The eye's ability to accommodate allows it to view focused images of both nearby and far away objects. The inability to provide a large variance in focal lengths leads to vision defects.

Myopia (Nearsightedness)

Nearsightedness is the inability of the eye to focus on distant objects. This condition is most common in youth and can be attributed to a bulging cornea or elongated eyeball. Light from distant objects is refracted more than necessary with a bulging cornea, as the convexity is greater, and the image is formed at a location in front of the retina. If the eyeball is elongated horizontally then the retina is placed at a further distance from the cornea-lens system and images of distant objects form in front of the retina. In either case, the image is not focused on the retina and this results in a blurry image of distant objects.

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To correct this condition, a diverging lens is used. The total refracting power of the eye is reduced when the light diverges before reaching the eye and then is converged by the cornea and lens to produce an image on the retina.

Hyperopia (Farsightedness)

Farsightedness is the inability to focus on nearby objects. This condition usually presents itself later on in life, when the ciliary muscles weaken and the flexibility of the lens is decreased. The lens can then no longer assume the highly convex shape, that is needed to focus nearby objects. The total refracting power of the eye has diminished and images are focused behind the retina. If this condition occurs in younger people, the cause is most likely because of a small, or short, eyeball. The retina then lies closer than usual to the cornea-lens system and images of nearby objects are focused behind the retina. In both cases, the image is not focused on the retina and a blurry image results.

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To correct this condition, a converging lens is used. This assists the eye's lens by refracting light before it enters the eye so that the image distance is decreased and the image of nearby objects is once again focused on the retina.

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