1. THE BASICS
1-1. How does the eye work?
It may
be an oversimplification, but the eye works very much like
a camera (Figure 1). There are a series of lenses: The
cornea, the aqueous humor, the crystalline lens, the
vitreous, humor. The cornea is, by far, the most powerful
lens, and the crystalline is second. Light enters through
the pupil, and is focused on the retina. The retina is
essentially the film in the camera. It records the image,
and sends it to the brain through the optic nerve.
Figure
1
1-2. What part of the eye is treated with LASIK?
LASIK
treats the cornea, the most powerful lens of the eye. The
cornea is a transparent tissue that covers the colored part
of the eye, much like the crystal of a watch covers the
face of the watch.
1-3. What is nearsightedness?
Nearsightedness,
or myopia,
is the eye condition in which the eye focuses well for near
vision, and poorly for distance vision. It is usually due
to the length of the eye (the front to back measurement)
being longer than normal. This causes the image to be
focused in front of the retina, rather than on the retina
(Figure 2).
Figure
2
1-4 What does the retina do?
The retina records the image. It is similar to the
“film” in a camera. In a nearsighted eye, since
the image focuses in front of the retina, it is out of
focus, or blurry, when it finally reaches the retina.
1-5. What is farsightedness?
Farsightedness,
or hyperopia,
is the eye condition in which the eye focuses well for
distance vision, and poorly for near vision. Some
farsighted people have poor vision for both distance and
near. It is usually due to the length of the eye being
shorter than normal. This causes an image to be focused
behind the eye, rather than on the retina (Figure 3).
Figure
3
1-6.
What is astigmatism?
Astigmatism
is the eye condition in which the front part of eye, the
cornea, is not perfectly spherical. One part of the image
is focused on a different spot than another part of the
image (Figure 4). This produces a blurred, or distorted,
image.
Figure
4
1-7. What does an astigmatic image look like?
An
astigmatic image looks like the image in a fun house
mirror. It can be elongated vertically or widened
horizontally. An astigmatic eye may produce rather subtle
distortions, perhaps more closely resembling an El Greco
painting than a fun house mirror.
1-8. What is 20/20 vision?
In the
United States, vision is generally tested at a distance of
20 feet from the eye chart. If you have 20/20 vision, you
can see the small letters a person with normal vision can
see 20 feet away from the eye chart.
1-9. What is the meaning of 20/40, and 20/400 vision?
If your
vision is 20/40, it means that someone with normal vision
can see an object 40 feet away, while you have to move up
to 20 feet from the object in order to see it. If your
vision is 20/400, a person with normal vision can see an
object 400 feet away, while you have to move up to 20 feet
away from the object in order to see it.
1-10. What is the meaning of 20/15 and 20/10?
20/15
means that what a normal person sees 15 feet away, you can
move back to 20 feet and still see the same object. If your
vision is 20/10, it is even better. What someone with
normal vision sees 10 feet away, you could back up to 20
feet and see the same object.
1-11.
What is 6/6 vision?
Most of the world (except the U.S., Britain, Canada, and
few other places), use the metric system. 6/6 is the metric
equivalent of 20/20; 6/12 is the metric equivalent of
20/40; 6/120 is the equivalent of 20/400; and so on.
1-12.
Are there other lenses in the eye?
While the cornea is the strongest lens of the eye,
accounting for about 40 diopters of power, there is a
crystalline lens inside the eye, usually referred to simply
as “the lens,” which has about 10 diopters of
power. Minor contributions to the lens power of the eye
come from two liquids inside the eye: The aqueous humor, a
water substance in the front part of the eye, and the
vitreous, a gel-like substance in the back of the eye.
