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The human eye is the organ that gives us the sense of sight, allowing us to learn more about the surrounding world than any of the other five senses. We use our eyes in almost everything we do, whether reading, working, watching television, writing a letter, driving a car, and countless other activities. Sight is the most precious of the five senses. The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or give off. The eye is able to see in dim light or bright light, but it cannot see objects when light is absent. The eye changes light rays into electrical signals then sends them to the brain, which interprets these electrical signals as visual images. The eye is set in a protective cone-shaped cavity in the skull called the orbit or socket and measures approximately one inch in diameter. The orbit is surrounded by layers of soft, fatty tissue which protect the eye and enable it to turn easily. Six muscles regulate the motion of the eye. Among the more important parts of the human eye are the iris, cornea, lens, retina, and the optic nerve. Normal Vision (emmetropia)– In an eye with normal vision, light is bent (refracted) by the curve of the cornea and lens to focus clearly on the retina. This results in objects looking crisp and clear.

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Nearsightedness (myopia) – A condition where either your cornea is excessively curved or the eyeball is too long. Light rays cannot reach far enough into the eye to focus on the retina. This causes objects near the eye to be in focus, but those at a distance appear blurry. There are varying degrees of myopia. The more myopic you are, the more distant objects appear blurred, the higher your eyeglass prescription and the thicker your glasses are. The severity of myopia can be classified as follows: Myopia (Nearsightedness) Mild Less than -3.00 diopters Moderate -3.00 to -6.00 diopters Severe -6.00 to -9.00 diopters Extreme More than -9.00 diopters

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Farsightedness (hyperopia) – A condition where either your cornea is too flat or the eyeball is too short. This causes light rays entering the eye to be focused behind the retina instead of directly on it. This makes objects in the distance appear clearer than objects near the eye. Some farsighted individuals, however, can use their focusing muscles to “pull” the image forward onto the retina. Once presbyopia sets in, around age 45, the focusing muscles of the eye get weaker and farsightedness seems to get worse – distant objects are seen more clearly than near objects. The severity of hyperopia can be classified as follows: Hyperopia (Farsightedness) Low Less than +2.25 diopters Moderate +2.25 to +5.00 diopters High More than +5.00 diopters

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Astigmatism – This occurs when the cornea’s shape is oblong like a football instead of round like a basketball. This causes an image to focus at two focal points, resulting in blurry vision. Astigmatism is the most common refractive condition and occurs to some extent in most eyes. The severity of astigmatism can be classified as follows: Astigmatism Mild Less than ±1.00 diopters Moderate ±1.00 to ±2.00 diopters Severe ±2.00 to ±3.00 diopters Extreme More than ±3.00 diopters

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Presbyopia – The loss of “accommodation” of the eye. This comes from a loss of flexibility of the lens inside the eye, as well as some weakening of the focusing muscles inside the eye. As people age, their eyes gradually lose the ability to change (accommodate) focus from distance to near. Although this is a gradual change throughout life, it usually is not noticeable until the mid-forties. Refractive surgery may not correct all vision problems that occur naturally with aging. If you are over 40 years old and have both your eyes fully corrected for distance vision, you will still need reading glasses for near work (i.e., reading). As an alternative to reading glasses for near vision, you may elect to leave one eye slightly nearsighted – this is called monovision.

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Monovision – The technique in which one eye is fully corrected for distance vision, and the other eye is under-corrected, or left slightly nearsighted so that eye has good reading vision without glasses. Monovision lets you maintain reading vision in one eye, to help avoid total dependence on reading glasses. Most presbyopic patients enjoy the convenience of not to having to constantly reach for reading glasses. It is best to evaluate the suitability of monovision for your situation in contact lenses with your eye doctor before having laser vision correction.

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EYE ANATOMY

Sclera – The white fibrous tissue that forms the outer wall of the eye. It provides structure, strength and protection to the eye, except in the front where clear cornea tissue takes its place, allowing us to see. Also called sclerotic, sclerotic coat. Iris – The round colored part of the eye (i.e., brown, blue, green, etc.)The primary function of the iris is to control the size of the pupil. Cornea – The clear dome on the front of the eye that covers the pupil and iris. The cornea accounts for the majority of the refracting (bending) of light rays that enter the eye and provides most of the focusing power for the eye. The cornea is composed of 5 layers of tissue, and is 550 microns (half of a millimeter) thick on average. The outer layer (the epithelium) is the eye’s protective layer. Most of the inner layers provide strength to the eye. The back layer (the endothelium) keeps the cornea clear by pumping excess fluid out. When the curvature, or “steepness,” of the cornea does not match the length of the eye, light rays do not focus clearly on the retina, and glasses or contact lenses are required to correct nearsightedness or farsightedness. The cornea is ideally shaped like a basketball, but if it is shaped more like a football, this creates astigmatism, which also requires correction with glasses or contacts. These conditions can also be corrected with a vision correction procedure, such as LASIK. Pupil – The black circular opening in the center of the iris of the eye, through which light passes to the retina. The primary function of the pupil is to control the amount of light entering the eye. When you are in a bright environment, the pupil becomes smaller to allow less light through; when it is dark, the pupil expands to allow more light to reach the back of the eye Lens – The clear body located inside the eye behind the iris and pupil that helps focus light rays entering the eye to form an image on the retina. It's primary function is to provide fine-tuning for focusing and reading. The lens can change shape to focus on near and distant objects. Around the age of 40 or 50, the lens loses this ability to change shape easily, resulting in the need for reading glasses. Conjunctiva – The mucous membrane that lines the inner surface of the eyelid and the exposed surface of the eyeball. Vitreous – A clear, jelly-like substance that fills the middle of the eye, in between the lens and the retina. This body provides a spherical (round) shape to the eye. Choroid – The dark-brown vascular coat of the eye between the sclera and the retina. Optic Nerve – The nerve that connects the retina to the brain; it transmits information to the brain about what the eye is seeing. Macula – A minute yellowish area containing the fovea centralis located near the center of the retina of the eye, at which visual perception is most acute. Retina – A thin light-sensitive membrane lining the inner eyeball. It captures the image formed by the cornea and lens, much like film in a camera. It is connected to the brain by the optic nerve.

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