A test to determine the curvature of the central 3 millimeters of the cornea. The cornea is like a cooking bowl sitting face down on your eyeball. If the bowl is deep, the curvature is very "steep" and if the bowl is shallow the curve is said to be "flat." For eyes with astigmatism the cornea is more curved in one direction than in another. The shape of the cornea is an important component in a person's clarity of vision, and an accurate reading of this curvature is critical to the success of refractive surgery. Optical surfaces should be smooth curves, ideally a parabola, but since eyes can be anything from spherical to oblate to toroidal the dey here is to have a regular astigmatic or smooth surface.