For you to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina, a layer of light sensing cells that lines the back of the eye. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images. This process is very similar to the way a camera takes a picture. The cornea and lens in your eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the retina or film receives a blurry image. This condition in the human eye is known as a refractive error. There are three types of refractive errors that can be corrected or reduced by refractive surgery. .
Myopia or nearsightedness is a refractive error that causes poor distance vision. If your eye is too long, or your cornea has too much focusing power, images focus in front the retina. If you have myopia, light rays have past the correct focal point by the time they reach the retina. The retina then sends this “over-focused,” blurry image to the brain. This condition affects over 25% of all people in the United States.
Hyperopia or farsightedness is the opposite of myopia. Distant objects are clear and close up objects appear blurry. This condition is a result of a eye that is too short or a cornea that lacks the necessary refractive power to focus images on the retina. If you have hyperopia, images focus on a point beyond the retina. This unfocused image captured by the retina is then sent to the brain and processed as an unclear picture.
Astigmatism is a condition which blurs and distorts both distant and near objects. A normal cornea is round with even curves from side to side and top to bottom. If you have astigmatism, your cornea is shaped more like the back of a spoon, curved more in one direction than in another. Light rays have more than one focal point and focus ondifferent areas of the retina.
(Aging eyes): Presbyopia is a normal condition after the age of 40, the lens becomes more rigid and can´t change shape as easily as it once did, it is more difficult to read at close range.
Eye glasses or contact lenses are the most common methods of correcting refractive errors. They work by refocusing light rays on the retina, compensating for the shape of your eye. Refractive surgery is also an option to correct or improve your vision by permanently changing the shape of the cornea, adjusting the eye´s focusing ability.