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Optical Terms
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
Short-sightedness
Also known as Near-sight or Myopia, short-sighted individuals have trouble
seeing things in the distance while anything close-up is clearer. This
occurs because light from the objects being viewed focuses in front of the
retina and not on it. It is believed that this mostly happens as a result of
the eyeball being slightly too long. The scientific term for this is Axial
Myopia.
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Long-sightedness
Also known as Far-sight or Hyperopia, long-sighted individuals have
difficulty with anything close-up while their general distance vision is not
as bad. Light from objects viewed will theoretically focus behind the retina
and not on it. However because the eye is able to naturally fatten the lens
and cause light to bend more, this error in focus is overcome, which
explains why distance vision is generally pretty good. When the same
individual tries to look at something up close, much more effort is
required, as light from a near object will fall much further back behind the
retina. Vision at near is therefore worse.
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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. This causes the light to focus in two different
positions such that the steeper or more curved area focuses further forward
and the less curved or flatter further back. 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. One popular way of explaining
how this happens likens the natural lens of the eye to a pillow with
feathers inside. The lens contains many fibres (feathers) which grow
throughout life in the same way that hair and skin continues to grow. This
is not surprising considering that these tissues all come from the same
founding cells. 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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How do lenses work?
Lenses have the ability to bend light or change its direction. A minus-powered
lens causes light to diverge once it has passed through, while a plus-powered
lens does the opposite by converging the light towards a point. A minus lens is
generally thicker at the outside edges than at the center and a plus lens is
thicker at its center than at the edge. This applies to both spectacle and
contact lenses.
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Minus Lenses and Short-sight
As the short-sighted eye focuses light in front or short of the retina it
makes sense to use a minus lens to take the focus back to the retina. It
changes the direction of light before it enters the eye by making it more
divergent. This, coupled with the normal focusing of the short-sighted eye,
serves to place the focal point back on the retina assuming the correct
strength of prescription has been used.
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Plus Lenses and Long-sight
The long-sighted person's focus is too far back so a plus-powered lens is
ideal in order to create more bending of light. This convergence of light
occurs in front of the eye and when the effect is coupled with that of the
eye's normal focusing the focal point is brought forwards onto the retina.
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Plus Lenses and Presbyopia
The visual experience of presbyopia is the same as that for long-sight such
that near objects are blurry. The difference is that long-sighted
individuals have a prescription for their distance as well as near vision
and that prescription is always plus-powered. Presbyopes have a more
plus-powered prescription up close only, while their distance vision could
be perfect, short-sighted or even astigmatic. So whatever the individuals
distance prescription may be, the near prescription is invariably always
different.
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Toroidal Lenses and Astigmatism
The astigmatic eye possesses two main areas of curvature causing light to
focus shorter along one curve and further back along the other. Since minus
lenses push the focus of light further back and plus lenses bring focal
points forwards, a combination of the characteristics of these lenses in a
single lens would solve the problem of astigmatism. This lens form is termed
toroidal and in contact lenses the term toric is often used.
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Lens Lingo
- Single vision - One focal distance is required for your eyes to
make adjustments at differing focal distances.
- 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.
- 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.
- 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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