 Your eyes are amazing tools that take in light and transmit
electrical impulses to the brain. The eyes are like a film camera.
Both are equipped with lenses, but neither really sees anything.
Rather, they collect information to be sent for processing. The
image that appears on the retina of the eye is transmitted to the
brain for processing where vision occurs.There are two main "lenses" of the eye: the cornea and the
crystalline lens. The cornea is a transparent window in front of
the eye and is responsible for about two-thirds of the eye's
focusing power. Just behind it is the iris, which gives the eye
its color. In the center of the iris is the pupil which allows
light into the eye. Light passes through the cornea into the
aqueous body, through the crystalline lens which gives the eye the
remaining one-third of the focusing power then passes through the
vitreous body before hitting the retina. The retina then processes
the light into electrical transmissions to the optic nerve. This
leads to the brain which then interprets and creates an image.
Myopia
People need to wear glasses or contact lenses to help the eye
focus when there is a refractive error. The most common type of
focusing problem is myopia (nearsightedness). Light coming into a
nearsighted eye will come to a focus in front of the retina and be
out of focus by the time it reaches the retina. The cause of
myopia in the majority of cases is that the eye has grown too
large. Myopic eyes can see clearly up close, but objects in the
distance are blurry. Minus-powered lenses are used to move the
focusing point farther back onto the retina.
Hyperopia
Another type of focusing problem is hyperopia (farsightedness). In
this case the light theoretically focuses behind the retina. The
typical farsighted eye is smaller than average. Hyperopic eyes see
better in the distance than they do up close, but neither distant
or near objects may be clear. Plus-powered lenses are used to
bring the focus point forward to the retina.
Astigmatism
Astigmatism is the remaining type of refractive error. The cornea
is irregular in shape being more oval than round. Instead of
having one focus point like the normal eye, an astigmatic eye has
two primary focus points. The more astigmatism there is, the
farther apart the two focus points are and the blurrier the vision
becomes. Astigmatism will affect both distance and near vision
equally. A lens that is curved like a cylinder is used to bring
the two focus points together to form one focus point.
Presbyopia
The word presbyopia is Greek for 'old sight'. The crystalline lens
inside the eye can adjust its power to change the eye's focus
between objects in the distance and up close. Therefore a young
child is able to bring objects very close to his/her eyes and keep
the object in focus. Unfortunately, as life progresses, the lens
becomes less flexible and cannot change shape as easily. This is
known as presbyopia. The early signs of presbyopia are needing
more light to read by and/or moving reading material farther away
to see clearly. It's at this point that reading glasses or bifocal
lenses are helpful. There are a few different solutions to
presbyopia: bifocals, reading glasses, and monovision. The
difficulty for some is choosing the best solution. We'll try to
help here.
Optical Remedies
Bifocals
A lot of people do not like to wear bifocals (including the
progressive lenses or no-line bifocals). Reasons for this vary
from difficulties adapting to the way things look through bifocals
(especially when negotiating stairs, curbs, etc.) to not wanting
to feel/look old. The progressive lenses will take away the line
of the bifocal, so cosmetically nobody can tell you're wearing
them. Progressive lenses also have the added benefit of being able
to provide clear vision at any distance. True bifocals are great
for near and far, but the intermediate range is neglected.
Reading glasses
Those individuals who have never worn glasses (or wear contact
lenses to correct their distance vision) will need to resort to
wearing reading glasses (or bifocals with no corrective power in
the top part of the lens). These are available in the drug store,
powers ranging from +1.25 for those just starting to need reading
glasses to +3.25. Choosing the right lens power is simple: Find
the lowest power that allows you to read at the distance you like
to read at. Over the years, the power of the readers will need to
be increased.
The problem with reading glasses is that they are often not
located where you need them to be when you suddenly need to see
something up close. For this reason, most people who wear reading
glasses will have multiple pairs of them; one for the office, one
in the living room, one on the nightstand.Research is underway in
the area of presbyopic treatments and although not available at
this time, technologies are being developed that will reverse
presbyopia and will allow surgery for those that require reading
glasses.Monovision may be an attractive alternative for this
problem.
Monovision
Monovision refers to having one eye that is nearsighted (allowing
that eye to read) and the other eye being focused for the
distance. A minority of people have monovision naturally, but most
people who enjoy monovision are doing it with contact lenses or
had their eyes corrected with laser eye surgery. One thing is
certain: Monovision is not for everybody! Both distance and near
vision will be somewhat compromised compared to having both eyes
focused at the same distance. The majority of people do not adapt
well to monovision, but for those who tolerate the difference
between the two eyes, there's no better way to go.
Who uses Monovision?
The most successful candidates for monovision are individuals who:
- Adapt well to change and can accept less than perfect
vision.
- Can concentrate on using one eye for reading and the other
for distance (some people have such a strong dominant eye that
the brain fails to recognize the other eye is clearer at a given
distance).
- Do not do long periods of near vision tasks - because most
people need to concentrate to see with the non-dominant eye (the
eye that is normally used as the reading eye in monovision), it
can be tiring if this must be done for several hours a day.
The best way to experience monovision before surgery is to have
a trial with contact lenses. If this is not possible, be sure to
discuss monovision with your eye care provider before making
your decision.
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