Astigmatism is a type of optical aberration,
and is therefore an important consideration in the design of optical systems
- especially those consisting of a series of lenses who purpose is to
form a sharp clear image of an object at a specific location in space.
An optical system with astigmatism is one in which rays
light propogating in two perpendicular planes reach points of focus (plural:"foci")
at different locations in space.[more
Biological eyes, including human
eyes, are sophisticated optical systems in that they control and direct
light entering the eye ("system") via a series of mechanisms,
such as :
- variation in the size of the pupil
to control the amount of light entering the eye,
- action of the ciliary
muscle to control the curvature of the lens, hence accomodation
for near- and distant- vision, and
- combination of curvatures of surfaces and refractive indices of the
tissues (esp. the cornea,
humour) acting together to direct light entering the eye to form
a sharp image on the retina.
As for other types of image-forming optical systems, an eye is said to
suffer from astigmatism if the image formed (on the
retina, in the case of an eye) is distorted, hence out-of-focus,
due to light rays entering the system along different (perpendicular,
e.g. horizonal vs. vertical) planes being brought to foci at different
In such cases, parts of an object may appear to be in focus while other
parts of the same small object appear to be blurred - because (in
the case of an eye) the optical focus of the rays from the "blurred"
parts of the object are located either in front of, or behind, the retina
- so complete information about those parts of the object is not transmitted
to the brain.
Astigmatism is a relatively common condition. If present
it is usually congenital (i.e. present from birth). In mild cases the
eye can adjust to focus light adequately so no treatment, or only mild
treatment, is required. However in many cases the patient also has other
visual conditions such as short-sightedness or long-sightedness. In that
case, it may be possible to treat the patient's personal combination of
visual conditions together, e.g. with a single prescription for spectacles
or contact lenses.
Causes of Astigmatism in Human Vision:
In the context of human vision / ophthalmology, astigmatism
is usually attributed to the shape (curvature) of the cornea and/or to
the the shape (curvature) of the lens.
Treatments for Astigmatism in Human Vision:
Possible treatments for astigmatism include prescription
of spectacles or contact lenses, or surgery e.g. laser eye-surgery using
an excimer laser to adjust the curvature of the cornea.
More about Ophthalmology: This section includes short definitions
of many diseases, disorders, and conditions of the eyes and visual system.
For definitions of other terms in this category, choose from the list
to the left (but note that this is not a complete/exhaustive list).
Other related pages include the diagram
of the eye, and definitions
of parts of the eye, a description
of the human retina, and definitions
of parts of the retina, and the section of short definitions
of clinical and surgical procedures re. eyes and human visual system.
For further information see also our section of Books