Astigmatism usually
occurs when the front surface
of the eye, the cornea,
has an irregular curvature.
Normally the cornea is smooth and equally curved in
all directions and light entering the cornea is
focused equally on all planes, or in all directions.
In astigmatism, the front surface of the cornea is
curved more in one direction than in the other. This
abnormality may result in vision that is much like
looking into a distorted, wavy mirror. Usually,
astigmatism causes blurred
vision at all distances.
Astigmatism causes
blurry vision when looking at objects
that are close (e.g.,
reading a letter) and objects that
are far away (e.g.,
reading a road sign).
Vision with astigmatism is not necessarily
uniformly blurry; some distances are more
out of focus than others.
Presbyopia
(aging of the lens in the eye)
After age 40, the
lens of the eye becomes more
rigid and
does not flex as easily. The result is that it is
more difficult to read at close range. This normal aging
process of the lens can also be combined with
myopia, hyperopia or astigmatism.
A person with presbyopia has difficulty focusing on close
objects. It is similar to hyperopia,
but the underlying cause is different.
Symptoms may seem to occur suddenly, but
presbyopia develops
over several years, as the
lens capsule loses its ability to expand as a
person ages. The result is that while
distant objects are seen
clearly, close objects
are blurred because the lens cannot resume
the shape that it needs to bring them into focus.
