People need glasses to correct blurred vision. Blurring occurs when the image of the object that you are looking at is not focussed in the correct place on the retina at the back of the eye.
There are four reasons why the image can focus in the wrong place!
1. Hypermetropia (long sightedness) Hypermetropia occurs when light focuses behind the retina. It is caused by the eye being too short or the cornea at the front of the eye being too flat and not focusing light as efficiently. Hypermetropia is corrected by a convex lens which converges light rays into the eye to bring the focus forward on to the retina.
2. Myopia (short sightedness) Myopia occurs when light focuses in front of the retina. It is caused by the eye being too long or the cornea at the front of the eye being too steep and focusing light too efficiently.Myopia is corrected by a concave lens which diverges light rays into the eye to move the focus back to the retina.
3. AstigmatismAstigmatism occurs when light focuses unevenly at the retina. It is caused by the cornea or crystalline lens of the eye being "rugby ball" shaped.Astigmatism is corrected by a cylindrical spectacle lens (cyl)
4. PresbyopiaPresbyopia occurs when the crystalline lens in the eye loses its ability to change shape. This means that objects which are close up cannot be focused on the retina properly but objects in the distance can be.Presbyopia is corrected by a convex lens which is only used for near viewing. It is sometimes corrected by bifocal or varifocal lenses when distance glasses are also required. |
