The cornea is a part of  the eye that helps focus light to create an image on the retina. It works in much the  same  way that the  lens of  a camera  focuses  light to create  an image on film.  The  bending and focusing of light is also known as refraction. Usually the shape of the cornea and the eye are not perfect and the image on the retina is out-of-focus (blurred) or distorted.

These imperfections in the focusing power of the eye are called refractive errors.

There are three primary types  of  refractive  errors: They are myopia,  hyperopia  and  astigmatism.

Myopia,  or nearsightedness,  -more difficulty seeing  distant  objects as clearly as near objects.

Hyperopia, or  farsightedness,  -have  more difficulty  seeing  near objects  as clearly  as distant  objects.

Astigmatism -a distortion of the image on the retin  caused by irregularities in the cornea or lens of the eye.

Combinations of myopia and astigmatism or hyperopia and astigmatism are common.

In  LASIK  surgery,  precise and controlled  removal of corneal  tissue by a special  laser, reshapes  the cornea changing its focusing power.  LASIK  is a procedure that  permanently changes the shape of the cornea,  the  clear  covering  of  the  front of  the eye, by  using  an  excimer  laser.  A  knife,  called  a microkeratome,  is used to  cut a  flap in the cornea.  A hinge is left  at one end of this flap.  The flap is folded back revealing the stroma,  the middle section of the cornea.  Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced.