How the eye functions

The eye functions on much the same principle as a camera.

The iris, or coloured portion of the eye, acts as the shutter to regulate the amount of light admitted. The cornea (the clear window at the front of the eye), and the lens (located behind the pupil), serve to focus light rays, from the object being viewed, onto the retina at the back of the eye. The retina then transmits the “picture” of the object viewed to the brain where the object is “seen”.

Refractive error
For the eye to present a clear image, the light rays entering need to come to focus on the retina. If rays of light do not focus on the retina, a blurred image is transmitted to the brain.

This is correctable by glasses or contact lenses and is known as a refractive error.

The three categories of refractive errors are: myopia (short-sightedness), hypermetropia (long- sightedness) and astigmatism.

All of these may be corrected by refractive surgery.

What is my refractive error?
The format of prescriptions can vary, but the information is the same. The Distance line refers to the refractive error of your normal vision The Near line refers to your Presbyopic error or reading correction.

Myopia is a very common defect affecting approximately one in five adults. It often commences in early teenage years and stabilises at around 20 years of age. Without their spectacles, short-sighted people are able to see near objects clearly if held at the appropriate distance, but usually objects beyond arms length are “blurred” or “out of focus”. Spectacles or contact lenses are used to correct myopia. People with moderate to high levels of myopia are unable to see anything clearly without their glasses.

If you wear myopic spectacles then by holding them a few inches away from a printed page you will note that they make everything seem smaller.

One or a combination of the following causes myopia:

  • Steep cornea
  • Long eyeball
  • Powerful lens

Consequently light rays focus in front of, instead of directly on the retina.

Excimer laser surgery corrects myopia by causing a degree of corneal flattening and decreasing the anterior corneal curvature. This reduces the focusing power of the cornea and allows light rays to be ‘refocussed’ onto the retina. Intraocular implanted lenses either replace or augment the patient’s own lens to achieve this ‘refocussing’.

One or a combination of the following causes hypermetropia:

  1. Flat cornea
  2. Short eyeball

Consequently light rays are focused behind instead of on the retina.

Long-sighted people are able to see images in the distance more clearly than they can see images close up. Hypermetropic people may however, be unable to see at any distance clearly without their glasses. Laser surgery corrects hypermetropia by making the cornea steeper so the rays are ‘refocused’ on the back of the eye.

It is possible to correct up to +6.0 dioptres of long-sight (including astigmatism) with the excimer laser. If you are above the level of treatment it is still worthwhile coming for a consultation. Our surgeons may be able to give you less dependence on glasses and contact lenses, or offer other treatment options such as Refractive Lens Exchange.

People with astigmatism have a cornea that instead of being shaped like a soccer ball has more of a rugby ball shape (i.e. it is not equally curved in all places). This means that the rays of light are bent unequally, therefore giving a distorted or blurred image at all distances.

Astigmatism can be found either alone, or in conjunction with myopia or hypermetropia. Laser surgery can treat astigmatism especially in conjunction with myopia or hypermetropia. Occasionally, arcuate keratotomy (corneal incisions) may be used to treat astigmatism alone.

Presbyopia is the term used to describe the need for reading glasses that usually occurs in middle age as a result of ageing changes within the eye’s natural lens.

Most people require reading glasses when they reach their late forties, however short-sighted people often do not require glasses for reading when they reach this age.

People with normal vision when younger will need to wear reading glasses from about their late forties onwards. These people may still have good unaided distance vision.

People who are mildly short-sighted will be unaffected by the symptoms of presbyopia in so much as they can read without glasses. People who have no prescription, are long-sighted or strongly short-sighted will suffer from the symptoms of presbyopia and need near vision glasses.

If a myopic patient is treated by excimer laser photokeratectomy, he or she will effectively become “normal-sighted” and whilst glasses may not be required for everyday tasks, once the treated person reaches 45-50, like most of the population, reading glasses will be required for near work.

Monovision describes a form of treatment for presbyopia, where one eye is used for distance vision and the other is used for near vision.

People with no refractive error will have only one eye treated, which will be made mildly myopic (short-sighted). This eye will then be used for near or short distance vision.
People with myopia (short-sight) will have both eyes treated. One eye will be fully corrected and the other will be under corrected. This will leave one eye mildly myopic (short-sighted) for near vision.

People with hypermetropia (long-sight) will also have both eyes treated. One eye will be fully corrected and the other eye will be over corrected. This will leave the eye mildly myopic (short-sighted) for near vision.

Many people will accept this situation happily. It may take several weeks to fully adjust to the situation but most people will find it a useful alternative to being presbyopic and wearing reading glasses.

Some disadvantages to Monovision are:

  • Decreased depth perception
  • Reading very fine print may still be a problem
  • Driving long distances especially at night may not be comfortable. This can be overcome by wearing glasses when required

There are several laser and surgical options for the correction of short-sight, long-sight and astigmatism. No single vision correction procedure will be suitable for all eyes and a procedure that may be right for one individual may not be right for another.

Unlike some centres that may be able to offer you only excimer laser treatment or only non-laser surgical options, our ophthalmic surgeons can offer a wide range of both laser and surgical treatments. This means that we can consider treating all degrees of short- and long-sight with or without astigmatism.

Before deciding upon which may be the best treatment option for your eyes it is important for you to have a thorough eye examination. Our experienced ophthalmic surgeons can then discuss with you the procedures that may be suitable for the correction of your sight.