Why are lenses with negative diopters selected for myopic people, and positive lenses for farsighted ones.

Myopia is characterized by the fact that visual acuity decreases and objects that are far away are poorly perceived. This process is associated with large refraction in the optical system of the eyes, which does not correspond to the length of its axis. The image focuses in front of the retina, not on it. Therefore, for myopic people, glasses with negative diopters are selected.
Hyperopia can be called the opposite of myopia, since the problem of hyperopia is associated with the perception of objects at close range. But, if there is an in-depth problem of farsightedness, then the perception of objects at a long distance is impaired. This problem occurs due to a short eyeball or flat cornea. Such a state does not allow light rays entering the eye to be refracted to the extent that would be sufficient for focus on the retina. Therefore, the image focuses not on the retina, but behind it. For farsighted people, glasses with positive diopters are selected.

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