Your eyes - vision disorders
Some people will be lucky enough to have perfect vision at all distances, however at least half the population will suffer from some form of error in focus or refractive error.
There are four main types of refractive error; Myopia, Hyperopia, Astigmatism and Presbyopia.
Myopia, commonly called shortsightedness, is a condition in which light is focussed in front of the retina, resulting in blurred vision. Shortsighted people can often see reasonably clearly at short distances, but will not be able to see distant objects clearly.
There is currently no cure for myopia, but spectacles, contact lenses and refractive surgery can all provide good distance vision for people with myopia.
Hyperopia or longsightedness is a condition in which the optical components of the eye are not strong enough, and so light is not focussed onto the retina. This results in blurred vision that is usually worse at shorter distances.
The eye's lens and cornea focus light into an image on the retina, just as a camera lens focuses light on to a film. In a resting hyperopic (longsighted) eye, the light is focused behind the retina and so the image is blurred.
People with hyperopia often have reasonable vision in the distance, but may find that their vision is blurred or that they experience feelings of eyestrain or headaches when doing near work such as reading. This is because an effort must be made to accommodate so the light can be focussed on the retina.
What do we do about hyperopia?
The optometrist has many things to consider when making a decision and symptoms are very important. In general, young people who are slightly hyperopic do not have problems. If they do, they may benefit from eye exercises or need spectacles, mainly for close work such as reading and using computers.
Older people, or young people with significant hyperopia, often have problems because focusing requires much effort. Their vision is more likely to be blurred, especially for close objects. They usually need spectacles for reading and sometimes for distance vision as well.
Presbyopia is a common condition that makes vision difficult at a normal reading distance. It is not a disease.
How will presbyopia affect my vision?
Close tasks such as reading and sewing become difficult, particularly in poor light. For example, you may find that you are holding your newspaper further away from your eyes to make the print clearer. Presbyopia does not affect distance vision. You may also have difficulty concentrating when reading or you may find periods of close work result in sore eyes, headaches or tiredness.
What causes presbyopia?
It is important to understand how your eyes change their focus for viewing close objects. Normally they are focused for distance vision. Inside the eye there is a lens about the size of a pea. To focus on close objects, a special muscle in the eye changes the shape of the lens. This process is called accommodation.
With age the lens loses its flexibility and is less able to change its shape. This is a completely normal ageing change, just like stiffening joints or greying hair. The loss in lens flexibility is the reason that close focusing becomes more difficult.
Who is likely to become presbyopic?
Everyone experiences the ageing process that causes presbyopia. The condition cannot be prevented.
How is presbyopia treated?
Presbyopia is corrected by a spectacle prescription designed especially for close distances, in the form of spectacles or contact lenses.
It is important that the prescription is calculated for the distance at which you do your close tasks. It is not yet possible to cure presbyopia by surgery.
Will wearing spectacles weaken my eyes?
No. Presbyopia will continue regardless of whether spectacles are worn. Wearing spectacles will not accelerate or slow the development of presbyopia.
Astigmatism is a condition where the optical power of the eye varies depending on the angle of light passing through it. Astigmatism produces blurred vision at all distances.
It is usually due to the shape of the cornea (the front surface of the eye). If the curvature of the cornea is not the same in all directions (like the side of an Australian football) it will bend the light passing through it by different amounts depending on the direction of the light, producing astigmatism.
Astigmatism is a focusing error which causes asymmetric blur. Some directions in an image are more out of focus than others. This can be contrasted with short-sightedness (myopia) where all directions are uniformly blurred.
How is astigmatism corrected?
Spectacles and contact lenses (hard and soft) can correct astigmatism. Sometimes correction of astigmatism can cause change in the apparent size and shape of objects and may affect judgement of distance.
A patient may feel taller or shorter, or walls may appear to slope and floors curve.In most cases, adjustment to these side effects takes only a week or so. Astigmatism correction may involve a compromise between optimal clarity and visual discomfort.
How is astigmatism detected?
Some people notice blur themselves. Only a proper eye examination will determine for certain if you have astigmatism.
Astigmatism is not an eye disease and any changes are generally gradual and not necessarily for the worse. Most people have at least very slight astigmatism.