Effect of refractive error in visual tests with sinusoidal gratings.

While sinusoidal grating stimuli are finding increasing application in clinical tests for visual disturbance, the possible effects on the test results of simple defocus of the retinal image, due to refractive or accommodative error, are not always acknowledged. Theoretical considerations and experimental measurements emphasize that the degradation in visual performance due to such defocus effects is minimal when the eye pupil is small and the spatial frequency of the grating test objects is low.