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Vision tests

Testing eyesight and prescribing and providing spectacles is the work of a specialist who is not necessarily medically qualified but is trained to diagnose eye defects.

Vision tests are designed to see how well the eyes focus light on the sensitive area at the back of the eye, the retina. Light is made up of individual rays travelling in straight lines. These can only change direction if they pass through certain materials, such as the glass of a camera lens or the transparent tissue of the eye. This bending of the light rays is known as refraction.

Refraction concentrates the rays of light within the eye so that they cast an image on the retina. It mostly takes place at the front of the eye, in the cornea, its transparent 'window'. There is a lens at the centre of the eye, but this is mainly concerned with fine focusing, achieved by using a circular muscle which surrounds the lens to subtly change its shape.

OUT OF SHAPE
Most vision problems are caused, not through any fault in the lens, but by a less than perfectly shaped eyeball. It should be spherical. If it is too long and thin, the cornea is too rounded, the light rays are bent too sharply, and the image is focused in front of the retina. This makes it difficult to see distant objects clearly, and is known as shortsightedness or myopia. Too short an eyeball means the cornea is too flat and the focused image falls, in theory at least, behind the retina. This makes it difficult to focus on fine detail close up. This is longsightedness or hypermetropia.

Other conditions that may show up in an eye test are presbyopia, a type of longsightedness that tends to develop as a person ages, and astigmatism, where the eye is unable to focus equally well on vertical and horizontal objects.

EYE CHARTS
An optician's test chart' is the basic tool of his trade. It is usually placed 6m (20ft) from the patient. In smaller rooms, mirrors are used to make this possible. The letter at the top is of a size which can be read
at 60m (200ft) by a person with normal eyesight. The next row can be read at 36m (120ft), the third at 24m (80ft) and so on to the seventh row, which normally sighted people can read at 6m (20ft).

Quality of eyesight is measured by two numbers. The first refers to your distance from the chart, the second to the row you can read. Persons with normal eyesight have 6/6 or 20/20 vision, depending on whether metres or feet are used to measure. Someone who could only read the top line of the chart would have 6/60 (or 20/200) sight.

PRESCRIBING LENSES
If your vision is abnormal, the optician will ask you to look at the chart again while wearing a special frame. By fitting combinations of lenses into this, and checking to see whether they make it easier or more difficult for you to read the chart, he can discover the strength of spectacle lens required for each eye - it is rarely the same in both - to bring vision back to normal.

He will also peer into your eyes with a retinoscope, a magnifying instrument with a light on the end which enables him to look into the eye and test for astigmatism.

   
   

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