Common Vision Defects

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This depicts the effects of rather extreme cases of nearsightedness, farsightedness and astigmatism on the paths of parallel input rays, which would be characteristic of objects at effectively infinite distance, like looking at the moon. In these extreme cases, neither the farsighted eye nor the nearsighted eye would see the moon clearly. The nearsighted eye refracts the light too much, focusing at a position before it reaches the image-detecting retina. The farsighted eye does not refract the light enough, and cannot bring the rays to a focus by the time they reach the retina.

The diagrams also assume a relaxed eye, in which case the focal length of the eye's interior lens is at a maximum. The eye's effort in accommodating to view closer objects involves muscle tension that results in the interior lens being more rounded and therefore of shorter focal length (greater refracting power). In the cases depicted here, accommodation might be sufficient to bring the moon into focus for the farsighted eye, but would work against the nearsignted eye.

If the object to be viewed is brought closer to the eye, then you would expect that the image would be worse for the farsignted eye and better for the nearsighted eye. An example for an intermediate distance might be instructive.

Intermediate distance example
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Correction of Common Vision Defects

This shows the nature of lenses which would correct the vision by providing the appropriate amount of total refraction to bring the parallel rays from a distant object to a point on the retina. That is the typical strategy for prescription of glasses or contact lenses: correct the eye for clear vision at a great distance and then rely upon accommodation to allow the person to see clearly at intermediate distances and up to a normal close-focus point.

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Hyperopia (Farsightedness)

Common vision defects
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Myopia (Nearsightedness)

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Astigmatism

This is of course a gross oversimplification of the complex problem of correcting astigmatism. If the elements of the eye are significantly asymmetric, then correction is difficult. For the case depicted here, one again tries to correct the vision for a distant object by trying to bring all parallel rays together at a point on the retina. A standard strategy is to start with the best vision that can be achieved with a single symmetric lens, and then add a smaller strength of a cylindrical lens which can be rotated to find the angle of greatest improvement. If that is satisfactory, then a single lens can be manufactured to match that "spherical" plus "cylindrical" correction.

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Nearsightedness and Farsightedness at Intermediate Distances.

It takes more refractive power to focus rays from a closer object since the rays are more divergent when they arrive at the lens. Since the nearsighted eye has excess refractive power that focuses parallel rays somewhere in the interior of the eye, bringing the object closer will cause the focal point to move toward the image-detecting retina and improve the vision. For the extremely farsighted eye above, things just go from bad to worse when you bring the object closer since you need more refractive power and you didn't have enough to begin with.

Common vision defects
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