The present invention relates to a process in which the visual effect ofdifferent higher order aberrations is normalised in relation to the visualeffect produced by defocus. In addition, it relates to a correcting factorthat will normalise RMS, a test chart for measuring the effects of higherorder aberrations, methods of testing for the effects of higher orderaberrations, models to establish the relative visual effect of aberrations, amethod for designing contact lenses, a method in which the VPDF is used tooptimise the design of both the front and back surface of the lens, methodsfor designing surgical procedure, and methods of ocular surgery.
