How Eyes Work
The eye is a sensor, producing focused images which are translated into electrical signals and then transmitted to the brain for interpretation. The eyeball is very similar to a camera and contains two focusing elements: the cornea and the lens.
The cornea is the most powerful part of the focusing system, while the lens is a focusing element capable of changing focus from distance to near. Therefore a normal eye has the ability to focus clearly over a wide range of distances.
Complex mechanisms control the growth of the eye ball in early childhood to achieve a stable focus, usually by the age of 18 to 21. Errors in these growth processes can lead to an eye with a sub-optimal focusing process, resulting in the need for glasses or contact lenses.
The cornea is a relatively stable optical element and does not seem to change much through life. However from the age of 20 onwards the lens of the eye thickens and alters in shape. These changes start to reduce the functional quality of vision by increasing the spherical aberration (a focusing error not corrected by contact lens or glasses and only identified on wavefront measurements) of the eye.
To continue the camera analogy, it was spherical aberration caused by errors in the manufacture of the focusing mirror resulted in the flaws in the early pictures from the Hubble Space Telescope. These ageing changes in the lens continue throughout one’s life, giving rise to presbyopia (a progressive loss of focusing power resulting in the need for reading glasses) in one’s 40s and eventually cataracts a few years later.
Cataracts are the world’s commonest cause of visual problems, with more than 20 million people affected globally. It is likely that every one of us will get a cataract if we live long enough.
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