Normal Sight
There are two focusing elements in the eye, the cornea (the clear ‘front window’ of the eye) and the lens. Together these focus light sharply onto the retina at the back of the eye which acts like the film of a camera. The cornea does most of the focusing for objects that are more than a few metres away, whilst the lens is responsible for focusing for closer activities such as reading.

In normal sight the image is
focused accurately onto the retina

Short Sight (Near Sight, Myopia)
Short sight affects about 25% of people in this country. Vision is blurred because the eye is too long and so objects are focused in front of the retina instead of onto it. In short sight, distance vision is worse than near vision.

In short sight the image is
focused in front of the retina

Long Sight (Near Sight, Hypermetropia, Hyperopia)
In long sight the eye is too short and so objects are focused behind the retina. Near vision is worse than distance vision but the effects vary with age. Young people may have clear vision but become dependent on glasses as they get older. Long sight is not the same as presbyopia which is discussed below.

In long sight the image is
focused behind the retina

Astigmatism
Astigmatism is caused by a distortion of the shape of the cornea. Normally the cornea is spherical but in astigmatism its curvature is greater in one region than another. An analogy is that in astigmatism the cornea is shaped like a rugby ball instead of a tennis ball. Vision is blurred at all distances. Astigmatism usually occurs with either short or long sight.

In astigmatism the image is focused
onto the retina but is distorted  

Presbyopia
This is the dependence on reading glasses or bifocals for close activities such as reading that comes on at about the age of 45. In presbyopia the shape of the eye is normal. The problem is that the lens can no longer change its shape to focus on close objects. Whilst presbyopia is not a disease but simply a manifestation of getting older like grey hair, it is a change that often causes great inconvenience.

In presbyopia the image of close
objects is focused behind the retina

After Cataract Surgery
Cataract surgery has improved so much in the last few years that it is possible to leave most patients only minimally dependent on distance glasses after the operation. However a proportion of patients suffer from substantial short or long sight or astigmatism after cataract surgery and need glasses for clear vision. For such patients refractive surgery often greatly improves unaided vision.

Medical Eye Disorders
There are various diseases in which the eye may suffer from severe short or long sight or astigmatism. For example patients who have received corneal transplants often have advanced astigmatism.