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