The most common eye disorders are refractive corrections, which mean that the image of the object a person is looking at is not focussed properly onto the retina (the light-sensitive tissue in the back of the eye).

If the image is not focussed exactly on the retina, then the image will be blurred, just like an out-of-focus photograph. In this case, the person is said to have a refractive correction.

 

Refractive corrections exist when there is a mismatch between the length of the eye, and its optical power. These mismatches usually originate during childhood, when the eyes are growing. The exact causes of refractive errors are still being studied, but it is known that both hereditary and environmental influences can affect their development.

Most people have some refractive correction, but in the majority of cases the correction is small, and does not cause any problems. Refractive corrections can usually be corrected using spectacles or contact lenses and in many cases by laser surgery.

The are four main types of refractive corrections are discussed below: