There are many reasons why an eye may not function properly.
The most common of these is when the light does not end up
perfectly focused on the retina. These are called refractive
errors.
Myopia Nearsightedness
In this condition, the image of a distant object
is focused in front of the retina, because the eyeball may
be too long or the refractive mechanism too strong.
Short-sighted people tend to have a blurred distant vision,
but generally good near vision. A minus (concave) powered
lens is required to correct short sight.
Hypermetropia - Farsightedness In this condition, the image of a distant object
is focused behind the retina, because the eyeball may be
too short or the refractive mechanism is too weak. Long-sighted
people Generally have difficulty seeing closer objects.
In young people, distance vision is usually good, but in
older patients, it tends to be reduced. A plus (convex)
powered lens is required to correct long sight.
Astigmatism
Astigmatism is a focusing error, which causes asymmetric
blur due to the uneven curvature of the front part of the
eye called the cornea. Some directions in an image are more out of focus than others. An astigmatic person
will see images distorted or sometimes even double. Certain
letters may be more difficult to read than others image are more out of focus than others.
An astigmatic person
will see images distorted or sometimes even double. Certain
letters may be more difficult to read than others, depending
on the orientation of the lines within them. If not corrected,
a person with astigmatism will experience symptoms like eyestrain
& headaches. A toric or cylindrical lens is used to correct
astigmatism.
Presbyopia - Focusing problems in
the 40's Presbyopia is a common condition, which makes vision
difficult at a normal reading distance, usually occurring
in those more than 40 years of age. In this condition, tasks
such as reading become difficult, particularly under poor
lighting conditions, but distance vision is fine. You may
also have difficulty concentrating when reading, or you may
find periods of close work result in eyestrain, headaches
or tiredness. This usually occurs because the lens within
the eye loses its elasticity with advancing age. A plus powered
lens is used to correct Presbyopia.
