Definition and Purpose

myopia
If the radius is too small, rays from infinity are bound before they even get to the retina. In this case the person can see close things without adjusting the lens, the working point is somewhere at near vision. This situation is called nearsigthedness (myopia). Moving the working point back to infinity means that the radius must be increased, so that the lens gets less steep and the rays are less broken. Refractive Corneal Surgery would try to flatten the cornea in this case.

hyperopia
Far-sightedness (hyperopia) means that the radius is too big, that means the rays are broken too loose, they combine behind the retina. In adding additional lenses the overall refraction radius becomes shorter and rays will be bounded in the retina. Here, Refractive Corneal Surgery would try to steepen the cornea.

astigmatism
It means that light rays are not broken uniformly. This can be due to the eye's lens or more often to a non-spherical cornea. In most cases astigmatism is regular, that is to say there exist two uniform axis: one at refractive minimum and the other at refractive maximum. Again, if the axis of the maximum is at vertical position and the axis of the minimum is at horizontal position, it is an (regular) astigmatism with the rule. Any regular astigmatism can be corrected with prosthetic devices. Irregular astigmatism is seldom and is either sign of some corneal disease, most likely a keratoconos, or due to corneal surgery. Irregular astigmatism is difficult to correct. Best results are obtained with rigid contact lenses.