It is common in Physiological Optics to fit the corneal and the lens surfaces
to conic-based surfaces (usually ellipse-based surfaces), obtaining their
characteristic radius of curvature and asphericity. Here we show that the
variation in radius and asphericity due to experimental noise is strongly
correlated. This correlation is seen both in experimental data of the corneal
topographer Pentacam and in simulations. We also show that the effect is a
characteristic of the geometry of ellipses, and not restricted to any
experimental device or fitting procedure.Comment: 5 pages, 5 figure
