We have perfect vision when the image of a distant object is focused correctly on the retina. This is known as Emmetropia and is the ideal state of the eye. In this condition the focus of the eye is naturally set for distance vision but, if the gaze shifts to something closer, the eye is able to focus on that object effortlessly. Emmetropia occurs when the curvature of the cornea, the shape of the lens and their distances from each other and the retina are in harmony, creating a perfectly focused, sharp image.
Unfortunately most people do not have perfect vision. In the UK about 60% of the population needs eye correction but this varies dramatically with age. Around a third of people in their teens, twenties and thirties need to wear glasses or contact lenses. However this increases significantly as people go through their forties, to the extent that about 90% of people over 60 need eye correction.
Hypermetropia means "long sight" and is where the image is formed behind the retina. This could be because the eye is too short, or the cornea or the lens does not refract (bend) the light enough. A hypermetropic person has blurred vision when looking at objects close to them, and clearer vision when looking at objects in the distance.
By placing a convex lens in front of a hypermetropic eye, the image is moved forward and focuses correctly on the retina. A hypermetropic person will then see distance and close objects clearly.
Myopia means "short sight" and is where light from an object forms an image before it reaches the retina. This could be because the eye is too long, or the cornea or crystalline lens refracts (bends) the light too steeply. A myopic person has clear vision when looking at objects close to them, and blurred vision when looking at objects in the distance.
By placing a concave lens in front of a myopic eye, the image moves back to the retina so it will be clear. A myopic person can then see distance and close objects clearly.
Astigmatism occurs when your cornea is shaped like a rugby ball with two different curvatures. Images appear blurred or ghost-like because light rays are refracted unequally. In extreme cases, images both near and far, appear blurred. Many people who have myopia also have astigmatism.
Positioning a cylindrical lens in front of an astigmatic eye will refract the light in one direction more than in the other. Basically the light passes through the cylindrical lens in two different directions. The power of the lens is not the same all over. It is stronger in certain areas to compensate for the irregular curves on the astigmatic eye. When corrected, an astigmatic person will than see distance and close objects clearly.
During middle age, usually beginning in their forties, people experience blurred vision at near points, such as when reading or working at the computer. This is known as Presbyopia and happens to everyone at some point in life, even those who have never had a vision problem before. The onset of Presbyopia is the main cause of the increase in the need for eye correction with age.
When people develop Presbyopia, they find they need to hold books, magazines, newspapers, menus and other reading materials at arm's length in order to focus properly. When they perform closeup work, such as writing, they may have headaches or eyestrain, or feel fatigued. Those who are already myopic will need eye correction for both conditions, most conveniently provided by bifocal or varifocal glasses.
