The cornea is the major refractive element of the human eye， being responsible for 2/3 of the eye′s total refractive power. During the past ten years， the development of computerized corneal topography has provided a complete understanding of corneal shape， especially the anterior surface. Asphericity （Q） is an important parameter describing corneal shape， and is widely described in corneal topography. This overview focuses on the development of the calculation method used for Q， especially the difference in the calculation between the sagittal and tangential radii.
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