The planned correction is programmed into the computer of the excimer laser.

The computer controls the amount and distribution of the laser energy to reshape the cornea.

The laser acts to disrupt the bonds between cornea cells: minute amounts of the cornea are sculpted away - less than the thickness of a human hair.

The cornea is thinned in a particular pattern (directed by the laser's computer) depending upon what your vision error is. 

In myopia the center of the cornea is thinned, like cutting the hole in a donut.

The thinned cornea corrects the blurred vision from myopia, weakening the near sighted (too strong) eye.

click on the picture to see full size   myopic laser correction

click on the picture to see full size   hyperopic laser correction

In hyperopia the cornea around the central zone is thinned, like cutting the donut itself. This causes a steeper central cornea, strengthening the farsighted (too weak) eye.

In astigmatism an oval reshaping is performed to make the eye more round.  Astigmatism correction - before and after

click on the picture to see full size  laser astigmatism correction