The ability to see is an amazing process made possible by many parts of the eye working together. For a person to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina. The retina begins the process of using the optic nerve to convert those light rays into images the brain understands, similar to how a camera takes a picture. The cornea and lens in the eye serve as the camera lens. The retina is like the film. If the image is not in focus, the end result is a blurred image. When this happens, eye care professionals term it a refractive error.
The prescription on a patient’s eye is measured in units called diopters. Diopters represent the amount of correction needed to allow the patient to have the best distance vision. If an eye is farsighted (hyperopic), a convex or plus lens is used. In addition, combinations of lenses are used to correct astigmatism. The final result of the measurement is referred to as a refraction or refractive error.
Myopia (nearsightedness). Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface.
Those with myopia see distant objects blurred. Near objects, however, can be focused clearly. Nearsighted people have difficulty reading highway signs and seeing other objects at a distance, but can see for up-close tasks such as reading or sewing. Nearsighted people often have headaches or eyestrain and might squint or feel fatigued when driving or playing sports.
Hyperopia (farsightedness). This vision problem occurs when light rays entering the eye focus behind the retina, rather than directly on it. The eyeball of a farsighted person is shorter than normal. For those with hyperopia, the nearer an object is, the blurrier the image gets.
Farsighted people sometimes have headaches or eyestrain and may squint or feel fatigued when performing work at close range.
Many children are born with hyperopia, and some of them "outgrow" it as the eyeball lengthens with normal growth.
Astigmatism. An irregularly shaped cornea is the common cause when an eye is astigmatic. With astigmatism, the surface of the cornea is shaped like an egg rather than a spherical ball so the eye does not have one point of focus.
Astigmatism is the most common vision problem and it may accompany nearsightedness or farsightedness.
Presbyopia occurs when a patient has increased difficulty bringing near objects into focus (such as during reading). During middle age, usually beginning in the 40s, people experience blurred vision at near points, such as when reading, sewing or working at the computer.
When people develop presbyopia, they find they need to hold books, magazines, newspapers, menus and other reading materials at arm's length in order to focus properly. When they perform near work, such as embroidery or handwriting, they may have headaches or eyestrain or feel fatigued.
It is a natural change as a person ages. The inability to focus up close develops because the crystalline lens becomes stiff with age, so it cannot change shape easily. Remember, changes in lens shape allow changes in focus. Those who have nearsightedness, farsightedness or astigmatism corrected with lenses or surgery can still expect to experience presbyopia as their eye ages.