Myopia & Hypermetropia

Are you long sighted, short sighted or are you lucky enough to have perfect eyesight In the perfect eye where light is brought into focus sharply on the retina, the screen at the back of the eye, where it stimulates receptor cells that in turn send impulses to the brain.

For this to work, the focusing power of the eye has to correspond to the eye’s length. In the model eye, we expect to find a total power of sixty dioptres, which requires the length of the eye to be exactly 22.22mm. It is amazing that nature can ever get her creations as perfect, however she often does. Actually it doesn’t matter if the eye is stronger as long as it’s length is less, but the length and power must correspond. A good example of the same process is when we set up a projector and, (where the projector represents the focusing system and the screen the retina).

It is possible to focus the picture sharply either, by moving the screen towards or away from the projector, or by changing the focusing lens power on the projector itself.

SO WHAT IS MYOPIA

If your close up vision is fine but you can’t see very well in the distance, you are myopic or short sighted. The shortsighted eye will be either, “too long for its power, or too powerful for it’s length”.
To correct a myopic eye we obviously can’t change its length, but we can affect its power with either contact lenses or spectacles.

Our shortsighted eye will require a lens which has a minus power to reduce the overall strength of the system. If your prescription is –2.00, it means that the focusing power of your eye is probably +62.00.

A common misconception is that when your prescription increases, say from –2.00 to –2.75, your eyesight has become worse, this is not the case, it simply indicates that your eye has changed shape. The important thing is the quality of your vision. As long as your sight with the new power is as clear as it used to be when you first had the old one, your sight has not deteriorated. It is only when you are unable to see as well with the correction that brings light accurately into focus on the retina that there is any cause for concern. It is normal for shortsighted eyes to become longer, or for the cornea to become more curved and powerful as we get older that is why we commonly experience a gradual and continued increase in the prescription throughout our early and mid life. However it is relatively unusual for this to indicate that the eye is “getting worse”.

HYPERMETROPIA

Those of you who have made it this far will not be surprised to learn that Hypermetropia, or long sight is exactly the opposite of Myopia. In Hypermetropia, the eye is either not long enough for its power, or not strong enough for its length. In other words, the light is not brought into focus soon enough to fall sharply on the retina and the picture is blurry.

For the correction of hypermetropia we must use lenses with a positive or plus power to increase the focusing strength of the eye. So if your prescription is +2.00, your eyes power is probably about 58.00.

What is strange is that hypermetropia also increases as we get older, that is to say, if our eyes start out too long they will continue to get longer, and if they start out too short they will continue to get shorter. Before you start to panic, we are only talking about changes of a few hundredths of a millimetre throughout a normal life.

So next time you have an eye examination and your optician tells you that your prescription has changed, you do not need to worry that your eyes are deteriorating, they have simply changed shape a little.

Paul Graves
Body’s Opticians