Aspheric RGP lenses have to be fitted a little steeper (have a shorter apical radius) than the base curve of a spherical lens to fit the cornea due to the rate of flattening of the lens which results in a lesser sagittal depth.
Aspheric lenses are used to attempt to get a better match between the lens and the cornea.
Due to the power change across an aspheric lens, patient's visual performance may not be the same as with a spherical base curve lens.
