Refractive Errors Explained: Myopia, Hyperopia, Astigmatism and More

If you need glasses or contact lenses, you almost certainly have a refractive error. These are by far the most common eye problems in the world. According to the World Health Organization, uncorrected refractive error is one of the leading causes of vision impairment globally, even though it can usually be corrected simply and cheaply with a pair of spectacles.

A refractive error is not a disease of the eye in the usual sense. It means that the eye does not bend (refract) light precisely enough to form a sharp image on the retina. This guide explains the main types, how an eye care professional measures them, and the options for correcting them, from glasses to contact lenses to laser and lens surgery. Each main type has its own detailed guide, linked throughout.

How a normal eye focuses

For clear vision, light from an object must be focused precisely onto the retina, the light-sensitive layer at the back of the eye. Two structures do the focusing: the cornea, the clear front window, which provides most of the power, and the lens inside the eye, which can change shape to adjust focus for near objects (accommodation). Our guide to how vision works explains this in detail.

An eye that focuses distant objects perfectly on the retina without any effort is called emmetropic. Any mismatch between the eye's length and its optical power is called ametropia, which is the medical term for refractive error. The most common reason is that the eyeball is slightly too long or too short; less often, the curvature of the cornea or the power of the lens is responsible.

The main types of refractive error

Myopia (short-sightedness)

In myopia, distant objects such as road signs, the board at school or faces across a room look blurry, while close objects remain sharp. It usually begins in childhood and tends to progress during the school years before stabilising in early adulthood. Myopia is becoming far more common worldwide, especially in East Asia, and researchers link this rise to more time spent on near work and less time outdoors in daylight. High myopia increases the long-term risk of retinal detachment, myopic macular degeneration, glaucoma and cataract, which is why slowing its progression in children has become an important goal.

Hyperopia (long-sightedness)

In hyperopia, the eye is relatively too short, so the image would fall behind the retina. Young people can often compensate by using their focusing muscle, so they may see clearly but feel tired, get headaches or find reading uncomfortable. As the focusing reserve decreases with age, blur becomes more obvious, first at near and later at distance. In children, significant hyperopia can cause a turning eye (accommodative esotropia) or amblyopia.

Astigmatism

In astigmatism, the cornea or lens is shaped more like a rugby ball than a football, curved more steeply in one direction than the other. Light is focused at two different points, causing blur and distortion at all distances. Small amounts are extremely common and often need no correction. Astigmatism often occurs together with myopia or hyperopia. A rapidly increasing or irregular astigmatism can be a sign of keratoconus.

Presbyopia

Presbyopia is the gradual age-related loss of near focusing that affects everyone, typically becoming noticeable in the early to mid-40s. The lens becomes stiffer and can no longer change shape enough for close work. People start holding books and phones at arm's length and need more light to read. Presbyopia adds to any existing refractive error: a short-sighted person may find they can read better with their glasses off, while a long-sighted person often notices presbyopia earlier.

Anisometropia

Anisometropia means there is a significant difference in refractive error between the two eyes, for example one eye is much more short-sighted than the other. Adults may notice eye strain or difficulty adapting to glasses, because lenses of very different powers produce slightly different image sizes in each eye (aniseikonia). In children, anisometropia is particularly important: the brain may favour the clearer eye and ignore the blurrier one, causing amblyopia (lazy eye), often without any visible sign. Early correction, sometimes combined with patching, gives the best results.

What causes refractive errors?

Refractive errors result from a combination of genetic and environmental factors:

• Genetics: having one or both parents with myopia increases a child's risk. Many genes affect eye growth.
• Eye growth: during childhood, the eye normally grows in a coordinated way to reduce hyperopia present at birth, a process called emmetropisation. If growth overshoots, myopia develops.
• Environment: strong evidence links less time outdoors to higher myopia risk in children; prolonged near work is also associated, though the evidence is less consistent.
• Ageing: the lens stiffens (presbyopia) and later may become cloudy, which can shift the prescription.
• Other conditions: diabetes with fluctuating blood sugar can temporarily change the focus; keratoconus and some eye surgeries alter the cornea's shape.

Symptoms to look out for

The main symptom is blurred vision, but refractive errors can also cause:

• Squinting or screwing up the eyes to see
• Headaches, particularly around the forehead after reading or screen work
• Eye strain, tired or aching eyes
• Difficulty driving at night, glare or haloes
• Double or ghosted images in one eye (often astigmatism)
• In children: sitting close to the television, holding books very close, rubbing eyes, losing their place when reading, poor concentration or avoiding schoolwork. Many children do not complain because they think everyone sees the same way.

How refractive errors are diagnosed

Refractive errors are diagnosed during an eye examination by an optometrist or ophthalmologist. The tests are quick and painless. You can read more about each step in our guide to eye examinations.

1. Visual acuity test: you read letters on a chart at a set distance, one eye at a time. This shows how well you see, not why. Results may be written as 6/6 in metric countries or 20/20 in the US, meaning normal vision at the test distance.
2. Objective refraction: an automated refractor or a handheld retinoscope measures how light reflects from the back of the eye to estimate the prescription without you having to answer questions.
3. Subjective refraction: the examiner fine-tunes the prescription by switching lenses and asking which looks clearer.
4. Corneal measurements: keratometry or corneal topography maps the curvature of the cornea, important for astigmatism, contact lenses and surgery planning.
5. Cycloplegic refraction: in children and young people, eye drops are often used to temporarily relax the focusing muscle. Without them, hidden hyperopia can be missed.
6. Eye health check: the examiner also looks at the front and back of the eye, because blur is not always due to a refractive error.

Online and app-based vision checks, such as an online visual acuity chart, can help you notice a change, but they cannot measure a prescription accurately or check eye health. They do not replace a professional examination.

Understanding your prescription

A spectacle prescription is written in dioptres (D). A minus sign (for example -2.50) indicates myopia, a plus sign (+1.75) indicates hyperopia. If astigmatism is present, you will see a cylinder value (CYL) and an axis between 0 and 180 degrees. An ADD value shows extra power for reading, used for presbyopia. Our guide on how to read your glasses prescription explains every abbreviation.

Correction options

All the options below correct the focus of light; none of them changes the underlying cause, except that surgery permanently reshapes the cornea or replaces the lens. The right choice depends on your prescription, age, lifestyle, eye health and personal preference.

Glasses

Spectacles are the simplest, safest and most widely used correction. They can correct virtually all refractive errors and carry no risk of infection. Options include single-vision lenses for one distance, bifocals and progressive lenses for presbyopia, and coatings to reduce reflections. Downsides include fogging, limited peripheral correction with strong prescriptions and inconvenience during sport.

Contact lenses

Contact lenses sit directly on the eye, giving a wider field of view and a more natural image size, which is especially helpful for high prescriptions and anisometropia. Soft lenses come in daily, two-weekly and monthly types, with toric designs for astigmatism and multifocal designs for presbyopia. Rigid gas-permeable lenses can give sharper vision in irregular corneas such as keratoconus. Special designs and orthokeratology are also used to slow myopia in children. The main risk is infection, particularly a corneal ulcer, which is strongly linked to poor hygiene, sleeping in lenses and exposure to water.

Refractive surgery

Laser and lens procedures can reduce or eliminate dependence on glasses for suitable adults whose prescription has been stable, usually for at least a year:

• LASIK: a thin corneal flap is created, the underlying cornea is reshaped with an excimer laser and the flap is replaced. Fast recovery.
• PRK / LASEK / surface ablation: the surface layer is removed and the cornea reshaped. Slower, more uncomfortable recovery, but no flap.
• SMILE (lenticule extraction): a laser cuts a small disc of tissue inside the cornea, which is removed through a small incision.
• Implantable collamer lenses (phakic IOLs): a lens is placed inside the eye in front of the natural lens; often used for high myopia or thin corneas.
• Refractive lens exchange: the natural lens is replaced with an artificial one, similar to cataract surgery; usually considered for older adults, especially with presbyopia.

Surgery has risks, including dry eye, glare and haloes, under- or over-correction and, rarely, serious complications. Not everyone is a good candidate. See our guide to eye surgery options for more.

What about eye exercises?

Despite many claims online, eye exercises cannot change the length of the eyeball or the curvature of the cornea, and there is no good evidence that they correct myopia, hyperopia, astigmatism or presbyopia. The Bates method and similar programmes have not been shown to work in scientific studies. Exercises can still be useful for relaxing the eyes during screen work and, under professional supervision, for some binocular problems such as convergence insufficiency. Read our article on what science really says about eye exercises.

When to see a doctor

Have a full eye examination if your vision has become blurred, if you get frequent headaches or eye strain, or if you have not had an eye test for two years or more. Children should have their eyes checked as recommended by local screening programmes, and sooner if parents or teachers notice signs of poor vision. Adults with diabetes, a family history of glaucoma or high myopia need regular checks.