Figure. 7:

Plot of the Strehl ratio as a function of defocus for an AO-corrected eye and for a typical eye (corrected for astigmatism). Strehl ratio is the ratio of the peak height of the PSF for the actual eye to the peak height of the PSF of a diffraction-limited eye with the same pupil size. Strehl ratios range from 0 to 1 where the higher number indicates a sharper image. The pupil size was 5 mm for both cases. At best focus, the image quality in the aberration-free eye far exceeds that of the typical eye, but image quality drops quickly with defocus. By the time the defocus is +/− 0.15 D or more, the aberration-correction actually results in worse image quality than if it were uncorrected. This illustrates how aberrations can serve to increase the depth of focus of the eye. It also illustrates the cost of correcting ones aberrations.