Why You Need Glasses or Contacts

Sight is our most precious sense. Our eyes enable us to take in the surrounding world. Without sight, the way we perceive the world would be forever changed. No wonder the eyes are often elevated in literature, art, religion, and philosophy to symbolize everything from the windows of the soul to supreme wisdom. Indeed, the eyes are a marvel of mechanics, performing many complicated functions in a very short time — in the blink of an eye, you might say.

However, changes within the eyeball may occur, resulting in impaired vision. Objects that we once viewed with crystal clarity may become blurred or distorted. To better understand how vision may change, let's first examine how vision normally happens.

How Vision Happens

You may have heard the comparison between a camera and the human eye. Just as a camera takes in light and transforms it into an image on film, your eye does virtually the same thing, only the "film" is your retina and your brain "develops" the image. We see objects when light, which is reflected by the objects, passes through the lens of the eye and strikes the retina, at the back of the eye. Our brains then interpret the shapes, colors, and dimensions of the objects we see.

A clearly focused object is the result of normal vision. However, just as an improper focus of light entering a camera lens will blur a photo, if light entering the eyeball does not focus properly on the retina, the result is blurred vision.

It is helpful to think of seeing as six distinct steps, outlined below, which are roughly similar to the way a camera processes an image (Figure 1).

Light Reflects Off an Object

Suppose you are on a tropical beach, looking at a palm tree as its fronds wave in the ocean breeze. You're actually seeing the light from the sun reflecting off the leaves. That is why you can't see in the dark: there is no light present to reflect off objects and enter your eye.

Light Enters the Eye through the Cornea

The light reflected from the leaves reaches your eye. It first enters the cornea. The cornea is the part of your eye you touch when you put in a contact lens. It is a transparent dome, the size of a dime and as thick as a credit card. The cornea provides most of the eye's focusing power, so small changes in its curvature can make an enormous difference in how clearly you see objects. The curved surface of the cornea bends the incoming light so that the rays converge together, instead of remaining parallel as they enter the eye. Thus, the cornea does the initial focusing work of the eye.

Light Passes through the Pupil

After passing through the cornea, the light reaches the pupil. The pupil is a hole in the iris, which is a thin layer of tissue in the middle of the eye. Muscles in the iris constrict or dilate the pupil to let the optimal amount of light pass through into the eye's interior, just like the f-stop in a camera.

Light Is Focused by the Lens

The light that passes through the pupil then enters the lens of the eye. The lens is behind the iris. It rapidly adjusts its focus for distance or near vision. The lens provides the final focus to create a clear image for the retina.

Light Reaches the Retina

The light reaches the back of the eye, where the retina lies. If you have normal vision, there is a clear image of the autumn leaves on the retina. The retina's photoreceptor cells, the rods and cones, convert the image of the leaves to signals that the brain can understand.

Light Signals Are Interpreted by the Brain

Finally, the signals are carried to the brain through the nerve bundle at the back of the eye — the optic nerve, which consists of millions of nerve fibers. The brain receives and interprets the signals, and it is at that point that you actually see.

Refractive Errors: Nearsightedness, Farsightedness, and Astigmatism

The eye is a marvelous, but complicated, part of our bodies. As you age, a variety of problems with vision may arise. These problems fall into two classes: problems that can be corrected with glasses or contacts and problems that can't be. If your vision can be corrected with glasses and contacts, your problem is called a refractive error, and we can usually get rid of your glasses and contacts with the appropriate surgical procedure. Refractive errors are focusing problems in the eye. If glasses and contacts don't correct your vision, you have a bigger problem, and you may need a different kind of surgery, such as cataract surgery.

If you have cataracts, you might want to consult my book, Cataract Surgery: A Patient's Guide to Cataract Treatment. The book you are reading now is mainly concerned with the first class of problems, refractive errors, and how to correct them permanently.

How well you see is determined, for the most part, by how accurately your eyes are able to focus, or refract, light. In a normal eye, the light that enters the eye is refracted to a precise point on the retina (Figure 2). Unfortunately, this precise focus often does not occur. The result is various forms of blurry vision due to refractive error. If you have a refractive error, you need glasses or contacts to see clearly. There are two basic types of refractive error: myopia] and hyperopia. Each of these can occur alone or in combination with astigmatism.

Myopia (Nearsightedness)

Also known as nearsightedness, myopia is a condition in which you can see nearby objects clearly, but objects at a distance appear blurred. This happens when light reflecting from a faraway object enters the eye and comes to a point of focus too soon, before it reaches the retina (Figure 3). Myopia may be due to any one of three causes. First, the cornea may have too much curvature, which causes the light to bend too much and focus in front of the retina. Second, the lens may be too strong, again focusing the light too quickly so the focus occurs before the retina. Third, the eyeball can be too long — the retina is too far back for the combined focusing power of the cornea and lens. Each of these three possible causes of myopia results in light coming to focus in front of the retina.

In general terms, nearsightedness can be corrected surgically in two ways: either move the retina forward toward the front of the eye, or move the focus of light back toward the retina. The retina can be moved forward by resecting a strip of sclera around the equator of the eye and sewing the remaining front and back halves of the eye back together. This operation was invented in Russia and is risky, so it has never been done in the United States (fortunately).

The other method is to reduce...