What are refractive errors?
Refractive error refers to a range of common conditions. You can be short-sighted (myopia), long-sighted (hyperopia), or have astigmatism and presbyopia. Refractive errors occur when light doesn’t focus correctly when it hits the retina, the light-sensitive layer at the back of the eye (E on the diagram below).
Refractive errors can cause vision to become blurred (out of focus).
How common are refractive errors?
Refractive errors are very common and affect millions of children and adults in the UK. A survey by UK Biobank found that 54% of participants aged 40-69 had a refractive error. A lot of the time, it only causes mild eye strain or the inconvenience of having to wear glasses or contact lenses.
But, for some people, the impact is much more severe.
“54% aged 40-69 had a refractive error”
Severe short-sightedness (high myopia) is associated with an increased risk of retinal degeneration (myopic maculopathy) or retinal detachment – a medical emergency that requires urgent treatment. Left uncorrected, refractive error can be damaging in children whose eyesight is still developing. That’s why funding vital research into refractive error plays a big part in our mission to create a future everyone can see.
What are the common causes of refractive errors?
There are different types of refractive error (see below). Causes of refractive error can be:
- Genetic: refractive errors can run in families
- Lifestyle: Growing evidence shows that too much close-up work and insufficient time spent outdoors can contribute to certain refractive errors
- Ethnicity: There is a higher prevalence of refractive errors among specific populations, although this thought to reflect differences in lifestyle
There are also some biological factors associated with refractive errors. Refractive errors are mainly associated with small variations in the shape of the eye. These changes aren’t noticeable without close-up examination, but they are significant enough to affect how light is focused on the retina as it passes through the eye.
A refractive error can occur because of one or some of the following variations or changes that affect the shape of the eye:
- Eyeball length: the eyeball is too long or too short, which can prevent the light entering the eye from focusing on the retina correctly.
- Irregular cornea: the cornea, which is the clear window at the front of the eye, is not curved equally in all directions, and it can’t focus the light onto the retina correctly.
- Ageing of the lens: the lens, which sits in the middle of the eye and helps to focus light onto the retina, becomes stiffer as we age – which may cause the eye to struggle to focus during daily activities.
What are the different types of refractive errors?
Refractive errors occur due to eye shape variations that stop light from focussing correctly on the retina. The term refers to four different conditions – all of which make it harder for a person to see clearly. These variations are:
Short-sightedness (myopia) is a common refractive error where distant objects appear blurred. The condition occurs when light focuses just short of the retina, usually because the eyeball is too long.
Myopia often runs in families and is also more common in people with a Far Eastern heritage. Research suggests that too much close-up work can cause short-sightedness, which often develops or worsens during the teenage years. Spending too little time outdoors during childhood is another important risk factor.
Myopia can develop in later life, too, including because of other eye conditions, such as cataracts.
Long-sightedness, or hyperopia, is the most common type of refractive error. The condition occurs when light focuses too far beyond the retina, either because the front of the eye isn’t curved enough or the eyeball is slightly short.
As a result, close-up objects can appear blurry, especially when reading for a long time.
Long-sightedness also often runs in families and tends to be more common in people with a European heritage. Babies are often born with some degree of hyperopia, but most will grow out of it by around age two as their eyes develop. Children with severe long-sightedness are more likely to develop other eye problems, such as squint or amblyopia (or ‘lazy eye’).
Presbyopia occurs in everybody because of ageing. As people get older, the lens inside their eye gets harder and less flexible, which makes it harder or impossible to correctly focus light on the retina – causing close objects to appear blurry.
Presbyopia usually develops after the age of 40 to 50 years old. People with a combination of short-sightedness and presbyopia may still be able to read by taking their glasses off.
Astigmatism can make both distant and close objects look blurry or distorted. It happens when the front of the eye is not evenly curved – so it is shaped a bit like a rugby ball rather than a football. As a result, light bends differently as it enters the eye causing more than one focal point on the retina, which makes certain areas of a person’s field of vision becomes blurry.
As with long-sightedness, astigmatism is common in infants but often corrects itself as the eye develops. Many people also develop the condition as children or young adults, particularly after an eye injury or surgery. It often occurs alongside short or long-sightedness.
What are the signs and symptoms of refractive error?
Symptoms can vary depending on the type of refractive error and its severity. The most common symptom is blurred vision. Some people will experience very mild symptoms that are only noticeable during tasks that require a lot of focusing, such as reading, using a computer, or driving. But for others, their vision can be severely affected.
Common symptoms of refractive errors include:
- Problems with focusing and seeing details: for example, when reading or looking at a computer.
- Blurred vision: this may affect distant or close-up objects or even both.
- Double vision: seeing two images when looking at a single object.
- Headaches: trying harder to focus and adjust the eyes often causes headaches.
- Eye strain: when the eyes feel tired or sore, particularly when performing tasks involving lots of focus.
- Squinting: this can be a sign that somebody’s having to work harder than normal to focus their vision (but this is different from an ‘eye turn’, also known as a squint).
- Seeing a glare or halo around bright lights: is a result of diffraction, an effect that occurs when the light bends while entering the eye.
Who is most at risk of refractive errors?
Anyone can have a refractive error, but the biggest risk factor is family history. If family members need glasses or contact lenses, checking your eyes regularly is essential.
Refractive errors can affect people of any age. Some refractive errors, such as short-sightedness, can begin in childhood. But other conditions, such as presbyopia, occur at an older age – in people over 40. As they are caused by the shape of the eye, cornea, or lens – or changes to these parts of the eye – there is currently no way to prevent refractive errors from occurring.
How is refractive error diagnosed?
A refractive error is usually diagnosed during a routine eye examination. It’s recommended that everybody has a routine eye test every two years, although those at higher risk of refractive errors may be eligible for more frequent tests.
An optometrist will assess a person’s ability to focus on both near and distant objects, which will help determine what type of refractive error they may have and how much it affects their vision. They will also examine how well the eyes work together and how they react to light.
Early diagnosis of refractive errors is particularly important for children because it may impact their educational progress. And if they are not addressed promptly, other conditions, such as amblyopia (lazy eye), might develop.
Children often aren’t aware if there’s a problem with their vision as they may never have known any different. So, it’s essential to look for possible signs they’re struggling to see clearly – such as sitting close to the television, squinting, or rubbing their eyes. The sooner problems are detected and children receive appropriate treatment, the better, so it’s always best to get things checked out. Routine eye tests by an optometrist are free for children.
How is refractive error treated?
If a refractive error is very mild, treatment may not be required, as a person’s eyes may be able to adjust. However, working too hard to focus can result in eye strain and headaches, so treatment may still be required even if the impact on vision seems small.
Most refractive errors are quite simple to correct. The most common treatment is wearing glasses or contact lenses. But surgery can sometimes offer another option for fixing the problem.
Glasses or contact lenses
These will help correct blurred vision and may be required all the time or just to wear while carrying out certain tasks, like reading and driving. An optometrist will prescribe the right glasses or contact lenses to achieve the clearest possible vision. Some people find their vision worsens as they age and may need stronger prescriptions. For refractive errors associated with ageing, reading glasses can help.
Young children must receive appropriate treatment, as not correcting refractive errors while their eyes and brains are still developing may lead to problems later on. For their brains to ‘learn’ how to see correctly, both eyes need to be working clearly and equally. Treatment for children is usually glasses.
For adults with more severe refractive error, some types of surgery, like laser eye surgery, that can correct the shape of the cornea may be an option. However, this isn’t suitable for everybody and will usually need to be paid for privately. Short-sightedness is sometimes also treated with surgery to replace the lens with an artificial one. This relatively new procedure can be very effective but isn’t always suitable for everyone.
Fight for Sight’s goal is to help further the understanding of refractive error and use this knowledge to develop prevention techniques and better treatments.
We’re funding several projects related to refractive error.
A Genetic Test for Myopia
Thanks to research funded by Fight for Sight, it will soon be easier to identify children at a high risk of short-sightedness or myopia in later life. A research team led by scientists at Cardiff University has developed a genetic test that will identify an increased risk of myopia.
Find out more: Fight for Sight
How does short-sightedness affect children’s learning at school?
We funded Dr Horwood from the University of Reading. Dr Horwood’s team found that 4 in 10 children with normal eyesight focus on at the wrong distance for seeing close work clearly at age 5-9. It seems like they may be putting-up with blurred near vision. Children with developmental problems or who are long-sighted also under-focus.
Think you have refractive error? Here's what to do.
Have your eyes tested every two years, even if you think your vision is fine. If you or your child experiences any vision changes or loss of sight, such as blurring, however minor, always get things checked right away – even if it’s less than two years since you/they last had tests. Anybody who may be more at risk of refractive error, such as people with a family history or children, may need more frequent examinations.
This College of Optometrists website can be used to find a nearby optometrist and to answer further questions:
Last updated May 2023
Approved by Professor Jeremy Guggenheim, Cardiff University