Defects of Vision and their Correction
Three common refractive defects of the eye:
- Myopia or Near-sightedness
- Hypermetropia or Farsightedness
- Near-sightedness : A person with Myopia can see nearby objects clearly
- A person with myopia cannot see faraway objects clearly.
- The far point for the myopic eye is nearer than infinity
- Occurs due to
- Excessive curvature of the eye lens
- Elongation of eyeball
- The image of a distance object is formed in front of the retina and not on the retina
- Defected is corrected by using Concave lenses such that the lens will bring the image back on to the retina.
- Far sightedness : A person with Hypermetropia can see faraway objects clearly.
- A person with Hypermetropia cannot see nearby objects clearly.
- The near point of the eye is more than 25cm
- This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.
- Focal length of the eye lens is too long
- Eyeball has become too small.
- The image of a distance object is formed in behind the retina and not on the retina
- Defected is corrected by using Convex lenses such that the lens will bring the image back on to the retina.
- The power of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.
- The near point moves away.
- Spectacles with convex lenses are recommended
Myopia and Hypermetropia
- Sometimes a person may suffer from both near sightedness and far-sightedness.
- Such people are advised to use bifocal lenses.
- Bifocal lenses consists of concave on the upper portion and convex on the lower portion.
- Concave supports distinct vision and convex supports near vision
Refractive eye defects can also be corrected using contact lenses or through specific surgical procedures.