Short sight occurs when light is focused in front of the retina and causes distance vision to become blurred. Near vision, however, is usually clear. This commonly develops in childhood or adolescence and is often first noticed by blurring of writing on the school blackboard.
Long sight occurs when the natural tendency is to focus light beyond the retina rather than on it, and the eye has to make a compensating effort to re-focus. With a younger person this may only be possible with effort and may, for example, cause headaches when reading. In an older person, as well as making reading very difficult, it may also cause distance vision to become blurred.
Astigmatism occurs when the front of the eye is egg shaped instead of being regularly curved like a ball. In this case both distance and near vision may be distorted. People with this condition may suffer from headaches or be unduly sensitive to light.
As we get older, the lens of the eye thickens and slowly loses its flexibility leading to a gradual decline in our ability to focus on objects that are close up. This loss of focusing ability is called PRESBYOPIA.