Understanding Refractive Errors
To understand what myopia means — or other technical terms like astigmatism — it helps to start with how a normal eye creates an image for the brain to interpret. The analogy I like to use is an old-fashioned film camera. Light enters the front of the eye, which acts like the camera lens, and is focused onto the retina at the back, which acts like the film. When the focus lands exactly on the retina, the image is sharp and the brain receives a clear picture. When the light is focused in the wrong place, the image becomes blurred. A refractive error is simply a mismatch between the length of the eye and its focusing power, so that light does not land precisely where it should.
Myopia
Myopia means near-sightedness, or short-sightedness. A near-sighted eye is usually slightly longer than average, so light is focused just in front of the retina rather than on it, and distant objects appear blurry. If the object is brought closer to the eye, the image gradually sharpens. How close it needs to be depends on how nearsighted the eye is — the higher the degree of myopia, the closer the object must come before it is in focus.
A child with myopia usually has no difficulty with reading or other close work, but will say that far things look blurry. They may struggle to see the classroom board clearly, and will rarely win the license plate game on a car journey.
Myopia is measured in dioptres (D) and denoted by a minus sign, usually in steps of 0.25 D. As a rough guide, I typically suggest glasses for children from around −0.75 D onwards.
You may also come across the term myopic defocus. This describes light being focused in front of the retina, which creates a particular kind of blur. When this blur occurs in the peripheral part of vision (the side vision rather than the centre), it seems to send a signal to the eye that it is already long enough, which slows down growth. This finding is the basis of several modern myopia control spectacle and contact lenses, which are designed to create myopic defocus in the periphery while leaving central vision clear.
In short, for myopia — near vision is clear, distance vision is blurred.
Hyperopia
Hyperopia (sometimes called hypermetropia) means far-sightedness, although the name is a little misleading in children, as you will see. A hyperopic eye is usually slightly shorter than average, so light is focused behind the retina, again producing a blurred image. Unlike a myopic eye, however, a hyperopic eye can use its own focusing power — known as accommodation — to pull the image forward onto the retina. There is a limit to this: each eye has only a certain amount of focusing power, which decreases with age, so if the degree of hyperopia is high, accommodation alone may not be enough to produce a clear image.
Because close work requires more focusing effort than distance vision, a child with hyperopia often finds reading and near tasks harder. Distance vision may appear fine, but the child may complain of tiredness, headaches, or difficulty concentrating during reading. Occasionally, they may report double vision, or their eyes may appear to cross. If you notice this, it is worth seeing an eye doctor promptly. Glasses usually resolve the problem.
Hyperopia is also measured in dioptres, denoted by a plus sign — for example, +2.00 D.
The counterpart to myopic defocus is hyperopic defocus, where light is focused behind the retina. When this occurs in peripheral vision, it appears to send the opposite signal — encouraging the eye to grow longer to compensate — and this is thought to be one of the ways myopia develops and progresses.
In short, for hyperopia — near vision is often harder or more tiring, and sometimes blurred; distance vision tends to be clearer, though not always perfect.
Astigmatism
Astigmatism is different from both myopia and hyperopia. Here, the length of the eye is not the problem. Instead, the outer window of the eye — the cornea — is shaped more like an American football than a soccer ball. It is curved more steeply in one direction than in the other. That football shape can be oriented in any direction, from lying on its side, to standing on its tip, to anywhere in between. This orientation is called the axis of the astigmatism, and is described in degrees from 0 to 180.
Because the cornea is unevenly curved, light coming from different directions does not focus to a single point. Instead, it is stretched out, producing a blurred or distorted image. And because the focus is neither too far forward nor too far back, moving the object closer or further away makes no real difference — the image stays out of focus.
A child with astigmatism therefore sees images that are blurry, shadowy, or slightly distorted, at both near and distance. Astigmatism is usually present from birth, which means the child's visual world has always looked this way and they have nothing to compare it with. They are therefore unlikely to complain, because nothing has changed from their perspective, so the problem goes undetected. This is one of the reasons why early vision screening in childhood matters.
Astigmatism can be denoted by either a plus or a minus sign, depending on the convention used, with the axis recorded alongside it.
In short, for astigmatism — both near and distance vision may be blurred or distorted, and the blur does not improve by moving the object closer or further away.
Can all three coexist?
Not all three at once. An eye cannot be both too long and too short, so myopia and hyperopia are mutually exclusive. Astigmatism, however, can occur alongside either one, and often does.
A note on testing in children
Testing children for refractive errors is a little different from testing adults. Because children have such strong focusing power, their eyes can mask a degree of hyperopia or make an eye seem more myopic than it really is during a standard test. To get an accurate measurement, eye doctors sometimes use drops that temporarily relax the focusing muscles of the eye. This prevents accidental over-prescribing of myopic glasses or under-prescribing of hyperopic ones.
These are often called dilating drops, because a side effect is that the pupils also widen. While the drops are working, your child will notice blurred near vision and sensitivity to bright light. This is expected and temporary — normal vision returns within a day or so, and reassurance is all that is usually needed. This procedure is known as cycloplegic refraction.