ASTIGMATISM 
In
simplistic terms, if there is no astigmatism, the cornea
of the eye is shaped like a baseball cut in half. The
curvature or steepness of the half-dome is the same
all the way around. Compare this to a cornea which is
similar to a football cut in half lengthwise (in the
long direction, through both pointy ends). The curvature
of the cornea in the long direction (along the seams)
is not as steep as along the short direction. Such a
cornea focuses light, not at a single point, but at
2 points. Someone who has uncorrected astigmatism may
see images that are fuzzy and doubled. A cornea shaped
like a football, cut lengthwise, has astigmatism.
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Eye
with astigmatism |
In
these diagrams of an eye with astigmatism, the rays
of light from the object are being brought into sharp
focus at two different points, one on the retina and
the other, behind the retina. This is an eye with a
cornea that has astigmatism, a non-uniform curvature
like the football cut lengthwise. The two different
curvatures results in two different focal points.
There
are several different combinations of astigmatism, depending
on where the focal points are located.
•
Simple myopic astigmatism: One point
in front of retina, other on the retina
• Compound myopic astigmatism:
Both points of focus in front of the retina
• Simple hyperopic astigmatism:
One point behind the retina, other on the retina
• Compound hyperopic astigmatism:
Both points of focus behind the retina
• Mixed astigmatism: One point
in front of the retina, the other behind the retina
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