Light passes through four major structures as it passes through the eye. These are the cornea, the pupil, the lens and the retina.

The very front surface of the eye is called the cornea. This clear layer acts like a fixed focus lens and is responsible for 70 percent of the total focusing ability of the eye. This is the most important layer in refractive surgery.

Behind the cornea is the colored part of the eye called the iris. The pupil is part of the iris. The pupil can change size to respond to different light situations, opening in darkness and closing in bright light. This helps to control how much light enters the eye.

Behind the pupil is the crystalline lens of the eye. This lens can change focus to respond to different focusing distances. This lens is the part that is affected in cataract formation or in presbyopia (loss of reading ability with age).

The back surface of the eye is the retina. This functions much like the film in your camera. The retina receives images from the cornea and the lens and sends the images to the brain.

To see an image clearly, light first passes through the cornea where it is partially focused. It then passes through the pupil and through the crystalline lens which adjusts its focus for the object's distance. This focused image is projected onto the retina and sent to the brain for interpretation. A normal eye is called emmetropic, meaning no correction with glasses or contact lenses is necessary. The image is very clear and sharp; no blurriness is noted.

There are four major refractive conditions that can be responsible for an image not coming into focus on the retina, or being blurry: myopia or nearsightedness, hyperopia or farsightedness, astigmatism and presbyopia. More than one condition can be present in the same eye. To understand each of these conditions, click on the highlighted links.