As with any refracting surface, the cornea must be both symmetrical and regular in order to produce a good quality image on the retina. Due to trauma, surgery, or naturally occurring reasons, the cornea may lose its symmetry or become irregular. When this happens, normal eyeglasses and soft contact lenses no longer work as they once did. The problem occurs when the cornea changes shape in such a way that the power of the correcting lens cannot follow it as precisely as it should in order to neutralize those complex changes.
