A sphere has the same radius of curvature at every position, but due to spherical aberration the refractive power is not the same everywhere.When radii are directly converted into power values according to the laws of Gaussian optics the refractive effect, as it is called, is not taken into account. In a topography map calculated without considering the refractive effect a sphere with a radius of, e.g., 8 mm will show the same power of 42.2 D at every point of its surface.

By convention most keratometers use the keratometer calibration index of 1.3375 to calculate power from the anterior radius alone. In doing so they assume, firstly, that the cornea has a single refracting surface and, secondly, that the ratio between the radii of the posterior and anterior surfaces is constant (82.2%). For untreated eyes this calculation method is usually sufficiently accurate. After refractive corneal surgery, however, it is no longer possible to calculate corneal power based only on the anterior surface, as the ratio between the anterior and posterior radius of curvature of the cornea has changed considerably. In these cases the corneal radii of the posterior surface must be included in the analysis.

For historical reasons, most Placido topographers and keratometers use a refractive index of 1.3375 for calculating corneal power. However, this refractive index is actually incorrect, even for the untreated eye (n ≈ 1.332). Most intraocular lens (IOL) power calculation formulas use n=1.3375 to calculate the desired ‘K-reading’, necessitating empirical correction to obtain the correct IOL power even in normal cases.

For post-LASIK patients this empirical correction can no longer be used because it is now necessary to take the radii of curvature of both the anterior and the posterior surface into account (see B). When the curvature values are converted to power values using the refractive indices of air, corneal tissue and aqueous fluid the resulting power values can no longer be put directly into the IOL formulas. Since the IOL formulas already provide for an empirical correction this would result in a double correction. For this reason power values calculated by this method must first be converted into equivalent K-values before they can be put into formulas that are based on a refractive index of 1.3375.

Some modern formulas are able to deal with the actual, measured curvature data of the front and back surface of the cornea, however. Examples are the BESSt II formula by Dr. Edmondo Borasio, PhacoOptics by Dr. Thomas Olson and Okulix by Dr. Paul-Rolf Preussner.

Calculation of corneal power by ray tracing involves sending parallel light through the cornea. It must be taken into account that each light beam is refracted according to the refractive indices for air (n1 = 1), corneal tissue (n2 = 1.376) and aqueous tissue (n3 = 1.336), the slope of the surfaces, and the exact point of refraction. This is necessary because the principal planes of the anterior and posterior surface differ slightly from one another (corneal thickness).

The conventional method of calculating true net power does not consider the slight difference in location of the principal planes of the anterior and posterior surface.