Astigmatism

Labelled Diagram of the Eye

Labelled diagram of the structure of the retina

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.

Definition: An optical system with astigmatism is one in which rays of light propogating in two perpendicular planes reach points of focus (plural:"foci") at different locations in space.

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, aqueous humour, lens and vitreous 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 locations. 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

For further information see also our pages of books about ophthalmology.

The following books may also be of interest:

The Aging Eye (Harvard Medical School)
MCQ Companion to the Eye (Textbook)
The Secret of Perfect Vision: How You Can Prevent and Reverse Nearsightedness
What Your Doctor May Not Tell You About(TM) Glaucoma: The Essential Treatments and Advances That Could Save Your Sight

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