The effect of varying lens shape factors and refractive index distribution on two lens performance characteristics (equivalent power and spherical aberration) are considered, using a mathematical model of the human lens which is based upon ellipsoidal iso-indical contours. Values of radii of curvature based on the work of various authors are used to determine asphericity and these inferred values compared with previous measurements. Discrepancies are found and although asphericity has no effect on equivalent power, it does affect the spherical aberration. Based on preliminary findings of the refractive index distribution in the equatorial plane of human lenses, the index distribution is described by a polynomial in the Y (the distance from the optical axis in the equatorial plane), the coefficients of which are polynomials in Z (the distance along the optical axis). The shape of the index profile was found to have a significant effect on the equivalent power and spherical aberration of the lens. The results indicate that more information on surface asphericity and accurate measurement of the index profile in the sagittal plane are required for more accurate modelling of the human lens.