Spherical aberration of aspheric contact lenses on eye. 
It is important to assess aspheric rigid contact lenses not only for fit, but also for optical performance.  Using ray tracing techniques, on-eye total longitudinal aberration induced by aspheric contact lenses and the variation of the tangential focus with ray height were determined.  Both the contact lens and tear lens were taken into account.  The front and back surface profiles of the contact lens and the corneal profile were modeled as conic sections.  On-eye aberration was found to become strongly more positive as the p-value of the contact lens front surface increased.  It became more negative as the back surface p-value increased, but the tendency was weaker.  These calculations indicate that selection of a contact lens back surface p-value according to fitting considerations without choosing a front surface p-value to minimize aberrations could, in some circumstances, lead to a reduction in visual performance.