Refractive Errors

Myopia, hyperopia, astigmatism, presbyopia


The eye itself is a complex optic system which usually has a refractive power of 60 diopters.

An eye that does not have a refractive power of 60 diopters is deemed to be ametropic as the image of the object is not formed on the retina – the object is blurred.

The main refractive defects are myopia, hyperopia, astigmatism and presbyopia. They occur naturally and if they are not corrected by a specialist, they can cause varying degrees of loss of vision.


Myopia causes the image to be focused in front of the retina. The use of glasses allows the image to be formed on the retina. In severe cases of myopia, the retina is very delicate and is at risk of tearing and retinal detachment. Preventative laser treatment is then required.


Hyperopia causes the image to be focused behind the retina. Most children are born far-sighted and eventually reach emmetropia or normal vision, by age six through accommodation. Over one third of children go on to become myopic as adults. Severe hyperopia can lead to visual impairment (amblyopia) and or strabismus. It then needs to be corrected as soon as possible with the help of glasses.


A refractive error characterized by irregular curvature of the cornea or lens resulting in blurred vision due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. Astigmatism is usually congenital and does not tend to change over time. Depending on the severity of the astigmatism, it must be corrected with eyeglasses or contact lenses.


Anisometropia is the condition in which the two eyes have a different refractive power, thus they are in varying stages of myopia (nearsightedness) or hyperopia (farsightedness). This increases the risk of developing amblyopia. It is therefore important to correct anisometropia as soon as it is discovered unless the difference between the two eyes is very small.


Presbyopia is a condition in which the lens of the eye loses its ability to focus, making it difficult to see objects up close affecting reading and sewing etc. It is not a disease but a natural ageing effect of the eye. It tends to affect people after the age of 45 when the lens starts to lose its "flexibility" and its sclerosis. Presbyopia corrects itself but after the age of 60, the patient must change their glasses every 2-3 years to stabilize this process.

The Scale of the Problem

The WHO estimates that 153 million people globally are blind or partially sighted due to uncorrected refractive errors. In addition to this, there are hundreds of millions of other people who are severely longsighted due to uncorrected presbyopia. In less developed countries, where there is an insufficient eye care infrastructure, the general population does not have access to the services they need to correct their refractive errors.

Poor vision - a public health problem

Appropriate treatment to prevent the development of amblyopia in children is required to allow them to benefit and succeed at school. For adults, the prevention of amblyopia is needed so they can continue to carry out their jobs, learn and develop their skills.

Throughout the world, programs are being put in place to improve the access to these vital services, especially in the poorest and most disadvantaged areas.

Prevention and Treatment

At the moment, there are no ways to prevent the onset of refractive errors. In developed countries, the only possibility available is optical correction. Vast screening programmes testing for refractive errors have been put in place in schools. This is what the OPC is involved in, in its new generation of programmes. We help to train the necessary personnel in optical correction whilst increasing the supply of glasses at an affordable price.

At what age can a child start to wear glasses ?
There is no minimum age for a child to start wearing glasses. If they need glasses, then the sooner they start wearing them the better.

See also : Sight Defect Corrections