People with extreme nearsightedness are diagnosed with over – 8.00 diopters of myopia, Those with extreme farsightedness have over + 2.75 diopters of hyperopia. In both cases, their vision is so impaired it is debilitating, and they need glasses or contacts just to function.
In extreme nearsightedness, the cornea is excessively steep and/or the eye is extremely long so light rays are focused in front of the retina. The eye cannot focus properly, especially on objects in the distance. In extreme farsightedness, the eye is extremely short and/or the cornea is excessively flat. When light rays enter the eye they are focused behind the retina giving blurred vision. The blur is even worse when looking at near objects.