Refraction Training Manual
5. Correction of Ametropia
Myopia
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Light from infinity is focussed in front of the retina.
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Light from the far point is naturally focussed onto the retina.
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Therefore, we want to provide a spectacle lens that will make light from infinity appear to come from the far point. By doing this we correct the refractive error. In myopia we need to use negative lenses.
Hypermetropia
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Light from infinity is focussed behind the retina.
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Light from the far point is naturally focussed onto the retina.
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Therefore, we, again, want to provide a spectacle lens that will make light from infinity appear to come from the far point. In hypermetropia we need to use positive lenses.
Astigmatism
Here we need to use a sphere and cyl combination to correct both principal meridians to make light from infinity appear to come from the respective far points.
