What Are Refractive Errors?
The cornea and
lens bend or refract light rays so they can be focused on the retina, the nerve
layer that lines the back of the eye. The retina receives the picture formed by
these light rays and sends the image to the brain through the optic nerve. A
refractive error means that the shape of the eye does not allow the light to be
properly refracted making images blurry. Refractive errors include:
What is myopia (nearsightedness)?
Nearsighted
people have difficulty seeing objects at a distance, such as highway signs, but
usually can see up-close for tasks such as reading or sewing.
Myopia
(nearsightedness) occurs when light rays are focused in front of the retina
instead of directly on the retina. Myopia is a vision problem experienced by
approximately one-third of the population. When the eyeball is too long from
front to back, the image of a distant object focuses in front of the retina,
instead of directly on it. As a result, the distant object appears blurred. The
more myopic the eye, the closer an object must be before it is in sharp focus.
What is hyperopia
(farsightedness)?
Hyperopia or farsightedness occurs
when light rays are not bent enough to focus on the retina. Hyperopia
is a common vision problem, affecting about one-fourth of the population. If
the eye is too short from front to back, light rays reach the retina before
they converge (focus). People with hyperopia can
sometimes see distant objects very well, but may have difficulty seeing objects
that are close.
Young eyes can
sometimes compensate for this refractive error --- depending on age and the
degree of hyperopia present. But with aging, the
human lens loses this ability and a hyperopic person eventually may have
difficulty seeing objects at a distance, as well as those that are nearby. In
fact by age 40, even those with little or no refractive error will begin to
experience difficulty focusing on close objects.
What is astigmatism?
Astigmatism
occurs when light rays are focused at more than one point on the retina.
Astigmatism is the most common vision problem. It occurs when the cornea
surface is not ideally rounded, but is curved more along one axis than the
other --- that is, when the eye is shaped more like the side of a football than
a basketball. Light entering the eye does not focus symmetrically on the
retina. The result is astigmatism, which blurs both near and distance vision.
What is presbyopia (age-related difficulty with near vision)?
With increasing
age, the lens inside of the eyes loses the ability to focus on nearby objects.
The problem usually manifests itself around age 40 and can be corrected with
bifocals or reading glasses. This is a normal aging process that all people
develop.
Some
people with myopia can use their natural nearsightedness to read without
glasses at an age when other people must wear reading glasses.
