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For people with normal vision, the light which rebounds off objects into the eye is focused onto the retina, meaning that they can see a clear image of that object. However, if that light is focused onto the area in front of or behind the retina, the image will be unclear. Such conditions are referred to collectively as refractive errors.

Refractive errors can be categorized into the following four main forms: short sightedness, far sightedness, astigmatism, and age related far sightedness.