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20/20 Vision
This is also known as perfect sight where no
prescription is required in order to see things
clearly and comfortably. The fraction 20/20 is
understood as follows: the bottom half represents
the distance in meters at which a perfect sighted
person is able to see clearly a target seen by
another at 20 feet. So if you are perfect sighted
you will see at 20 feet exactly what another perfect
sighted person will see at 20 feet. However, if you
have trouble seeing things in the distance, you
might see at 20 feet what a perfect sighted person
sees at 40 feet. So this means your vision is
measured as 20/40.
Assuming you have perfect sight then when you are
looking at something in the distance, light from the
object will focus on your retinas very precisely.
When seeing something close-up the focusing is
blurred very momentarily, but this is unnoticeable
because your eye has an automatic refocusing ability
that makes the target clear. This refocusing ability
is called 'Accommodation' and it works by making the
natural lens of the eye fatter, or more convex in
shape. As we get older we lose more and more of this
ability, which is why things close-up start to blur
after about 45 years of plodding on the planet.
Short-sightedness means that the light focuses in
front of your retinas or 'too short' and
long-sightedness means it focuses 'behind' or 'too
long'.
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Sight Conditions
Near-sightedness
Short-sighted
individuals have trouble seeing things in the distance
while anything close-up is clearer.
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Far-sightedness
Long-sighted individuals
have difficulty with anything close-up while
their general distance vision is not as bad.
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Astigmatism
Often the eye is shaped like an egg or football.
This is what we call astigmatism. It is quite a
normal condition and most of us have to some
degree. Like an egg, two main areas of curvature
are found: one is flatter and the other is
steeper.The effect
on vision is to distort shapes so similarly
shaped characters become confused. The letter G
may be confused with a C and the number 2 may be
mistaken for the letter Z.
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Presbyopia
As our eyes grow older the ability to focus clearly on
anything close-up starts to diminish. This occurs
because we lose our ability to accommodate or make the
lens of the eye fatter in shape. When the lens is younger
and has fewer fibres it is more flexible and can
change its shape more easily just as a pillow
with fewer feathers can. When the lens is older
and has many more fibres this flexibility is
obviously lost which causes the near vision to
blur.
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Lens
Types
- Single vision
- One focal distance is required for your
eyes to make adjustments at differing focal
distances.
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Bifocals -
The gradual decrease in ability of the eye to
focus is a natural part of the aging process.
Bifocals contain two prescriptions for
correcting vision at different distances.
Bifocals aid both near and far vision, with a
visible line dividing the two areas of power
prescription.
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Trifocals -
Trifocals are prescribed to aid near, far and
middle distance seeing. The trifocal lens design
has three distinct areas of power to be used,
with visible lines dividing the three distinct
areas of power prescription.
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Progressive no lines
- Advanced technology allows these lenses to
gradually change in power from the distance
segment to the near vision segment Your specific
prescription is ground from a series of custom
lens designs and powers.
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