| Refractive Errors What is normal vision?
In order to better understand how refractive errors affect our vision, it
is important to understand how normal vision occurs. For persons with normal vision, the
following sequence takes place:
1. Light enters the eye through the
cornea, the clear, dome-shaped surface that covers the front of the eye.
2. From the cornea, the light passes
through the pupil. The amount of light passing through is regulated by the iris, or the
colored part of your eye.
3. From there, the light then hits the
lens, the transparent structure inside the eye that focuses light rays onto the
retina.
4. Next, it passes through the virtuous humor, the clear,
jelly-like substance that fills the center of the eye and helps to keep the eye round in
shape.
5. Finally, it reaches the retina, the
light-sensitive nerve layer that lines the back of the eye, where the image appears
inverted.
6. The optic nerve is then responsible for
interpreting the impulses it receives into images.
What are refractive errors?
The following are the most common refractive errors, all of which affect vision and may
require corrective lenses or surgery for correction or improvement:
astigmatism
Astigmatism is a condition in which an abnormal curvature of the cornea can cause two
focal points to fall in two different locations -- making objects up close and at a
distance appear blurry. Astigmatisms may cause eye strain and may be combined with
nearsightedness or farsightedness. Eyeglasses, contact lenses, or corrective surgery may
help to correct or improve the condition.
- hyperopia
Commonly known as farsightedness, hyperopia is the most common refractive error in which
an image of a distant object becomes focused behind the retina, either because the eyeball
axis is too short, or because the refractive power of the object is too weak. This
condition makes close objects appear out of focus and may cause headaches and/or eye
strain.
Eyeglasses or contact lenses may help to
correct or improve hyperopia by adjusting the focusing power to the retina. Corrective
surgery may also help by changing the shape of the cornea to a more spherical, round shape
instead of an oval shape.
- myopia
 
Commonly known as nearsightedness, myopia is a condition
in which, opposite of hyperopia, an image of a distant object becomes focused in front the
retina, either because the eyeball axis is too long, or because the refractive power of
the object is too strong. This condition makes distant objects appear out of focus and may
cause headaches and/or eye strain.
Eyeglasses or contact lenses may help to
correct or improve myopia by adjusting the focusing power to the retina. Corrective
surgery may also help by changing the shape of the cornea to a more spherical, round shape
instead of an oblong shape.
- presbyopia
Another type of farsightedness, presbyopia is caused when the center of the eye
lens hardens making it unable to accommodate near vision. This condition generally affects
almost everyone over the age of 50 -- even those with myopia. Eyeglasses or contact lenses
may be prescribed to correct or improve the condition.
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