In order to see clearly it is essential that all the lights rays entering the eye come into focus on a single point onto the retina. In patients with normal vision (emmetropia) all the rays of light entering the eye can automatically focus onto the retina when the eye is in unregulated state (i.e. the muscles holding the lens are related).

Hyperopia occurs when the eyeball is too small or the power of the cornea and/or lens is weak. As a result parallel rays of light will come into focus behind the retina when the eye is in unregulated state. In people with only a small degree of farsightedness or in young people, the lens is flexible and changes its shape to make the rays of light come into focus on the retina and give clear vision even without glasses. However, this exposes the eye to stress due to the need for constant use of the eye muscle, causing eye fatigue and headaches. As the natural lens stiffens with age hyperopic patients first find themselves more dependent on reading glasses than people with normal sight, and then find that they need glasses for the distance too.

Myopia occurs if the eyeball is too long or the cornea (the clear front cover of the eye) is too curved/too powerful. As a result, parallel rays of light entering the eye isn’t focused correctly onto the retina, instead they are focused in front of the retina.People with myopia can see close objects clearly, but distant objects appear blurred. For example they may have difficulty clearly seeing TV screen or driving without wearing their glasses.

This is caused by an irregularly shaped cornea or lens. Normally, the cornea is symmetrically round shape and curved equally in all directions (like a baseball). However, in people with astigmatism the cornea is shaped more like an American football which is more curved one one direction (line connecting the 12 and 6 o’clock) than the other direction (line connecting 3 and 9 o’clock). In an eye with astigmatism, light does not come into focus at a single point, instead multiple focus points occur, either in front of the retina or behind it (or both) depending on the angle of the light entering the eye. For example, the figure below shows that vertical light forms an image behind the retina (in the same way as farsightedness) while horizontal light forms an image in front of the retina (in the same way as nearsightedness).