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Causes of different focusing problems of the eye
Nearsightedness, or Myopia
 This is the most common focusing problem, affecting a significant proportion of the Indian population. Nearsighted people can see various ranges of near objects clearly but not distant ones. Nearsightedness usually results from an eyeball that is too long. Because of this extra length, light from distant objects converges to a point in front. Since the retina only captures the quality of the image that reaches it, the brain receives a blurred image of what the eye is trying to see.
Farsightedness, or Hyperopia
 This is a focusing problem caused by an eyeball that is shorter than normal. Due to the reduced length, the lens fails to bring light rays to a focal point by the time they reach the retina. The effects of farsightedness vary with age because of the diminishing flexibility of the lens. Young people may not notice any effects, but as the eye begin to age, near objects become increasingly difficult to see. Later in life nearly all focal range may be unclear.
Astigmatism
 Astigmatism is the result of the general inability of the eye to clearly focus image from any distance. It results from uneven curvatures of the cornea. Instead of being spherical, it is more similar to the side of an egg. This focusing problem usually occurs with nearsightedness,
or farsightedness.
Is that all?
Modem technology has advanced so much that now we know, in addition to the above three refractive errors there may be pre existing or induced imperfections in the focus of the eye which are known as higher order
wave front aberrations such as coma, trefoil etc. These aberrations were known to physicists who designed instruments like telescopes, cameras and microscope. Now these can be measured in the eye also. These aberrations are not correctable by spectacles. |
