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Cornea: The cornea is sometimes referred to as the "window of the eye". It provides most of the focusing power when light enters your eye. The cornea is composed of 5 layers of tissue. The outer layer ( the epithelium), is the eye's protective layer. This layer is made up of highly regenerative cells that have the ability to grow back within 3 days, and therefore, allow for fast healing of superficial injuries. Most of the inner layers provide strength to the eye.

Pupil: The pupil is the 'black circle' that you see in a person's eye. 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.

Iris: This is the colored portion of a person's eye ( i.e., blue/green/brown/hazel). The primary function of the iris is to control the size of the pupil. This is achieved through contraction or expansion of the muscles of the iris.

Lens: The lens is the clear structure located behind the pupil. Its primary function is to provide fine-tuning for focusing and reading. The lens performs this function by altering its shape. At about the age of 40-50, the lens becomes less flexible and presbyopia sets in. At about the age of 60 or 70, the lens may become cloudy and hard (cataract formation), preventing light from entering the eye.

Retina: The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain.

Optic Nerve: The optic nerve carries images from the retina to the brain.

Sclera: This is the 'white part' of an eye. The sclera's purpose is to provide structure, strength and protection to the eye. Vitreous: This is the clear 'gel like' substance located inside the eye's cavity. Its purpose is to provide a spherical shape to the eye. The vitreous may develop small clumps known as 'floaters', which are more common in nearsighted people than in the rest of the population.

Visual refractive disorders of the eye:

Diopters: Visual refractive disorders of the eye (i.e. myopia, astigmatism, hyperopia and presbyopia) are measured in units called 'diopters'. Diopters represent the amount of correction needed in corrective lenses to normalize vision. The more nearsighted or farsighted you are, the higher your prescription in diopters.

Your prescription is written in three numbers: 1st Number; Identifies your degree of nearsightedness or farsightedness. The sign identifies whether you are nearsighted (-sign) or farsighted (+sign) 2nd Number; Identifies your degree of astigmatism. The sign can be either a plus or minus. 3rd Number; is the axis which indicates the direction of your astigmatism.

For example, -5.00 -1.50 X 180 represents a typical prescription.

Myopia: More than 70 million people in North America are nearsighted. Myopia is the medical term for nearsightedness. Myopia occurs when an eye is too long for the cornea's curvature. Light rays entering the eye do not come to a sharp focus on the retina at the back of the eye. Instead, they focus further forward, producing a blurred image.

The term 'nearsighted' means that you can see 'near' objects clearly without your glasses, but objects further in the distance are blurry. There are varying degrees of myopia or nearsightedness. The more myopic you are, the more blurred distant objects appear, the higher your prescription in diopters and the thicker your glasses. Of all myopic people, about 90% have corrections less than - 6.00 diopters.

Mild Myopia; Less than - 3.00 diopters. Moderate Myopia; -3.00 to -6.00 diopters. Severe Myopia; -6.00 to -9.00 diopters. Extreme Myopia; more than - 9.00 diopters.


Astigmatism: Many patients with myopia have some degree of astigmatism or ovalness to their corneas. Astigmatism occurs when the cornea is shaped more like a football than a basketball. As a result, patients with astigmatism experience distortion or tilting of images because of unequal bending of light rays entering the eye. Patients with high degrees of astigmatism have blurred vision not only for distant objects, as with myopia, but for near objects as well. Astigmatism is also measured in diopters. Of all myopic people, 50% have astigmatism. Most of these people have corrections of less than 1 diopter.

Mild Astigmatism Less than 1.00 diopter Moderate Astigmatism 1.00 to 2.00 diopters Severe Astigmatism 2.00 to 3.00 diopters Extreme Astigmatism More than 3.00 diopters

Hyperopia: Hyperopia is the medical term for 'farsightedness'. It occurs when an eye is too short for the cornea's curvature. Light rays entering the eye focus behind the retina and as a result a blurred image is produced.


Presbyopia: Presbyopia is the normal process of aging, in which the natural lens of the eye loses some of the flexibility that characterizes a younger eye. This usually occurs at age 40-50. Everyone experiences presbyopia, some sooner, some later. Because of this normal process, nearsighted people begin to wear bifocals and those who never needed glasses may require reading glasses.

copyright Scott L. Jaben, M.D.