The main effect of optical defocus in the eye is always a decrease in retinal image contrast, but when the pointspread function has sharp edges (as it does for defocus levels above 1 diopter), defocus also produces frequency-specific spatial contrast sign-reversals (e.g., 1 + cos(fx) becomes 1–cos(fx)). Such "spurious resolution" distortions occur whenever the optical transfer function imposed by defocus creates a half-cycle phase shift (e.g., 1+cos(fx) becomes 1+cos(fx+π) = 1-cos(fx)). Correcting such phase reversal errors computationally in simulated out-of-focus retinal images (e.g., of printed text) can produce decisive improvements in stimulus recognizability. Mathematically, the same correction can be performed in advance (e.g., in a video display) by altering the phase spectra of visual stimuli to anticipate phase reversals that will be produced by subsequent defocus (e.g., to precorrect phase reversal at frequency f, replace stimulus 1+cos(fx) with 1+cos(fx-π)). In practice this works—e.g., a phase-precorrected point stimulus viewed out-of-focus does appear as a small bright spot on a uniform background. But in comparison to the dramatic improvement produced by phase-correcting images after defocus, pre-defocus phase correction generally produces a post-defocus retinal image with disappointingly low contrast. Analysis to be reported here shows why this is so, and allows one to estimate that peak contrast in the defocused image of a phase-precorrected stimulus can never be greater than around 25%. |