Corneal surfaces in eyes with regular astigmatism are thought to be toroidal surfaces . based on mathematical models for corneal surfaces before and after treatment , a corneal ablation model is given for such refractive errors as spherical myopia , compound myopic astigmatism , and simple myopic astigmatism . a similar corneal ablation model is also given for such refractive errors as hyperopia , compound hyperopic astigmatism , and simple hyperopic astigmatism
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