If you need eyewear to see clearly, you are either myopic or hyperopic,
and perhaps astigmatic as well (assuming there is no eye disease).
These three conditions are called the Lower Order Aberrations and can
usually be corrected by LASIK or another laser vision correction procedure.
The eyes have an ability called Accommodation, which is switching focus
between different distances. In a 20/20 eye objects are clear at all
distances. However:
If you have myopia, close objects are clear but distant objects are blurry
If you have hyperopia, far objects are clear but close-up objects are blurry
The structure in the eye that has this accommodative ability is the
lens. It can change its shape, its curvature, to bend light at
different angles. The eyes have a second lens known as the cornea. This
is the clear front covering, over the iris and pupil. It bends
(refracts) light entering the eye, but it cannot change its curvature.
The cornea does about 60 percent of refraction and the lens follows up
with the other 40 percent. In a normal eye, they work well together to
bring all objects to a clear focus on the retina.
The degree to which the eyes can refract light is known as their
optical power, their focusing power, or their refractive ability. Those
terms are all synonymous. Please see our Patient Education section to learn more about how the eyes work.
Distance, Light Refraction, and Myopia
Myopia is nearsightedness, also called shortsightedness. A myopic eye
bends light too much and focuses it before it reaches the retina at the
back of the eye. Other ways of saying this are:
A myopic eye has too much optical power (refractive power) for its length (front to back); and
A myopic eye is too long for its degree of optical power.
Light rays from distant objects need less refraction to focus on the
retina than light rays from close-up objects. If your eyes refract all
incoming light fairly sharply (because of corneal steepness), then far
objects may appear blurry. Close-up objects are clear because they need
the sharp light refraction a myopic eye gives them.
Severe myopia has a smaller range of distances in which things look clear
Mildly myopic eyes can see clearly further away and need less correction
Myopia is corrected by reducing the eye’s optical power – reducing how
sharply it refracts light. The LASIK laser does this by slightly
flattening the cornea according to your very precise treatment plan.
Small amounts of tissue are removed from the center area of the cornea.
When the corneal curvature is flatter, light is not bent so sharply as
it passes through and distant objects can come into focus.
Distance, Light Refraction, and Hyperopia
Other names for hyperopia are farsightedness and long-sightedness. A
hyperopic eye is not bending light rays enough to focus them on the
retina. Instead, they are focusing behind the retina (or would be if
they could pass through the dense retinal tissue). The other ways of
expressing this are:
Hyperopic eyes have too little optical power (refractive power) for their length front to back; and
Hyperopic eyes are too short for their strength of optical power.
Since light rays from close-up objects need stronger optical power
(focusing ability) than distant objects, a hyperopic eye does not see
them clearly; but it does see distant objects clearly, since they do
not need more optical power than the hyperopic eye has.
Hyperopia can be corrected by steepening corneal curvature. The LASIK
laser does this by removing tiny pieces of corneal tissue from around
the corneal periphery. Then light will be more sharply refracted and
close-up objects will focus on the retina.
Glasses and contact leses modify the eye’s focusing power by placing a third lens in front of the
cornea. Laser vision correction does the modification permanently,
right on the cornea, and frees you from the daily chores associated
with eyewear.
If you would like to schedule a personal consultation with one of our LASIK doctors, please contact us today. We have many locations in North Carolina and we hope to meet with you soon.
