The effects of higher-order ocular aberrations on functional visual performance were assessed for seven young healthy subjects, as a function of light level. Contrast acuity thresholds were measured for a functional vision test with and without correction of higher-order aberrations and at a number of different pupil sizes in three of the subjects investigated. A visual simulator incorporating an adaptive optics (AO) system was used for this purpose. The results show that when light level is decreased, the drop in neural contrast sensitivity limits the impact that increased optical degradations have on vision. The expected AO benefit for functional vision is given for scotopic, mesopic and photopic regimes.