Refractive errors

Refractive errors happen when the shape of your eye stops light from focusing correctly, causing blurry vision. Refractive errors are very common and often run in families. There are different types of refractive errors. You can be nearsighted (myopia), farsighted (hyperopia), have astigmatism, and have presbyopia.

Refractive errors can be diagnosed at routine eye examinations, an important part of eye care for everyone, including children. You can correct refractive errors by wearing prescription glasses or contact lenses. Some people choose to have refractive surgery, a longer-term treatment option.

Types of refractive errors

Refractive errors occur when the shape of the eye means light rays don't focus in the correct place on the retina at the back of the eye. This prevents you from seeing clearly. Different types of refractive errors affect vision in different ways. The four most common refractive errors are:

Symptoms of refractive errors

Not everyone realises they're living with a refractive error. The symptoms of refractive errors can develop gradually. They might not be obvious at first, but getting a diagnosis and treatment can make your eyes more comfortable and help you see more clearly.

Common symptoms of refractive errors include:

• Blurred vision
• Glare or halos around lights
• Double vision
• Squinting
• Headaches
• Eye strain

Symptoms of refractive errors often start in childhood, but presbyopia develops later in life, typically over the age of 40.

Even if you've been diagnosed with a refractive error and had it corrected, your vision can change over time. That's why people with refractive errors should have regular eye tests and new prescription glasses or contact lenses when needed.

Testing and diagnosis for refractive errors

Opticians can diagnose refractive errors as part of a routine eye exam. Eye examinations generally include the following tests:

• Visual acuity test – you read an eye chart with or without lenses depending on whether you already have a prescription.
• Refraction test – typically, this involves looking at an eye chart while wearing lenses of different strengths and telling the optician which lens makes the chart look clearer.
• An astigmatism test - opticians use a retinoscope to shine a light into the eye and measure the curve of any astigmatism.
• Other tests to review your eye health include examining the front and back of the eye with a slit lamp ophthalmoscope (specialist torch) and testing the pressure of the eye.

When you're diagnosed with a refractive error, you'll get a prescription with the following numbers:

• Spherical – a positive number means that you're farsighted, a negative number means you're nearsighted. The higher or lower the number, the stronger the prescription.
• Cylinder – this measures astigmatism in dioptres. If the number is 1.5 or more, you'll probably need eyeglasses or contact lenses.
• Axis – describes the location of astigmatism on the cornea.

How to treat refractive errors

Refractive errors are normally easy to correct with prescription glasses or contact lenses. Which you choose can depend on your budget, lifestyle, and personal preference. Surgical correction is also an option but usually isn't paid for by the NHS.

What causes refractive errors?

As light enters the eye, the cornea and lens bend the light, focusing it on the retina at the back of the eye. The retina contains the light-sensitive cells we need to see clearly.

Refractive errors happen when the shape of the eye means that light rays don't focus correctly on the retina. This can be caused by the eyeball being too long or short, the shape of the cornea, or the ageing of the lens.

There are some risk factors associated with refractive errors, including:

• Family history; refractive errors can happen to anyone but are more likely if you have a family history.
• Research suggests that children who don't spend enough time outdoors and do a lot of reading or using screens are at greater risk of myopia.
• Some eye conditions are rarely associated with having hyperopia, including narrow-angle glaucoma, corneal dystrophy, congenital cataracts and retinitis pigmentosa.
• Astigmatism can occur as a result of eye surgery or an injury to the cornea.

When to get medical advice 

Children and adults should have regular eye tests. Routine eye exams are an important part of looking after your eye health and can detect refractive errors and a range of common eye diseases. If you wear prescription eyeglasses or contact lenses, regular eye tests will check your prescription is up to date and your vision as clear as possible.