Effect of Focus on the Visual Response to a Sinusoidally Modulated Spatial Stimulus*

The recent development in the use of sinusoidal gratings for the analytical study of optical systems has been applied to the effects of focus and pupil aperture on visual resolution. For an eye with a dilated pupil, the in-focus position is shown to depend upon the spatial frequency of the test target. The effective refractive power of the eye increases for the detection of low-frequency gratings. If the eye is corrected for this change in effective refractive power, an improvement of about 70% in contrast sensitivity occurs for low spatial frequencies. The implications of these findings on the phenomenon of "night myopia" are discussed.