Astigmatism (lect 11)

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    • Emmetropia is when light rays are brought to focus at retnia.
      It forms a crisp image.
      NO spectacle lens correction needed.
    • Mypoia is when light rays are focused INFRONT of retina.
      Causes blur circle at retnia.
      due to axial length of eye being too long OR dioptric power of cornea being too strong.
      Correct using negative lens!! (spherical)
    • Hyperopia is when light rays are focused BEHIND retnia.
      due to axial length of eye being too short OR dioptric power of cornea too weak.
      Correct using positive lens !! (spherical)
      Can also correct using accommodation - crystalline lens
    • Astigmatism has two focal lines
      causes blur ellipse
      Midway point between two focal lines, produces a blur circle called "circle of least confusion"
    • Astigmatism forms when the cornea is not spherical, causing light rays to diverge.
    • Astigmatism forms ellipse images rather than point images.
    • Spherical eye = Normal eye
    • Spherical cornea = radius of curvature is SAME in ALL directions (power is same in ALL directions)
    • Astigmatic cornea = radius of curvature NOT same all directions !!
      (power NOT same all the way round)
    • 3 MAIN TYPES LENSES TO correct ASTIGMATISM:
      • Cylindrical lenses
      • Sphero-cylindrical lenses
      • Toric lenses
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