Shortsight, Longsight, Astigmatism
Myopia is the number one reason people need glasses or contact lenses and it is typically first noticed in teenage years. A common complaint is not being able to see the board at school. Short sighted prescriptions are minus numbers (i.e. -2.50ds).
In order to see perfectly the light/image entering your eye must fall exactly onto your retina. With myopia the light is focussed in front of the retina and hence things in the distance appear blurred. This means that the eye is too powerful for its size. No one knows the exact cause of myopia but it can often run in families and may be more common with people who do lots of close work in early and teenage life.
Long sightedness (Hypermetropia)
Hypermetropia typically affects older people and is often first noticed as people reach their mid 30’s. It normally initially affects your near vision but over time your distance vision can also become blurred. Long sighted prescriptions are always plus e.g. +2.00.
In order to see perfectly the light entering your eye must fall precisely onto your retina. If you are long sighted the light is focussed behind the retina, meaning that the eye is too weak for its size. People assume that long sightedness describes a condition whereby people are unable to see things close up but have perfect distance vision. Unfortunately hypermetropia is not quite as simple as this.
Many people who are long sighted can see perfectly well without spectacles.
Whether or not your vision is affected is down to 2 factors:
- Age: If you are young then you may well be unaffected by your long sightedness as the natural lens in your eye is able to refocus (accommodate) the light from behind the retina onto the retina (the lens cannot do this for short sightedness). Young long sighted people may complain of frontal or temporal headaches but see clearly. As you get older the lens in your eye begins to lose its ability to refocus this light and therefore things start to become blurred. This initially begins with loss of near vision but soon extends to distance vision.
- Amount of long sightedness: If you are very long sighted (i.e. +4.00ds or more) then you are likely to have to wear glasses from an early age as your distance and near vision will most likely be affected.
Contrary to what many people believe astigmatism is not a disease and it is an extremely common condition. Having astigmatism is no different to being long or short sighted; it is just a type of prescription. Over 80% of people who wear glasses have astigmatism to some degree and it is normally combined with either long sightedness or short sightedness.
Depending on the degree of astigmatism you may or may not need to wear spectacles all the time. If your astigmatism is mild you may just need to wear your glasses when performing certain tasks such as reading or computer use.
You may have heard the phrase ‘rugby ball shaped eye’ to explain astigmatism. With astigmatism it is the shape of your eye that causes the blurred vision. What this means is that the curvature of your eye / power of your eye is different horizontally compared with vertically. For example, the curve of the eye horizontally might be steeper (more powerful) than the curve of the eye vertically resulting in an out of focus image.
If you have astigmatism, the prescription your optician issues for you will have the ‘cyl’ box filled out. This is short for cylinder and is the astigmatic component of your prescription. The following gives an example:
Sph Cyl Axis
-1.50 -2.00 180
The sphere component relates to whether you are long sighted or short sighted. In this case as it is minus, you are short sighted. The cyl relates to your astigmatism. In this case it is -2.00 which is moderately astigmatic. The axis relates to the direction of your astigmatism.
Astigmatism can be corrected with spectacles or toric contact lenes.
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