Not all eyes form perfectly, and many people require some visual enhancement, such as in the cases of long-sightedness, short-sightedness and astigmatism:
- Long-sightedness (Hypermetropia). This is where the eyeball is slightly too short to correctly focus light on the retina at the back of the eye. Close-in objects become blurry, but far objects are much less effected. Spectacles have a converging lens to enhance the focusing effect of the cornea, fixing this.
- Short-sightedness (Myopia). The exact opposite, the eyeball is too long in this case, and far away objects are blurry, with closer objects less affected. Spectacles have diverging lens to reduce the focusing effect of the cornea, fixing this.
- Astigmatism. This is where the cornea is not spheroidal in shape, instead, it is not evenly shaped, and shapes can appear wrong. Cylindrical lenses can fix this, but surgery to the cornea can also be used to fix these abnormalities.
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