It is well established that the anterior surface of the cornea is the major refractive element of the human eye, being responsible for approximately 75% of the eye’s total un-accommodated refractive power.^{1} It is currently believed that the human anterior corneal shape is closely modeled by a conic section which can be fully described by the vertex radius of curvature ($r0$) and a shape factor.^{2}^{,}^{3} The shape factor of a conic represents the variation in curvature form the apex towards the periphery. Five different parameters are used to express the shape factor of a conic: the shape factor $E$ and its derivatives $p$, the eccentricity $e$, the conic constant $k$, and asphericity $Q$. The formulas of conversion between them are: $E=e2$,$Q=\u2212e2$, and $K=p=1+Q$. Figure 1(a) shows a conic section referred to Cartesian coordinates with the vertex at the origin $O$. The equation of the conic section is given by $Y2=2r0Z-pZ2$, where the $Z$-axis is the optical axis, $Y$-value is the semi-chord diameter, and $Z$-value is the sagittal depth of the section.^{4}^{,}^{5} A conic section is obtained by cutting a cone by a plane including sphere, ellipse, hyperbola, and parabola [Fig. 1(b)]. Figure 1(b) is a version of Figure 5.14 from Smith and Atchison’s book, “The Eye and Visual Optical Instruments. ”^{6} Bennett^{7} derived the conic equation $rs2=r02+(1\u2212p)y2$ to calculate the $p$-value by sagittal radius ($rs$) from keratometry. Since then, many researchers have studied the corneal shape by asphericity ($p$ or $Q$) calculated by sagittal radius according to Bennett’s equation.^{8}^{–}^{12}