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SPECTACLES AND REFRACTIVE ERRORS: Refractive errors are errors in the mechanism of focusing light onto the retina at the back of the eye. They are discussed in more detail on another Answer . This topic deals with spectacles and how they correct refractive errors. A nearsighted person's eye can be thought of as either the eye being too long, or the bending or focusing powers of the cornea and lens being too strong.
A nearsighted person has the following symptoms: he can read and see quite well close up, but has difficulty in seeing at distance. Spectacles will have to be prescribed which diminish this powerful bending of light rays so that the focused light converges on the retina instead of in front of the retina. The following consultation is typical:
Patient: Doctor, I can see well close up, but I seem to be having difficulty in seeing in the distance. It is especially noticeable while I'm watching TV or driving.
Doctor: It sounds as though it could be a simple refractive error. After I have examined your eyes thoroughly to exclude any other condition, we can test your eyes, or as we call it, do a refraction, and see if glasses will help.
After the general ocular examination is completed and the patient is found to be normal, a refraction or eye-test is carried out. If the refraction shows that the patient is indeed nearsighted, spectacles will be prescribed to restore normal vision.
Doctor: How does that look now? How well can you see?
Patient: I can see absolutely clearly. It's perfectly in focus. Why?
Doctor: These are negative or minus lenses. They reduce the bending or refracting powers of the eye and shift the image further back onto your retina so that it is in focus for you.
People who suffer from farsightedness have symptoms which are the opposite to those who suffer from short-sightedness. They can sometimes, though not always, see quite well in the distance, but vision is blurred at near. They thus have more difficulty with their near vision than their distance vision. Their eyes can be regarded as either being too short, or light rays are bent insufficiently by their cornea and lens . They need plus or positive lenses to assist the eye in bending the light rays more so that the image is moved forward onto the retina.
Patient: Doctor, I cannot see close up, although my vision seems to be better for distance.
Doctor: Your examination revealed no medical problem with the eyes, but a refraction shows you to be farsighted. A plus lens will correct your problem.
Patient: Do I have to wear these glasses all the time?
Doctor: It depends on your needs. These glasses will make your near vision much clearer and your distance vision more comfortable.
Astigmatism is a problem which can complicate both near- and far-sightedness. This is a condition where the shape of the cornea is not perfectly round, but rather oval. Objects are focussed partially on and partially off the retina. People with this problem complain of blurred vision, eye strain or that they see straight objects as skewed or obliquely distorted.
Patient: Doctor, I don't see very clearly.
Doctor: Well, we've done a complete examination and there is no medical problem. At what distance is your vision blurred? Distance or nearby?
Patient: In fact, I see unclearly for both near and far vision.What could that be?
Doctor: How do you see upright objects, such as street lamps?
Patient: Funny you should mention that. They all appear to be leaning slightly towards one side.
The refraction reveals that the patient had astigmatism.
Doctor: You have astigmatism.
Patient: What can I do about that? It sounds serious.
Doctor: We will have to have glasses made which have a certain shape to correct the oval shape of your cornea. You should see perfectly well with them. In the future you might need to replace your glasses though, because the abnormal shape which your cornea has, can increase or change to a different position.
Patient: I don't like all this talk about being abnormal. Is it a rare condition?
Doctor: No, it is very common. Most people have at least a mild degree of astigmatism. You're definitely not abnormal or the only person with this condition.
The last condition that we will discuss is presbyopia or far-sightedness of the middle-aged.
Patient: I could always see perfectly and have never worn glasses in my life before. Now , since I've reached my forties over the last few months, I'm having difficulty in reading the newspaper. I can still see well in the distance, but things up close tend to blur.
Doctor: Is this worse in dim light?
Patient: Yes, it definitely is. It is especially bad when I'm reading the paper and that I normally do at the kitchen table when I get back from work. The light is not very bright, I must confess. Is the only problem then the lighting?
Doctor: No. You have reached middle age now and your lens is not as pliable as it was previously. All you are going to need is a pair of reading glasses. In time, you will probably have to replace them with a stronger pair. This is a normal condition and everyone will experience this at some time during the middle years.
Patient: But I have a friend who wears glasses all the time, except when he reads the paper.
Doctor: Yes, but he is most probably nearsighted and with his glasses he sees normally for distance and when he wants to read he removes them and then he can see clearly. It is exactly the same as someone who is nearsighted and doesn't need glasses for reading, but then does for distance.
Patient: I understand. That makes sense now.
The above-mentioned cases are typical of the type of complaints which eye care practitioners encounter daily.
If you have any further questions regarding spectacles and refractive errors, please consult your eye specialist.