The human eye's ability to accommodate allows
it to view focused images of both nearby and distant objects. As mentioned earlier in Lesson 6, the
lens of the eye assumes a large curvature (short focal length) to bring nearby
objects into focus and a flatter shape (long focal length) to bring a distant
object into focus. Unfortunately, the eye's inability to a provide a
wide variance in focal length leads to a variety of vision defects. Most often,
the defect occurs at one end of the spectrum - either the inability to assume a
short focal length and focus on nearby objects or the inability to assume a
long focal length and thus focus on distant objects.
Farsightedness or hyperopia is the
inability of the eye to focus on nearby objects. The farsighted eye has no
difficulty viewing distant objects. But the ability to view nearby objects
requires a different lens shape - a shape that the farsighted eye is unable to
assume. Subsequently, the farsighted eye is unable to focus on nearby objects.
The problem most frequently arises during latter stages in life, as a result of
the weakening of the ciliary muscles and/or the decreased flexibility of the
lens. These two potential causes leads to the result that the lens of the eye
can no longer assume the high curvature that is required to view nearby
objects. The lens' power to refract light has diminished and the images of
nearby objects are focused at a location behind the retina. On the retinal
surface, where the light-detecting nerve cells are located, the image is not
focused. These nerve cells thus detect a blurry image of nearby objects.
The cure for the farsighted eye centers around assisting the lens
in refracting the light. Since the lens can no longer assume the convex and
highly curved shape that is required to view nearby objects, it needs some
help. Thus, the farsighted eye is assisted by the use of a converging lens.
This converging lens will refract light before it enters the eye and
subsequently decreases the image distance. By beginning the refraction process
prior to light reaching the eye, the image of nearby objects is once again
focused upon the retinal surface.
While farsightedness most often occurs among adults,
occasionally younger people will suffer from this vision defect. When
farsightedness occurs among youth, the cause is seldom related to the inability
of the lens to assume a short focal length. In this case, the problem is more
closely related to an eyeball that is shortened. Because the eyeball is
shortened, the retina lies closer than usual to the cornea and lens. As a
result, the image of nearby objects is formed beyond the retina. The
traditional correction for such a problem is the same as for adults - the use
of a converging lens.