EMMETROPIA (Normal Vision):
Emmetropia is the term used to describe an eye that
requires no correction. Light rays that enter the
eye are focused on the retina.
MYOPIA (Nearsightedness):
Myopia is characterized by blurred distance vision
at all times, while near vision is in most cases
clear. With myopia, the image focuses in front of
the retina as demonstrated above. Myopia can be
caused by an eyeball that is too long (axial myopia)
or the refractive power of the cornea is curved
excessively (refractive myopia). Myopia usually
increases during adolescence and stabilizes about
the age of twenty-five. Symptoms include blur at
a distance.
HYPEROPIA (Farsightedness):
Hyperopia is characterized by blurred near vision,
while distance vision is in most cases clear. With
hyperopia, the image focuses behind the retina,
as demonstrated above. Hyperopia can be caused by
an eyeball that is too short (axial hyperopia) or
the refractive power of the cornea is too flat (refractive
hyperopia). Common symptoms include headaches and
blur with prolonged reading or near tasks.
ASTIGMATISM:
Astigmatism is distorted vision caused by a variation
in refractive power, along different meridians of
the cornea - usually 90 degees apart. As noted in
the above diagram, the yellow ray of light focuses
on the retina, while the pink ray of light focuses
in front of the retina (two different focusing locations).
Large amounts of astigmatism can be corrected with
glasses or specialized contact lenses--soft toric
or rigid gas permeable contacts.
PRESBYOPIA: The onset of this condition is
usually around the age of 40 years. It is caused
by the loss of accommodation or elasticity in the
crystalline lens inside the eye. Symptoms are first
noticed when one begins to push small print away
to achieve a better focus. Multifocal lenses, either
in the form of spectacle lenses or bifocal contacts,
are the prefered treatment.
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