By Dr. Charles J. Bock

Understanding Refractive Error

Refractive error is the term used to describe a patient's glasses prescription, whether it is farsighted or nearsighted, with or without astigmatism. All of these terms will be discussed below.

Hyperopia

Hyperopia, or farsightedness, refers to an eye in which the light rays entering the eye do not come into sharp focus before landing on the retina, which is the structure in the eye which senses light. As a result, the image the patient sees is blurred, and additional lens power is needed to focus the light entering the eye. People who are farsighted wear lenses with a positive number (a plus symbol in the first number of the prescription).

The vast majority of children are farsighted but do not need glasses. This is because most children are only a little bit farsighted and have strong focussing muscles in the eye that can help the lens of the eye bring images into focus. However, some children are very farsighted in both eyes, unequally farsighted in one eye versus the other, or have crossed eyes (called esotropia). These children need glasses to help them see better and, if the eyes are crossing, to help straighten the eyes.

Almost all children who need glasses for farsightedness need to wear them at all times and should do so unless you have been told otherwise.

Myopia

Myopia, or nearsightedness, refers to an eye in which the light rays entering the eye come into focus before landing on the retina, so that by the time they reach the retina, they are out of focus. In this sense, nearsighted eyes are "too long," and need glasses with negative power (a minus symbol in the first number of the prescription) to help decrease the natural focus of the eye so that images focus on the retina.

Myopia tends to run in families and becomes more common as children progress into adolescence. Many children who are only a little bit farsighted feel that they need their glasses only for activities such as seeing the board in school or watching television or a movie. For many of these children, part-time wear of the glasses is fine.

Astigmatism

Astigmatism exists when the cornea is not perfectly spherical (like the cross-section of a basketball) but is instead oval (like the cross-section of a football). No one's cornea is perfectly round, but the eye and brain can tolerate some imperfection. When the cornea is markedly "oval," corrective lenses are needed. The astigmatism correction is the second number in the prescription. Most children who have significant astigmatism need to wear their glasses full time.