Schematic representations of corneal recontouring by the excimer laser. A, Correction of myopia by flattening the central cornea. B, Correction of hyperopia by steepening the central corneal optical zone and blending the periphery. C, Correction of astigmatism by differential tissue removals 90 degrees apart. Note that in correction of myopic astigmatism, the steeper meridian with more tissue removal corresponds to the smaller dimension of the ellipse. D, In LASIK, a flap is reflected back, the excimer laser ablation is performed on the exposed stromal bed, and the flap is then replaced. The altered corneal contour of the bed causes the same alteration in the anterior surface of the flap.