Sometimes, for different
reasons, the eye doesn't focus quite right:
The surfaces of the lens or
cornea may not be smooth, causing an aberration that results in a streak of
distortion called astigmatism.
The lens may not be able to
change its curve to properly match the image (called accommodation).
The cornea may not be
shaped properly, resulting in blurred vision.
Most vision problems occur
when the eye cannot focus the image onto the retina. Here are a few of the
most common problems:
● Myopia (nearsightedness)
occurs when a distant object looks blurred because the image comes into
focus before it reaches the retina. Myopia can be corrected with a minus
lens, which moves the focus farther back.
● Hyperopia
(farsightedness) occurs when a close object looks blurred because the image
doesn't come into focus before it gets to the retina. Hyperopia, which can
also occur as we age, can be corrected with a plus lens. Bifocal lenses,
which have a small plus segment, can help a farsighted person read or do
close work, such as sewing.
● Astigmatism is caused by
a distortion that results in a second focal point. It can be corrected with
a cylinder curve.
In addition, lenses can be
made to correct for double vision when the eyes do not work together
("crossed eyes"). The lenses do this by moving the image to match the
wayward eye.
Corrective lenses, then,
are prescribed to correct for aberrations, to adjust the focal point onto
the retina or to compensate for other abnormalities. You can read more about
vision problems in How Refractive Vision Problems Work.
How a Lens
Works
The best way to understand
the behavior of light through a curved lens is to relate it to a prism. A
prism is thicker at one end, and light passing through it is bent
(refracted) toward the thickest portion. |