Long and short-sightedness

Associate Professor Gordon Sanderson from the Ophthalmology Department at Otago University explains how the growth of the eye during childhood can affect our eyesight in adulthood. If the eye grows too long, then short-sightedness (myopia) results. Too short in length results in long-sightedness (hypermetropia or hyperopia).

Transcript

ASSOCIATE PROFESSOR GORDON SANDERSON
There’s one specific point where the image is in focus, and that is referred to as the focal point, and the distance from that point to the lens is referred to as the focal length. Now that will give us a very clear image. If I hold that at the correct distance, I’ll get a very clear image of that light on the piece of paper. But if my eye grew too long, the image is now no longer in focus.

So if you are short-sighted or if you are myopic or if your eye has grown too long, you then have difficulty seeing in the distance. However, you can see up close, so your near vision is not affected.

The reason is that the eye when you’re born, is about two-thirds of the adult size, the eye’s a bit small when you’re a child. As you grow, your eye also grows. Now, usually it grows to the right length.

And people who are short-sighted generally have experienced that, at one point in their lives, they could see quite clearly but then they gradually deteriorate. And that has to be corrected with glasses or with a contact lens. And the way to correct that is to use a concave lens, which has the effect of removing some of the power, if you like, of the convex lens, and that will ultimately put the image back in focus on the retina.

Now the opposite also occurs where the eye, instead of growing to its full length, stops before it reaches that full length. Now that is referred to as long sight or hypermetropia.

Now hypermetropia is a little bit more complicated because what we didn’t mention before was inside the eye is the lens, which is capable of changing its power. Now if the eye is too short, the lens can actually do something about it. The lens can make itself get fatter and can actually correct the problem. It’s called accommodation. Now it doesn’t really fix the problem, it merely puts it right temporarily. Usually after prolonged accommodation, your eyes start to hurt and you start to get a headache. So accommodation for hypermetropia is not a good idea. Accommodation is intended for near vision – it’s intended for focusing.

The difficulty that the public have in understanding this is that the glasses that we give them to fix their hypermetropia doesn’t actually make their vision any better, they can see just as well by accommodating. Most of the public assume that glasses are to improve your vision. Well in this case, they’re not. The glasses are simply to alleviate the symptoms that you’ve got by having to use your accommodation all day long.

Acknowledgements:
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