The effect of defocus on the contrast and phase of the retinal image of a sinusoidal grating.

The variation with defocus in the contrast and phase of the retinal image of a grating stimulus is computed from a knowledge of the measured wavefront aberration of individual eyes. Contrast or modulation transfer is generally lower than in the aberration-free, diffraction-limited case, even at a pupil size of 3 mm, and marked spatial phase shifts, which can be seen as sideways shifts in the retinal image, frequently occur. A simple demonstration of these effects is given. The possible significance of the focus-dependent spatial phase shifts for the phase discrimination of the visual system and accommodation is discussed.