This study examines the extent to which a Zernike-based optical correction can restore acuity in keratoconus as a function of disease severity and contrast level. Increasingly complete Zernike corrections in the presence of Zernike-fit error were simulated. Acuity for keratoconic eyes with $<60\mathrm{D}$ maximum corneal power reached $20∕13$ with a fifth-order Zernike correction under high-contrast conditions and exhibited a loss of 0.1 logMAR (minimum angle of resolution) (from $20∕32$ to $20∕40$ ) for low-contrast conditions. Acuity for keratoconic eyes with $>60\mathrm{D}$ maximum corneal power approached $20∕13$ with sixth–tenth-order corrections under high-contrast conditions but did not return to similar levels with a tenth-order correction for low-contrast conditions. The results suggest that fit error affects visual performance for more difficult tasks and that restoring high-contrast acuity ( $20∕16$ or better) using a fifth-order Zernike correction is not limited by Zernike-fit error for over 88% of keratoconus cases.