The cornea is the clear front surface of the eye that bends (or refracts) light rays as they enter the eye. For clear vision, the cornea must have the correct shape and power to focus incoming light rays precisely on the retina at the back of the eye.
If the cornea is too steep, too flat or irregularly shaped, it cannot bend light at the angle needed to focus on the retina. In a nearsighted eye, for example, the cornea's shape causes incoming light rays to focus in front of the retina. Eyeglasses or contact lenses can refract the incoming light rays at the angle needed for clear vision.
All corneal modification procedures alter the curvature of the cornea so that incoming light is refracted at an angle that allows it to focus precisely on the retina. Such a procedure may eliminate the need for eyeglasses or contact lenses or reduce the power of the prescription.
People who are interested in any of the corneal modifications should first have a comprehensive eye examination to evaluate their eye health and determine their vision needs.