If we are to see clearly, both the cornea (a) and the lens (b) (figure 1) have to refract the rays of light (thereby the term refraction) so that they focus on the retina, a layer of sensitive cells which furnish the eye fundus.

There the light is converted into electric impulses which are transmitted via the optic nerve to the brain.

If the rays of light are not correctly focused on the retina, the image will be hazy. We call this fault a refraction deviation.

Glasses, contact lenses, or refractive surgery corrects or reduces this refraction deviation by changing the refraction of the rays of light in such a way that they are focused on or closer to the retina.

Myopia or short-sightedness

This is created by the projection of a distant image in front of the retina.

This makes the image appear hazy.

This can be corrected optically by fitting a negative lens (glasses or contact lenses) in front of the eye so that the image is projected further back and falls onto the retina.

Hypermetropia or long-sightedness

This is the name which is given to the deviation whereby a distant image is projected behind the retina so that it appears to be hazy.

As long as the lens has a normal ‘zoom’ capacity, the image slides forward by means of accommodation and becomes clear on the retina.

This deviation is corrected optically with positive lenses (glasses or contact lenses) in front of the eye.


This arises because the cornea is not perfectly round. This causes an uneven projection of the image around the retina.

This hazy image can only be corrected by the use of cylinders (in glasses or contact lenses).


Presbyopia typically begins around the age of 40 when the accommodation capacity becomes too weak so that the person can no longer read with the distance correction.