Gradual age-associated loss of the eye’s ability to focus on near objects
After about the age of 40, almost everyone starts to notice increased difficulty in reading small print because of the development of presbyopia. A person with normal vision is able to see close objects clearly because the elastic lens of the eye changes shape, becoming thicker and more curved when focusing on near objects. The thicker lens brings light rays from close objects into sharp focus on the light-sensitive retina at the back of the eye in a process known as accommodation. As we age, the lens becomes less elastic and the power of accommodation is reduced. Eventually, light rays from near objects can no longer be focused on the retina and the objects we see appear blurred.
Since presbyopia develops very slowly, most people are unaware of the initial stages of this condition. However, the symptoms usually become noticeable between the ages of 40 and 50. Longsighted people (see Hypermetropia) may have noticeable symptoms from an earlier age. Common symptoms of presbyopia include:
The need to hold newspapers and books at arm’s length so that you can read them.
Increased difficulty in focusing on near objects in poor light.
If you are shortsighted (see Myopia), the need to take off your glasses to see near objects clearly.
If you develop any of these problems, consult your optometrist.
Presbyopia can be corrected by wearing glasses with convex (outward-curved) lenses, which bring light rays from near objects into focus on the retina. If you are longsighted, shortsighted, or have astigmatism, you may be prescribed glasses with a different power in different parts of the lens. For example, bifocals have an upper lens to correct distance vision and a lower lens to correct presbyopia. Varifocal lenses that gradually alter the focusing power from top to bottom are also available. Presbyopia can sometimes be corrected with contact lenses, but glasses may still be necessary for reading.
Presbyopia tends to worsen with age, and you will probably need to have your lens prescription updated every few years. You should see your optometrist regularly in order to have vision tests. The condition eventually stabilizes at about the age of 60 by which time little natural focusing power is left. By this stage most of the focusing work is done by your glasses instead of the eye.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.