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.
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.
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.
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.
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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