What are errors of refraction?
A situation where a clear image is not formed on the retina. Myopia, astigmatism, hypermetropia all types of errors of refraction. Most of errors of refraction are very small changes in the eye, often in the order of fraction of millimeters. You only have to look at the difference in thickness in the center of a contact lens and the periphery to begin to appreciate how small the physical problems really are.