blur computationally for 4 types of aberrations (defocus, astigmatism, coma, and spherical aberration) present individually or in combinations. Phase errors in the computed images were corrected (by setting phase to zero), or avoided, by removing the affected frequency components (by setting modulation to zero). The resulting images served as visual stimuli to determine the effects of phase errors on visual acuity for single letters, letter clusters, and faces. The results show that 180° phase reversals induced by optical aberrations reduce visual acuity, when there is sufficient contrast in the affected frequency components. In the presence of positive spherical aberration, acuity loss due to phase errors was more for hyperopic defocus than for myopic defocus, because the contrast of phase-reversed components was much higher for hyperopic defocus. Phase shifts introduced by coma are less than 180° and consequently have a smaller impact on acuity. Although visual acuity improved the most when all frequency components were phase-corrected, phase-reversed components were nevertheless found to aid visual acuity, demonstrating phase-reversed resolution.