Some elderly people wear both glasses for myopia and glasses for presbyopia. When they look at things, they often switch between these two different pairs of glasses. As we all know, myopia glasses are concave lenses, while presbyopia glasses are convex lenses. Does this mean that myopia and hyperopia can occur simultaneously?
In fact, this is a misunderstanding. Although both presbyopia and hyperopia require convex lenses for correction, presbyopia and hyperopia are two completely different concepts. Presbyopia occurs because the elasticity of the lens gradually decreases with age, and its ability to become more convex also decreases. At this point, it is necessary to wear lenses of a certain degree to help the eye increase its focusing ability so that the image can still fall clearly on the retina, which is presbyopia. The anterior-posterior diameter of the eye in presbyopia does not become shorter. Vision formation is related to the adjustment state of the lens of the eye. When looking at things up close, the lens bulges, and when looking at things far away, the lens flattens, allowing light to converge on the retina.
However, there is a situation where true myopia and hyperopia coexist, which is called astigmatism. The corneal surface of astigmatism eyes is an oval spherical surface, with unequal curvature on different meridians; while the corneal surface of normal eyes is a regular spherical surface, that is, the curvature of all meridians on the surface of the cornea is the same. In normal eyes, the light reaching various points on the corneal surface can be focused on the retina after refraction by the eye. However, in astigmatism eyes, due to uneven curvature, some incident light is focused in front of the retina, some is focused behind the retina, and of course, some is focused exactly on the retina. As a result, the light cannot be focused on the same plane, causing blurry vision or deformed images. This type of astigmatism is called regular astigmatism, which can usually be corrected with cylindrical lenses. The causes of irregular astigmatism are more complex.