Understanding The Eye

This section provides more information about how the eye and vision work.. It expands on the brief explanation given on the previous page.

To Recap:

The previous page has already explained the basic structure of the eye. The main parts of the eye are:

• The cornea - the clear window at the front of the eye.
• The sclera - the white outer wall of the eye.
• The iris - the coloured part of the eye that controls the amount of light entering the eye.
• The lens - located behind the pupil helps focus the eye.
• The retina - like the film in a camera is the vital 'seeing' layer at the back of the eye.

Beneath the retina is a layer of dark pigment (called the retinal pigment epithelium). This absorbs any stray light within the eye. Deeper still beneath the retina is the choroid. This is a thin layer rich in blood vessels which provides nutrition to the over lying retina.

The interior of the eye is divided in two by the lens. Behind the lens is the main cavity of the eye. This is filled with a clear jelly like substance called "the vitreous". Immediately in front of the lens is the iris. Between the iris and the cornea, at the front of the eye, is a small fluid filled space called "the anterior chamber". The fluid within the anterior chamber is called the aqueous humour. It is continuously replenished. The aqueous humour is secreted (by the ciliary body) behind the iris, passes through the pupil and is drained away in the angle between the cornea and the iris.

Obviously the structures of the eye are all very small but when examined under the microscope they are quite complex. For example the retina is only a fraction of a millimeter thick but has 10 separate layers! The eye is about 1 inch in size from front to back (20 -25 mms). Short sighted eyes tend to be a bit larger and long sighted eyes a bit smaller.

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How the Eye is Focused

Two parts of the eye contribute to focusing an image onto the retina. These are the cornea and the lens. Most of the focusing is performed by the cornea; about ¾ of the total. The remaining ¼ is done by the lens. The focus of the cornea is fixed but in younger life the focus of the lens can be adjusted. A small muscle within the eye is able to subtly alter the shape of the lens and this adjusts its focus power. This ability means that younger people can see clearly in the distance but are able to adjust the focus of the eyes and also see clearly for near. Unfortunately the lens becomes less pliable with age and its shape is then less easily altered. This is why most people, even if they have good distance vision without spectacles, will need reading spectacles from middle age onwards. The ability to adjust the focus of the eye is called 'accommodation'.

The focus of the eye may be altered by surgery. For example lasers can be used to slightly alter the shape of the cornea. To correct short sight (myopia) the cornea is made a little flatter, and to correct long sight (hyperopia) it is made a little steeper. In cataract surgery the natural lens of the eye is removed. This is usually replaced by an artificial lens and the strength of this lens will determine the overall focus of the eye after the operation.

The image that is formed on the retina by the cornea and the lens is actually upside down and back to front. The brain compensates for this so that we appreciate the visual world correctly. Also because we have two eyes a small distance apart they "see" the world from a slightly different position. The image formed in each eye is therefore slightly different. For those whose eyes work together as a true pair this allows the brain to develop a keen sense of depth perception called 'stereopsis'.

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How Vision is measured

When a doctor or an optician examines the eyes the very first thing they will want to know is how well the eye is able to see. The usual and simplest way to measure vision is to use a letter chart. The most widely used is the Snellen chart devised by the Dutch ophthalmologist Hermann Snellen in 1862. This chart has a pyramid of progressively smaller lines of letters. The chart is designed to be read at 6 metres. How well the patient is able to see the chart is expressed as a fraction, e.g. 6/6. The top number is the distance (in metres) at which the chart was read. The bottom number indicates the smallest size of letter that could be seen at that distance. The single large letter at the top of the chart is the "60" letter. In order of decreasing size the letter sizes are then 36, 24, 18, 12, 9, 6, 5 and 4. The letter size number has to do with how far away that letter can be seen by a normal eye. For example the large "60" letter can just be seen by the average normal eye at 60 metres, the 36 size letter at 36 metres and so on.

Someone with normal vision will be able to read the size 6 letter at 6 metres and is therefore said to have a visual acuity of 6/6. Some people, particular the young, are able to see better than this and may have a visual acuity of 6/5 or occasionally 6/4. In the USA Snellen visual acuity is measured in feet rather than metres. The chart is then read at 20 feet; which is nearly equivalent to 6 metres. Normal vision is then 20/20 which is taken to mean the same as 6/6.

Vision may be tested several times. It may be assessed unaided or with spectacles (or contact lenses) or whilst looking through a tiny hole, called a 'pinhole'. 'Unaided' means reading the chart without any optical aid likes spectacles or contact lenses. The pinhole device is a simple way to eliminate blur that may be reducing vision and to help the examiner to find out what the best vision would be if the eye were properly in focus. For example if the patient is short sighted (myopic) but is not wearing their spectacles or contact lenses the chart will be blurred and they will be unable to read all the letters. If they look through a pinhole held close to the eye their vision will be much clearer and a truer impression of their best vision will be obtained. What is particularly important to an eye specialist is the "best corrected visual acuity (BCVA)". This is what the patient can see with any focus error, such as short sight, long sight or astigmatism, corrected with spectacles or contact lenses.

There are other aspects of vision that are tested in different ways, e.g. colour vision and width of vision (field of vision) but this are not covered here.

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Driving Vision Standards

In the UK the standard of vision legally required for driving is based on the ability to read a number plate at a certain distance; rather than on the ability to read a Snellen chart. However in Snellen letter chart terms the vision standard for driving a private car is equivalent to 6/10 with both eyes used together. This is though only a guide as the ability to see clearly within the controlled lighting of an eye clinic may not be the same as the lighting conditions on the road.

There is also a legal requirement for a certain width of vision. This is referred to as the 'field of vision'. To meet the standard for driving there should be an unimpeded field of vision of 120 degrees horizontally and 20 degrees above and below the centre of vision.

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Refractive Errors

To see clearly the eye must be in focus. This means that the image of the visual world is brought into focus onto the retina at the back of the eye. If the eye is not in proper focus it is said to have a "refractive error". There are three types of refractive error. These are myopia (short sightedness), hypermetropia (long sightedness) and astigmatism. There is a fourth condition where the aging eye loses its ability to adjust its focus for near vision. This is called presbyopia.

Myopia (short sighted)

In this condition the visual image comes into focus in front of the retina. It is as if the focus of the eye is too strong. By the time the image reaches the retina it has gone out of focus again. The result for the person affected is that distance vision is blurred but they are able to see clearly for near.

Hypermetropia (long sighted)

The term 'hypermetropia' is sometimes shortened to 'hyperopia'. In this condition the focus of the eye is too weak and the visual image is never brought into focus on the retina. If the retina were not in the way the image would come into focus in a position behind it. Extra focus is needed to bring distance vision into focus. If the hyperopia is not too great young people are able to over come it even without realising they are doing so. This is because in youth the eye has a natural ability to adjust its focus. By making a focusing effort the under focus of the eye can be corrected and vision becomes clear. Sometimes this effort causes eye or headache.

'Long sight' is not a very good term. It does not really explain the visual problem. It implies that the person affected will have good distance vision. Unfortunately this is not always the case especially with increasing age when it becomes harder to make the focusing effort to naturally compensate for the condition. What is true though is that with hyperopia (long sight) distance vision tends to be better than near vision.

Astigmatism

Astigmatism is where the focus of the eye varies in different orientations. For example the focus of the eye in the horizontal direction may not be the same as the focus vertically. The usual cause of astigmatism is that the curvature of the cornea, the window of the eye, is not perfectly spherical. Instead of being like the curve on a soccer ball, which is the same in all directions, astigmatism is the result of the curvature being slightly like that of an egg or rugby ball. The curvature around the point of a rugby ball is different (steeper) to the curvature around the waist of the ball. If the eye is like this it causes the focus in one direction to be different from the focus in another direction. For example if an eye with vertical/horizontal astigmatism looks at a letter "H" either the upright lines will be in focus or the cross line but not both at the same time. Astigmatism may be either of a short sighted or longsighted variety, or mixed.

Presbyopia: The bane of middle age.

As already mentioned the eye contains a lens, rather like a lens inside a camera. In children this lens is soft and very pliable. A small muscle inside the eye is able to change the shape of the lens and this alters its focus power. This is why young children can see clearly in the distance but are also able to naturally focus the eye to be able to see just as clearly three inches from the end of their nose! This ability to adjust the focus of the eye is called "accommodation". Unfortunately the lens of the eye gradually becomes less pliable and more stiff with ageing. The muscle is no longer able to so effectively alter its shape and focus. Usually by mid forties (assuming there is no other focus problem) clear vision at a comfortable reading distance is becoming hard to sustain and from then on some artificial focusing help is required, i.e. reading spectacles. By mid sixties all natural focus adjustment of the eye has been lost.

Measurement of Refractive Errors

Refractive errors are measured in Dioptres. A lens of one dioptre power has a focal length of 1 metre and a lens of 2 dioptres a focal length of half a metre, and so on. Short sight is corrected with minus (-) powered lenses and long sight and presbyopia with plus (+) powered lenses. Astigmatism may be corrected with either a minus or a plus powered lens depending on its type but also has an 'axis' to signify its orientation.

A spectacle prescription is written with three components:

Sphere (SPH)      Cylinder (CYL)      Axis

The 'sphere' signifies the short or long sightedness and so is a minus or plus number respectively. The 'cylinder' is the term used to describe the astigmatism. The axis signifies the orientation of this astigmatism. The axis is measured in degrees and is a number between 0 and 180. However if there is no astigmatism the cylinder and axis are left out. Lens powers are measured in quarter dioptre steps and always written to two decimal places, e.g. +1.25, -3.75.

If a lens correction for presbyopia is required this is referred to as the 'Add' because it is a plus powered lens on top of (added on to) the distance prescription.

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