Refractive Errors


Errors in refraction are problems that arise when the focussing power of the eye is changed. They cause problems with visual acuity, most frequently a blurring of vision.

The eye is responsible for allowing us to visualise what is in our environment. Light from the outside world enters through the cornea, which is the front covering of our eye. The cornea is responsible for the majority of the focussing power of our eyes which some say is as much as 70%. From here the light passes through a clear fluid, known as the aqueous humour. The further 30% of the focussing power is due to the lens, which is directly behind the aqueous humour. The thickness of the lens is determined by ciliary muscles, which in turn control the focussing power of the eye. For the image to be seen, it must be focussed on the retina.

Broadly speaking, errors in refraction can be classed into 2 groups, spherical or cylindrical. Spherical errors arise due to the focussing power of the eye being either too small or too great. If the focussing power or optical power of the eye is too great, the image is focussed in front of the retina, as opposed to being focussed on the retina as in normal vision. This is called myopia or short sightedness and causes images from objects at a distance to appear blurry. Hyperopia is the opposite of myopia, meaning the image is focussed behind the retina. This is known as long sightedness and occurs when the focussing power is too small for the eye. This also leads to a blurred vision, but only when looking at objects that are close, such as reading.

Cylindrical errors occur when the focussing power of the eye varies within different areas of the eye. People with this condition see parts of objects in focus, whilst other parts are blurry. This is known as astigmatism. It is estimated that billions of individuals worldwide suffer from refractive errors. The incidence of these errors is linked to both genetic and environmental factors. The majority can be corrected by using either spectacles or contact lenses.

See myopia, hyperopia, astigmatism and presbyopia.


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