Refractive Error Revealed

What causes people to need glasses? What in the world is “hyperopia”? Why do kids suddenly have trouble seeing the blackboard around the age of eight? Why are all of my middle aged friends sporting specs these days?

The answer is refractive error.

It’s one of the most common vision problems, but strangely enough, most of us – even those who have it, which is almost everyone who wears glasses or contacts – don’t fully understand it.

Staying Focussed

Our eyes are incredible, finely tuned machines. Refractive error is a condition that occurs when the eye’s focusing system isn’t working properly, resulting in blurry vision. To understand refractive error, you need to understand this amazing system.

In the front of your eye, the system consists of the cornea (the outermost, transparent layer) and the lens (another transparent structure just behind the pupil), which are both responsible for focusing (refracting) light rays coming from what you see. And in the back of your eye, the retina is the surface where the image you see is perfectly focused when everything is working properly.

The lens in our eyes works like the lens in a camera. Just as you can adjust a camera’s focus to accommodate objects at various distances, the lens in your eye is flexible and can focus on objects at different distances as well (called the focal distance). When your lens thickens, it helps you to see something close up. And when it shrinks, it helps you focus on things in the distance.

The whole system has to work in harmony. Your cornea has to be gently curved equally in all directions to focus light (so your eyeball is shaped like a sphere). You lens must stay flexible to adjust the focus further. And, just as there are limits to how far or close a subject can be from your camera, the focal distance can’t be too long or too short. When your focusing system works perfectly, it is said to be emmetropic (normally refracted).

Four Ways to Err

There are some eye conditions that can be corrected with glasses or contact lenses. These are called refractive errors. Refractive errors include three correctible eye conditions in which the eye has difficulty focusing light onto the retina.

  • Nearsightedness (myopia): Images are focused so they fall in front of the retina instead of right on it. It could be that your cornea is curved too much, or the distance between the front and back of your eye is too great, or a combination of the two. When you have nearsightedness, you can see things clearly when they are close, but not when they are far away.
  • Farsightedness (hyperopia): Images are focused so they fall behind the retina instead of right on it. It could be that your cornea is too flat, that the distance between the front and back of your eye is too short, or a combination of the two. When you are farsighted, you can see things at a distance, but have trouble focusing on close up tasks like reading.
  • Astigmatism: An irregularly shaped cornea (the outer transparent layer of the eye) focuses the light rays into more than one point causing image distortion. They might be more oval in shape, elongated in one or more directions, or dimpled. When you have astigmatism, it means that there are multiple focal points in your eye instead of just one. And some of these focal points might not be on the retina – most people with astigmatism also have farsightedness or nearsightedness.
  • Presbyopia: As we get older the lenses inside our eyes become less flexible and less able to bend and focus light. The lenses in our eyes grow larger over time, which reduces their flexibility, and their room to thicken and focus on close-up tasks. As a result, close-up tasks such as reading and sewing will become more difficult.

Top left, presbyopia. Top right, myopia. Bottom left, hyperopia. Bottom right, astigmatism.
A diagram illustrating four refractive errors. Top left, presbyopia. Top right, myopia. Bottom left, hyperopia. Bottom right, astigmatism. From The Merck Manual of Diagnosis and Therapy, Edition 18, p. 882, edited by Mark H. Beers. Copyright 2006 by Merck & Co., Inc., Whitehouse Station, NJ. Available at: http://www.merck.com/mmpe/sec09/ ch099/ch099a.html#S09_CH99_F001. Accessed July 5, 2007.

These four eye conditions are usually correctible with glasses or contact lenses.

Ages and Stages

Our eyes grow as we do. And when they grow, our focusing system is affected. This makes refractive error a strange little beast – it can develop or change within us at different times in our life.

Most babies (about 95 per cent) are born farsighted and most also have astigmatism. However, as we grow (and the distance between the front and back of our eyes lengthens), most of us lose our astigmatism by the age of three and farsightedness by the age of six. In contrast, very few babies are born with nearsightedness.

The age of six is a special time in the development of our eyes. At six, most of us have the best focus we will likely experience in our lifetime.

By six, farsightedness and astigmatism have cleared up for many children. And for those who still have them, they won’t get any worse.

On the other hand, after the age of six many children (even some who had been farsighted) begin to develop nearsightedness. Early onset myopia (also known as school age myopia) typically occurs between the ages of 6 and 14. There is a second (late-onset) wave, which starts in the late teens and can even appear in a person’s 20s.

By our mid-forties, presbyopia is beginning to set in for most of us (and it affects 100% of us by 55). While the lenses in our eyes have been growing less flexible throughout our lifetime, most of us begin to notice the effects at this age.

Symptoms and diagnosis

Sore eyes or headaches can occasionally accompany the blurry vision of refractive error. In children, watch out for frowning or other signs of difficulty while reading, and excessive blinking or eye rubbing. Regular eye examinations are always a smart idea to keep on top of any unwelcome developments, no matter what your age.

Refractive error is typically diagnosed by three tests:

  • With an autorefractor machine, you place your chin on a chin rest and focus on an image inside the device. The eye doctor will press a button and the machine will give an estimate of your refractive error.
  • In a retinoscope test, your doctor will shine a light in your eye to watch how it reflects off the retina.
  • A phoropter machine further refines the measurement from other tests. One eye at a time, your doctor will give you different lenses to look through until you find one that allows you to see most clearly. Because the test involves patience and careful observation, it is rarely used with young children.

Based on the measurement your doctor comes up with, you will receive a prescription to correct your refractive error.

Colour photo of an eye doctor using a phoropter to determine a young girl’s eyeglass prescription.
Colour photo of an eye doctor using a phoropter to determine a young girl’s eyeglass prescription. Photo Credit: National Eye Institute, National Institutes of Health

The Last Word

Refractive error is easily treatable. For children and young adults, treatment involves glasses or contact lenses because their eyes are still growing. Once someone is in their mid-twenties, another option is laser surgery, which can reshape the cornea to reduce or eliminate nearsightedness, farsightedness and astigmatism.

Treatment for presbyopia involves the use of glasses and to a lesser degree contact lenses. Laser surgery cannot be used since the problem is in the lens and not the cornea. Those who have not previously had vision correction will need to wear reading glasses and those who already have corrective lenses will need bifocals with the bottom portion designed to boost near vision.

Once treated, most people with refractive error can see with perfect or near-perfect vision. This is why refractive error is said to be a “correctible” form of vision loss. And it’s not even a consideration in most measures of vision, including for a driver’s license where your vision is considered to be what you see after “best possible correction.”​​​​