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Refractive Errors

How do we see things? / Principle of image formation

The retina, the innermost structure of the eye, behaves like a projection screen which receives the inverse images and the cerebral centre of the brain redresses (form an erect) the image.

Types of refractive errors:

  • Emmetropia.
  • Hyperopia or Farsightedness.
  • Myopia or Nearsightedness.
  • Astigmatism.
  • Presbyopia

Emmetropia(Normal vision):

  • Emmetropia.
  • A condition where no refractive error is presents.

Hyperopia or Farsightesness:

  • Eyeball is too short.
  • Clear distance vision but suffer with eyestrain for near work.
  • Correction by means of spherical plus lenses.

Myopia or Nearsightedness:

  • Eyeball is too long.
  • Clear near vision and blurred distant vision, people generally squeeze their eyes to see the distant object
  • Correction by means of spherical minus lenses.

Astigmatism:

  • A point object will not have a point image.
  • Blurred vision for distance and near.
  • Generally have eyestrain and headache.

Presbyopia:

  • Lens begin to lost its elasticity/accommodating capacity.
  • Difficulty in focusing near object and clear distant vision.
  • Symptoms begins at the age of forty.
  • Corrected by means of bifocal/progressive lenses for near and intermediate vision.
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