The key to understanding LASIK, the most advanced laser vision correction procedure, is to understand how the eye works. Sharp, clear vision is the result of light rays passing through the cornea, pupil and lens and focusing directly upon the retina. If the cornea is not round, or it is too steep or too flat in relation to the length of the eye, light rays focus either in front of or behind the retina resulting in "refractive errors" such as nearsightedness, farsightedness and astigmatism. Laser vision correction improves the way the eye focuses light by modifying the curvature of the cornea.