How are shape factors and K Values Calculated?
The Shape Factor (k) is defined as follows: k = 1 – a^2 / b^2 Where a and b are the half axes of the equivalent elliptical surface (a is the radial half axis and b is sagital half axes). For a normal prolate eye b > a and therefore k > 0. A spherical eye would have k = 0 and and oblate eye would have k < 0 The Eccentricity of an ellipse is the standard mathematical definition: E = {1 - Min(a,b)^2 / Max(a,b)^2}^(1/2) Note that eccentricity does not give an indication of whether the eye is oblate or prolate. There are numerous ways of determining the best fit equivalent elliptical surface. All of these are approximations because real eyes are rarely symmetrically elliptical. The Medmont E300 finds the ellipse that gives the same apical curvature and curvature at a specified chord. We have found in practice that this method gives repeatable and reliable shape factor readings. Finding the ellipse that has the same apical curvature and sag height at a given chord is another method - however
