How the Eye Sees

Why People Need Glasses: Nearsightedness, Farsightedness & Astigmatism

Myopia (Nearsightedness)

Myopia, or nearsightedness, is the condition in which vision is better at near than at a distance. This situation results whenever light rays are focused in front of the retina, rather than at the normal location that is on the retina. In most individuals this is caused by a cornea that is too steeply curved and/or an eye that is too long. Myopia usually begins in the early teenage years, and reaches its full extent by the early twenties. Most individuals simply wear glasses to correct their vision, although many wear contact lenses.

Although nearsightedness tends to run in families, its pattern of heredity is very complex. There is no good evidence that there is anything that an individual can do to prevent this condition, or to make themselves more or less nearsighted. Therefore, how you use your eyes, whether or not you wear glasses or contact lenses, take vitamins, etc. will not make your nearsightedness better or worse. Nearsighted individuals who are in their mid-forties or older can often read without using glasses. However, if laser vision correction is performed to improve distance vision without glasses, glasses may then be needed for reading.

Myopia or nearsightedness. This diagram shows how light rays from far away are focused in front of the retina in myopia. This results in blurred vision for distant objects, but clear vision for near.

Back to top

Farsightedness

Farsightedness, or hyperopia, is present when distance vision is better than near vision.

This does not necessarily mean that distance vision is clear in farsighted individuals. It may also be blurred, but it is clearer than near vision. In most individuals this is caused by a cornea that is too flat and/or an eye that is too short. Farsightedness can slowly increase throughout life in some individuals, but it usually does so very slowly over many years. One form of farsightedness, presbyopia, frequently develops at approximately 44 years of age. This causes individuals who have previously been able to see well without glasses to gradually lose their ability to read unless they wear glasses for this purpose. The cause of this condition is a natural hardening of the human lens. This hardening causes the lens to lose its ability to change shape, and it therefore cannot focus the light rays which enter the eye from a near object. Many individuals become so farsighted that they require glasses to see clearly in the distance as well as at near. If dissatisfied with glasses or contacts lenses, these patients often have laser vision correction, CK or lens implantation to improve their vision.

Farsightedness or hyperopia. This diagram shows how light rays from far away are focused behind the retina in farsightedness. This results in blurred vision for both distant and near objects.

Back to top

Astigmatism

Astigmatism also causes blurred vision but in a different manner. The most common cause of astigmatism is a non-spherical corneal curvature. In other words, although most people have a corneal whose shape is round (like a basketball), many people with astigmatism have a cornea that is shaped somewhat like a football. Light rays cannot be focused in one spot, and a certain amount of blurred vision is therefore always present. This condition is usually treated by prescribing glasses that are actually made in the shape of a football that is oriented in the opposite direction of the patient’s football-shaped cornea. Therefore, if the patient’s football shape is horizontal, the football shape in the glasses is made vertical. The net result of this is a basketball-type effect on the light rays that can then be focused clearly onto a single spot.

As a practical matter, most patients with astigmatism are also either nearsighted or farsighted as well. Therefore, their eyeglasses are usually made in such a manner as to correct both conditions. Laser vision correction can also reduce or eliminate astigmatism.

Measuring Eyeglass Strength

The strength, or prescription, of a contact or spectacle lens can vary greatly from person to person. In order to understand the accuracy and limitations of laser treatment, we must first discuss how lens strength is measured.

Lens strength is measured in units called “diopters”, and are given a numerical value. For those interested in such details, a one diopter lens causes parallel rays of light to be focused at a point that is one meter from the lens; a two diopter lens causes focusing to occur at 1/2 meter; a three diopter lens at 1/3 meter, etc. In addition to having different possible dioptic strengths, these are also different types of lenses. These are:

1. Concave shaped or “minus” lenses that are used for nearsightedness. These lenses cause light rays to move apart (“diverge”).
2. Convex shaped or “plus” lenses that are used for farsightedness. These lenses cause light rays to come together (“converge”).
3. Astigmatic lenses that are used to correct astigmatism. Astigmatic lenses may cause light rays to either diverge or converge, and are unique in that they do so to a varying degree through different sections of the lens.

 Back to top