Understanding refractive errors
What is a Refractive Error?
Myopia is a condition often referred to as "Short-Sightedness", meaning that the individual can see at short distances or close-up, but not far distances. Typically, children become myopic at 8-10 years old, but there is a wide variation, some having myopia from even younger and others not becoming myopic until later on in their school years. A small number of people will become myopic as young adults. Myopia is commonly caused in old age by changes in the lens inside the eye as it starts to develop a type of cataract known as nuclear cataract.
Hyperopia is a condition where the eye sees best far away. The eye, in its relaxed state, is out of focus for distance and near vision, but, by using its focussing mechanism, it can work to bring things into focus. In a normal eye this focussing mechanism is only used for changing focus to see things close up. In an eye with hyperopia, unless the focussing mechanism is used, everything is blurred, especially close up.
Mild hyperopia in a young person may not cause any problems at all. The more severe the hyperopia, the more work that must be done to see clearly. More effort is needed to focus for near than for far away. If the hyperopia is severe enough the effort of continually focussing may produce headache or "eyestrain" or blurring of vision, particularly for closework.
As we get older our ability to change focus for near gets weaker. People with hyperopia who could see clearly when young, will need reading glasses much sooner than others. Soon after, they will need glasses for distance as well.
Presbyopia is the result of our ability to change focus for near getting weaker. Young children can see very close up. As we get older, we gradually lose our ability to change focus. By our mid 40's, we start to have to hold things further away to read, particularly in dim light. This is called presbyopia.
Astigmatism is a condition of focussing of the eye where vision is never clear, regardless of distance. Light from objects is focussed at different points, rather than all being brought into focus at one point. Only correction with a lens, in the form of glasses or a contact lens, or surgery, can correct the focus.
Mild astigmatism may cause only slight blurring. More severe astigmatism may cause severe blurring and make it impossible to carry out normal activities without correction.
Astigmatism is caused by distortion of the cornea (the window at the front of the eye) or occasionally the lens inside the eye. The distortion can be likened to the shape of part of a rugby ball, rather the normal shape, which is more like part of a football.