How does my eye work?
The primary function of your eye is to focus light. You need glasses or contact lenses when your eyes cannot focus light properly.
Your eye works like a camera. Light rays enter the eye through the cornea, which provides most of the focusing power. Light then travels through the lens where it is fine-tuned to focus properly on the retina on the back of the eye.
The retina acts like film in a camera. Light is changed by the retina into electric impulses that are carried by the optic nerve to the brain. Light must be focused precisely on the retina for you to see clearly.
More than 70 million people in North America (about one in four) are nearsighted. Myopia is the medical term for nearsightedness. It occurs when your eye is too long in relation to the curvature of your cornea. Myopia causes light rays entering the eye to focus in front of the retina, producing a blurred image.
The term “nearsighted” means that you can see objects that are “near” to you more clearly than distant objects. The more myopic you are, the more blurred distant objects appear, the higher your prescription in diopters, and the thicker your glasses.
Hyperopia is the medical term for “farsightedness.” Hyperopia occurs when your eye is too short in relation to the curvature of your cornea. Light rays entering your eye focus behind the retina, producing a blurred image. Some farsighted people can use their focusing muscles to pull the image forward onto the retina, allowing them to see clearly. But others, who cannot overcome the effects of severe hyperopia, need reading glasses or bifocals.
Many patients with myopia and hyperopia have some degree of astigmatism, or ovalness, to their cornea. It occurs when your cornea is shaped more like a football than a basketball. As a result, you experience distortion or tilting of images due to the unequal bending of light rays entering your eyes. People with high degrees of astigmatism have blurred vision for both near and distant objects. Astigmatism is also measured in diopters.
Presbyopia is part of the normal process of aging and is not corrected with the laser. It develops as the lens of the eye loses some of the flexibility that characterizes a younger eye. Everyone experiences the effects of presbyopia, typicallly between the ages of 40 and 50. Nearsighted people who become presbyopic may require bifocals in their forties, and those who never needed glasses before may require reading glasses.
Mild myopia counteracts presbyopia. That is why, if you’re slightly myopic, you can remove your glasses and still be able to read, even after presbyopia sets in. After having laser vision correction, your myopia may be gone, and you will need reading glasses for fine print to correct your presbyopia like other normally sighted individuals. Since the excimer laser has no effect on your lens or your focusing muscles, it cannot be used to treat presbyopia.