Monochromatic aberrations provide an odd-error cue to focus direction.

Monochromatic aberrations that exist in the human eye will cause differences in the appearance of the point-spread function (PSF) depending on whether there is positive or negative defocus. We establish whether it is possible to use these differences in the PSF to distinguish the direction of defocus. The monochromatic aberrations of eight subjects were measured with a Hartmann-Shack wave-front sensor. Subjects also performed a forced-choice psychophysical task in which they decided whether a blurred target was defocused in front of or behind the retina. The optical system for the psychophysical task was designed to isolate the blur due to monochromatic aberrations as the only odd-error cue to the direction of defocus. Shack-Hartmann measurements showed that monochromatic aberrations increase as the pupil size increases. On average, the correct/incorrect responses for discriminating differences in the PSF for different directions of defocus were 54/46 for a 1-mm pupil and 83/17 for a 5-mm pupil, representing more than an eight-fold increase in discriminability. This discriminability extended for large amounts of defocus and also for more complex targets, such as letters. Sensitivity to the differences in the PSF for different directions of defocus increased as monochromatic aberrations increased, particularly for the even-order aberrations, which give rise to an odd-error focus cue. It was found that the ability to discriminate PSFs for different directions of defocus varied among individuals but, in general, depended on the magnitude of monochromatic aberrations.