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Myopia, Hyperopia, and Astigmatism

These are among the more fundamental principles of ophthalmic science, yet among those things most easily confused by the general public. It is not that these things are difficult to understand, just that they are often poorly explained by eye care practitioners, particularly when other disease processes may be in place.

In order to explain these three principles, we must begin with a simple foundation. The eye is set up like a hollow globe, like a basketball. The inner surface of the eye is lined with a very thin film called the retina, which functions as an optical processor, changing a light image and converting it to an electrochemical message. All of the nerves from the retinal cells conglomerate at the optic nerve head, and travel to the brain for processing. All other things functioning up to par, the retina must have a clear image on it in order to most effectively process the information.

So how does the eye insure that a clear image is focused on the retina? There are essentially two surfaces that bend light in the eye, the cornea and the lens. Ideally, the light rays will bend so that they focus right on the retina. Imagine using a magnifying glass. If you hold the glass just in the right spot, the image is focused, but move the lens too close or too far away, and the image blurs. Since the eye can not change length like this, the lens and cornea must be just the right power for that distance, or refractive errors (hyperopia, myopia, and astigmatism) will result.

Myopia:

This is also called near-sightedness. With myopia, the power of the eye is too great for the focal distance, so the image focuses in front of the retina. In order to compensate for this, minus power eyeglasses are prescribed (i.e. -2.50D). Someone who is near-sighted may be able to see objects that are close to him or her as clear, but this depends on the degree of near-sightedness. If you are not near-sighted, but would like to know what it is like, simply take a magnifying glass and hold it against your eye (with your glasses or contacts in). Everything will be blurry, just like it is for someone who is near-sighted.

Hyperopia:

This is also called far-sightedness, and is the exact opposite of myopia. In this case, there is not enough power in the eye, so the image focuses behind the retina. To compensate for this, plus lenses are prescribed (+2.50D). Some people who are far-sighted, particularly those with low amounts of far-sightedness or who are under age 40, will use the focusing ability of the lens in their eye to compensate. This can lead to headaches, eyestrain, or eye turns, so even though their vision without glasses may be 20/20, they still need glasses in order to relax this focusing system and optimize their visual functioning. In order to simulate far-sightedness if you are not far-sighted, find a friend who is near-sighted and borrow their glasses. Put them on over your glasses or contact lenses, and this is what far-sightedness is like.

Astigmatism:

In biological systems, there is virtually no such thing as perfection. For an object to be perfectly imaged in one plane on the retina, the optical system (cornea and lens) must be perfectly spherical. Oftentimes they are not perfectly spherical and are a little more squished in shape, like a football cut in half. In this case, there are two focus points for the light rather than one. This is called astigmatism, and is corrected with the cylinder portion of the eyeglass prescription (i.e. -2.50-0.50x180). About 85% of the population have some degree of astigmatism, and it affects vision both at distance and at near since the eye can not accommodate two focal points at once. To simulate this, use a magnifying glass held at the correct distance from a page of print, then tilt the lens at a 30-degree angle. You will see that the image blurs and distorts.

J. Hensil