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Refractive errors explained

22/01/2021

What is a refractive error?

With normal vision, light reflected from an object enters the eye through the cornea (the clear, curved surface that protects the front of the eye). The cornea bends (refracts) these light rays through to the eye’s natural lens, where they are bent again to focus at a sharp point on the retina (back layer of the eye containing light-sensitive cells). The retinal cells convert the light rays into electrical impulses that are sent to the brain (via the optic nerve) and processed to form an image.

Refractive errors occur when the eye is imperfectly shaped in such a way that light rays are not focused on the retina correctly.

Common refractive errors

All refractive errors affect the eye’s ability to see clearly at a range of distances. However, the way vision is compromised, and the accompanying symptoms, vary significantly between the different types of refractive errors. Generally speaking, the eye’s ability to focus light is affected by the shape of the cornea, the curvature of the lens and the length of the eye.

What is myopia?

diagram of an eye with myopia
Patients with myopia (also known as ‘short-sightedness’ or ‘near-sightedness’) can see nearby objects clearly, but struggle to focus on objects that are far away. Myopia most commonly occurs when the cornea or lens is too curved and bends light too strongly, if the eye is too long or due to a combination of these factors. As a result, light rays focus in front of, instead of on, the retina.

Myopia typically develops in early childhood and can worsen with age as the shape of the eye changes over time. Excessive screen-time and not enough time spent outdoors can lead to more severe cases. Myopia usually gets progressively worse until a person reaches their early 20s. For some people, it can progress into their late 20s and beyond.


What is hyperopia?

diagram of an eye with hyperopia
Hyperopia (also known as ‘long-sightedness’, ‘far-sightedness’ or ‘hypermetropia’) is a condition in which distant objects can be seen clearly, but close objects appear blurred. This usually occurs when the cornea (or lens) is less curved than normal, the eye is too short, or a combination of these two factors. The flattened cornea can’t bend light rays strongly enough, meaning light focuses behind the retina, rather than directly on it.

Long-sightedness can occur at any age but often gets worse after the age of 40.


What is astigmatism?

diagram of an eye with astigmatism
Astigmatism is characterised by blurred or distorted vision, usually at all distances. Astigmatism occurs when the cornea is imperfectly rounded (i.e. shaped more like a rugby ball than a soccer ball). As a result, light rays that pass through the cornea are bent unevenly and do not focus at one point on the retina.

Astigmatism is a common diagnosis and most people have some degree of corneal asymmetry. While people with mild astigmatism may not know they have it until they have a regular eye exam, more severe cases can result in very distorted vision – as though looking through a funhouse mirror.


What is presbyopia?

Presbyopia develops gradually over time as the eye’s lens becomes more rigid, and the muscles around the lens weaken. When looking at near objects, the lens in the eye changes shape to refract (bend) light more. Specifically, it becomes more convex. With age, the lens loses this ability and, as a consequence, the image does not focus on the retina.

Presbyopia causes progressive visual decline, typically between the ages of 40–65, and presents with blurry vision when performing nearby tasks, such as reading. It is common for older people to develop presbyopia alongside an existing refractive error such as myopia or astigmatism.

What are the symptoms of a refractive error?

  • Blurry or distorted vision
  • Difficulty focusing on near or far objects
  • Squinting to read
  • Eye straining or tiredness

If you’re experiencing any of the above symptoms, you should visit an optometrist for a comprehensive eye exam.

How are refractive errors diagnosed?

Refractive errors are diagnosed during a comprehensive eye exam with an optometrist or orthoptist.

During the exam, the eye care professional will perform a visual acuity test (using a Snellen chart) to check how clear your vision is. Refraction tests are also commonly used to check how well your eyes can focus light, and if corrective glasses or contacts are needed to adjust the refractive power.

Several other tests may also be included to determine the health and function of other parts of the eye. More information on comprehensive eye exams is available on our general eye health page.

Since the shape of your eye and strength of refractive errors can change over time, you should have a comprehensive eye exam every one to two years.

Managing a refractive error

Refractive errors can be corrected with glasses and contact lenses, or treated with laser eye surgery or refractive lens exchange, depending on the severity of the refractive error, lifestyle choices and a person’s age.

Laser eye surgery involves permanently reshaping the cornea to adjust the refractive power so it can properly focus light on the retina. This type of vision correction surgery is generally suitable for people with myopia, hyperopia and/or astigmatism, and presbyopia. You can find out more about laser eye surgery and the procedures we offer here on our laser eye surgery page.

Refractive lens exchange is another form of vision correction surgery that is usually reserved for people over the age of 45 years old and at risk of developing or already experiencing presbyopia. This procedure involves replacing the eye’s natural lens with an artificial one (known as an intraocular lens). For more information, and to learn about the different lens options available, please visit our cataract & lens surgery page.

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The information on this page is general in nature. All medical and surgical procedures have potential benefits and risks. Consult your ophthalmologist for specific medical advice.

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