11 how can the visual deficiencies known as myopia

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11. How can the visual deficiencies known as myopia and hypermetropia be optically explained? Myopia is the visual condition in which images are formed before (in front of) the retina. Hypermetropia is the visual condition in which the point of image formation is beyond (behind) the retina. In reality, myopia is due to an increase in the distance between the retina and the crystalline lens, mainly caused by a slight flattening of the eyeball. In hypermetropia, the retina is too close to the crystalline lens due to a slight shortening of the eyeball.
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In myopia, the near point and the far point of vision are closer (the refractivity of the crystalline lens corresponding to the maximum distension capacity of the ciliary muscles is not enough to provide visual accommodation). In hypermetropia, the ciliary muscles are not able to contract more to compensate for the inadequate position of the retina, meaning that the near point becomes more distant. Human Eyes Review - Image Diversity: myopia hypermetropia 12. What are presbyopia and astigmatism? Presbyopia is the visual impairment in which the strength of the ciliary muscle is lost, thus reducing the capability of the crystalline lens to adjust images of close objects onto the retina. In presbyopia, the near point of vision becomes more distant. This disease generally occurs in the elderly. Astigmatism is caused by refractive structures of an irregular shape, mainly the cornea. In astigmatism, a single object-point may produce more than one image on the retina and, as a result, the vision becomes distorted. Human Eyes Review - Image Diversity: presbyopia
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