Chapter 3 erected the relationship between refraction error and cornel ablation depth in theory . the paper proposed various mathematical models which can be used for proceed the simple myopia , simple hypermetropia , compund myopia astigmatism , compund hypermetropia astigmatism , simple myopia astigmatism , simple hypermetropia astigmatism , mixed astigmatism . the influence of the astigmatic axis and the degree of astigmatism on the correction field is discussed . wavefront aberration guided excimer laser cornea ablation is a main method of customised corneal ablation