Myopia: Short-sightedness or Myopia is a condition when the person is able to see the objects that are nearby very clearly and is unable to view the distant objects easily. This defect arises as the image of the object that is supposed to from at the retina forms in front of the retina. The reasons behind this defect is the elongation of the lens.Since thinner the lens,less is the focal length,so that's why the focal length of the lens decreases and consequently causes the focus,which was firstly on the retina,comes nearer to the lens.
Hypermetropia: Long- sightedness or Hypermetropia is a condition where the person is able to see the objects that are distant clearly and is unable to view the nearby objects clearly. This defect arises as the image of the object that is supposed to from at the retina forms behind the retina. A person with Hypermetropia can read comfortably only when reading material is kept beyond 25 cm from the eye. The reasons behind this defect is thickening of the eye lens.Since,thicker the lens,more is the focal length,so that's why the focal length of the lens increases and consequently causes the focus ,which was firstly on the retina,to move away, fartther from the lens.
Presbyopia: Ageing leads to a decrease in the power of accommodation of the Human eye. Most of the people with this defect find it extremely difficult to see the nearby objects with an ease. The diminishing flexibility of the eye lens and the weakening of the ciliary muscles leads to this defect. This defect can be corrected with the help of corrective eye glasses. Bi-focal lenses are used to help people who have both the defects of vision (Myopia and Hypermetropia).
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