TL;DR
If you wear glasses or contact lenses, there’s a good chance you have a refractive error. These are the most common type of vision problem, and they happen when the shape of your eye prevents light from focusing correctly on the retina. The result is blurred vision, whether that’s at a distance, up close, or both.
There are four main types: short-sightedness (myopia), long-sightedness (hyperopia), astigmatism and presbyopia. Each one has a different cause, but they all come down to the same basic issue. Something about the structure of the eye, whether it’s the length of the eyeball, the curve of the cornea or the flexibility of the lens, is stopping light from landing where it needs to.
How your eye focuses light
To see clearly, light needs to pass through the cornea (the clear front layer of your eye) and the lens (a flexible structure just behind the pupil), and then land precisely on the retina at the back. The retina converts that light into electrical signals, which travel along the optic nerve to your brain.
If the eyeball is slightly too long or too short, or if the cornea or lens is an unusual shape, light will focus in front of or behind the retina instead of on it. That mismatch is what we call a refractive error, and it’s the reason things look blurry.
1. Short-sightedness (myopia)
Short-sightedness means distant objects look blurred while close-up vision stays clear. It happens when the eyeball grows too long from front to back, causing light to focus in front of the retina instead of on it.
Myopia usually develops during childhood, most commonly between the ages of 6 and 14. It tends to run in families, so if one or both parents are short-sighted, their children are more likely to be as well. But genetics aren’t the whole story. Research from Great Ormond Street Hospital has shown that rates of short-sightedness are rising in the UK, with twice as many children affected today compared to 50 years ago.
Evidence suggests that spending more time on close-up work (like screens and books) and less time outdoors may be contributing factors. Researchers from the University of London note that children who spend more time outside are less likely to develop myopia, though the exact reason is still being studied.
Severe myopia (called high myopia) carries additional risks, including a higher chance of retinal detachment, glaucoma and myopic maculopathy later in life. If you’ve noticed your vision changing gradually, it may be worth getting your prescription checked, as myopia can progress over time.
2. Long-sightedness (hyperopia)
Long-sightedness is the opposite of myopia. Close-up objects look blurred, while distant vision may appear clearer. It happens when the eyeball is slightly too short, or when the cornea doesn’t curve enough, causing light to focus behind the retina.
Many children are born with a mild degree of long-sightedness, and their eyes naturally correct this as they grow through a process called emmetropisation. For some people, the correction doesn’t fully happen, and they remain hyperopic into adulthood. Estimates suggest that between a fifth and a quarter of people in the UK are long-sighted.
Genetics play a significant role. In mild cases, your eye can compensate by adjusting the shape of the lens (a process called accommodation), which is why some people with hyperopia don’t realise they have it until they’re older and that ability declines.
3. Astigmatism
Astigmatism causes blurry or distorted vision at all distances. It happens when the cornea or the lens has an uneven shape. Instead of being round like a football, the surface is curved more like a rugby ball. This means light bends unevenly as it enters the eye and can’t focus on a single point on the retina.
Astigmatism can be present from birth, or it can develop during childhood or early adulthood. It often occurs alongside myopia or hyperopia. In most cases, the cause isn’t known, although eye injuries or conditions like keratoconus (where the cornea thins and bulges) can make it worse. Mild astigmatism is often so subtle that people don’t realise they have it until an eye test picks it up.
4. Presbyopia
Presbyopia is different from the other refractive errors because it’s caused by ageing, not the shape of the eye. As you get older, the lens inside your eye gradually stiffens and loses its ability to change shape. This makes it harder to focus on nearby objects, which is why people in their mid-forties often start holding books and phones at arm’s length.
Virtually everyone develops presbyopia eventually, regardless of whether they’ve had perfect vision their whole life. Even if you’ve already had laser eye surgery to correct distance vision, presbyopia will still happen, because it’s the lens, not the cornea, that’s responsible.
Related read: Common Eye Problems In Seniors
Risk factors that affect all refractive errors
Family history is one of the strongest predictors of refractive errors. If close relatives wear glasses, you’re more likely to need them too. Age also matters. Myopia tends to develop and worsen during childhood, while presbyopia becomes noticeable from middle age. Extended screen time, limited outdoor activity in childhood and prolonged close-up work have all been linked to higher rates of myopia.
A common concern is whether wearing glasses can make your eyesight worse. But contrary to what many people think, this is not true. Glasses correct the way light enters your eye, and they don’t change its underlying shape. Your prescription may change over time, but that’s down to natural development or ageing, not the glasses themselves.
When to get your eyes tested
Refractive errors are picked up during a standard eye test with an optician. Refractive errors are among the most common eye conditions in the UK, and many people don’t realise how much their vision could be improved.
Adults should have their eyes tested at least every two years, and children should be tested before starting school and regularly after that.
If you’ve noticed any blurriness, headaches after reading or screen use, or difficulty seeing at certain distances, an eye test is the simplest way to find out what’s going on and get it sorted.
