Refractive Errors

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How the eyes process light

Rays of light come to our eyes from different sources such as the sun, light bulbs, fire, and so forth . Light rays travel through space until they hit an object such as a dog or a house. If the object is opaque, the light is either absorbed or reflected. The color we see is determined by the color of the light bands reflected from the object.

If the light rays hit something that is transparent such as water, glass, or the cornea of the eye, the light rays pass through it and their path is bent or refracted. Refraction is the bending of light as it passes from one transparent substance to another.

If a person looking at an object has normal vision, the light rays reflected by the object are bent by the cornea and lens until they come to a single focal point on the retina. If the image is blurred, there is probably something wrong with the way the light was bent or refracted; this is called a refractive error and can be corrected with glasses. The refractive error may be due to the shape of the cornea, the power of the lens, or the length of the eyeball. If all three of these factors are accurate, the light rays will be focused on the retina properly and the person will see clearly. However, if there is some variation in any one factor such as the cornea being too steep or too flat or the eyeball is shorter or longer than normal, the person may have a focusing problem.

Accommodation is the process the eye uses to focus for near vision. Binoculars and cameras can’t change their viewing distance without a dial or lever being turned. The "dial" in the eye is a ring-shaped muscle located around the edge of the lens inside the eye. This muscle is part of the ciliary body. When the muscle contracts, it makes the ring smaller, which makes the lens become thicker. The thicker lens has more power and can bring close-up objects into focus for clear viewing. When the ciliary muscle relaxes, the lens becomes thinner, and the person can view distant objects clearly.

At the same time the eyes bring an object into focus, they turn in or converge so both eyes are looking at the object. Convergence and accommodation work hand-in-hand so the eyes work together and see objects clearly.

Why do some people need glasses and others don’t?

Our ability to see depends in part on the shape of our eyes. To be able to see, our eyes must bend or "refract" light to focus it on the retina. Both the cornea and the lens refract light rays before they reach the retina. In the normal eye, light rays reach their sharpest point of focus on the retina. If the eye doesn’t refract light properly, you have a refractive error. Light reaches its sharpest point of focus somewhere in front of the retina or at a point located behind the retina. In either case, the image the person sees is blurred because it isn’t coming into focus on the retina. Refractive errors are eye disorders, not eye diseases.

Myopia

Myopia is better known as nearsightedness. People who are nearsighted see objects located close to them more clearly than they see objects far away . Myopia occurs when the eyeball is too long or the cornea is too curved. In other words, the cornea and lens in combination have too much refracting power. Light rays coming into the eye focus somewhere in front of the retina instead of on the retina. Distant objects appear blurred.

More than seventy million people in North America have myopia, which is inherited. Nearsightedness often becomes evident between the ages of six and sixteen, and it generally worsens during the teen years as the body grows rapidly. It is sometimes called "school age myopia" because it develops during the school years. Students with myopia will probably need to change their glasses every year or two.

As children become older, their vision is usually challenged more. They need better vision to see more details in a world that seems to keep on expanding and expanding. The printing in their textbooks and on blackboards gets smaller. The balls they play with not only get smaller, they can hit them farther. They begin to need glasses that weren’t required earlier in life

Sometimes myopia is picked up during eye screening tests at school. Or, the parent may notice the child is squinting or squeezing their lids closed to see better. Squinting takes advantage of the optical principle called the "pinhole effect". Viewing things through a small opening like a pinhole or nearly closed eyelids makes out-of-focus images appear somewhat clearer.

Glasses for myopia have concave lenses. In other words, they are thinner in the center than they are on the edges. They don’t cure or eliminate the cause of the myopia. Instead, they bend the light rays from distant objects so they focus on the retina. People with myopia will be able to see better only when they have their glasses on. Their vision will not be any better when they remove their glasses.

Hyperopia

Hyperopia is popularly known as farsightedness. You might think farsighted people can see objects far away better than those close up, but it isn’t that simple. Most children with hyperopia see things equally well or poorly without their glasses. Hyperopia occurs because the eye is too short or the cornea is too flat. You might say the cornea and the lens have too little refracting power. Light rays entering the eye haven’t reached a focal point by the time they reach the retina, and objects close to a person suffering from hyperopia appear blurred. Hyperopia is usually inherited.

Farsighted people, like nearsighted people, avoid blurred vision by accommodating. With accommodation, there is a fixed degree of convergence or turning of the eyes. Often during this process, one of the eyes turns in slightly while the other eye looks straight ahead. The condition in which the eyes look in two different directions is called strabismus. Not all children with hyperopia have strabismus, and not all children with strabismus have hyperopia, but the crossing of one eye is often a sign of significant farsightedness. Usually it occurs about age two when the child begins to accommodate to see fine details.

Glasses for hyperopia are thicker in the center, which makes the person’s eyes appear larger. Glasses for hyperopia tend get a little stronger until about ages six or seven, then a little weaker until the child stops growing.

Astigmatism

The cornea of people who suffer from astigmatism is shaped like a football. Their corneal curve is steeper in one direction than the other. Like the back of a spoon, the cornea has two different curvatures. In people who have astigmatism, the light rays reach a focal point at multiple spots in front of and behind the retina, distorting both their near and far vision . Light doesn’t come to a single focal point; it is more like a focal circle, and the image is still blurred no matter how close a person gets to it. Images appear to be blurred or ghost-like because light rays are refracted unequally. In most cases, images, both near and far, appear blurred.

Many people have both myopia and astigmatism, and some have both hyperopia and astigmatism. Almost everyone has some degree of astigmatism, but it is not severe enough to cause a problem. Blurred vision is the only symptom.

The front surface of the lenses used to correct astigmatism won’t be symmetrical. However, the asymmetry is so slight it won’t be noticed looking through the glasses. The degree of astigmatism someone has tends to remain about the same throughout a person’s life.

Presbyopia

Presbyopia, also known as "old man’s eyes," occurs as a result of aging. The person’s natural lens hardens and is less flexible, and the patient's ability to accommodate begins to decrease. Therefore, middle-aged people are unable to focus their eyes as well as they did earlier in their lives. Presbyopia is sometimes confused with hyperopia. However, the presbyopic eye doesn’t possess a refractive error such as in hyperopia; it simply doesn’t adjust to nearby images by increasing its refractive powers. Presbyopia begins between the ages of thirty-eight and forty-five and is experienced by nearly everyone by age fifty.

Generally, sufferers of presbyopia need to use some of kind of glasses to help them see things close to them. Many people select reading glasses — weak magnifying glasses that are thicker in the center than on the edges. Others select bifocals which combine reading glasses with lenses for distance vision for nearsighted people, or with clear glass for the person who doesn’t have any refractive problems. Most people will need to get stronger reading glasses as time goes on and their power to accommodate decreases. At about age sixty all accommodation is gone, and the person won’t need to change his or her prescription any more.

Determining the right prescription for glasses

In addition to looking for diseases that affect the eyes, doctors perform eye examinations to find out the type of refractive error a person has and the amount of refractive error. Refraction determines how a person’s eye bends light rays. The doctor uses the results of an exam to determine what kind of lenses will correct the error in the person’s optical system and make the light rays focus correctly on the retina.

Measuring refraction and visual acuity aren’t the same thing. Visual acuity is the smallest letter or figure that can be seen, while refraction determines the prescription for the glasses.

The eye professional uses a refractor to determine refractive error. The refractor is a device with built-in lenses. The dials are moved to change the lenses. Some refractors determine refractive errors automatically, then they are fine-tuned by asking "which is better, one or two."

In very young children, the doctor uses a retinoscope to determine refractive error. The amount of error can be determined without any response from the patient. The retinoscope sends a streak of light into the pupil through all the optical surfaces. The light is reflected back from the retina. If a refractive error is present, the appearance of reflected light won’t be quite right. The eye professional will hold different lenses in front of eye until the reflected light is right. The lens that makes the streak of light right identifies the type of refractive error and the strength of lenses needed to correct the refractive error. The retinoscope is very accurate.

Doctors use drops to dilate the pupils. It lets them perform a thorough examination of the interior of the eye with an ophthalmoscope. The eye drops temporarily paralyze the ability of the eye to accommodate. If the paralysis of the eye didn’t occur, the focus of the eye would change constantly, making measuring the eye’s refracting abilities inaccurate.

Measuring of refractive error

Refractive index is measured in diopters. A diopter is a unit based on the focal length (power) of a lens. A person with one diopter of myopia sees about 20/50 without corrective lenses. This means they see objects at twenty feet that people with normal vision see at fifty feet. For people who have one diopter of myopia, objects would begin to blur at three to four feet and they would need glasses for board work, movies, and driving. A person with three diopters of myopia would have 20/400 vision without correction. He or she would need to wear corrective lenses all the time. Without glasses, these people would only be able to see about one foot without a blurring of their vision.

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