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What is Astigmatism?

By January 25, 2015FAQs

Astigmatism is one of the least understood concepts in all of eye care. Partly because it can be difficult to put into words (and even more difficult to explain in writing) and it takes time to it explain well. For that reason, optometrists will often brush it off or just use a not-so-helpful analogy like: “Your eye is shaped like a football”… which probably raises more questions than it answers.

I will do my best to explain astigmatism.


I am not a doctor and this article is not a substitute for professional medical advice. You are not being a diagnosis in this article. Please read Contacts Advice Terms of Use before continuing.


We first have to understand some very basic concepts in visual optics. Stay with me here, it’s not as bad as you think!

  1. We see because light from the outside world enters our eye and stimulates the retina.
  2. We see clearly when our eyes focus said light onto the retina.
  3. The eye has 2 parts that focus light: the cornea, and the crystalline lens. Astigmatism is mostly due to the cornea, so we will forget about the crystalline lens for now.
  4. The cornea focuses light because of two simple properties: it is transparent, and it is curved. Astigmatism is related to the curvature of the cornea, so we can forget about the transparency for now.
  5. When the cornea only has a single curve all the way around, it is shaped like part of a perfect sphere. That will cause light to be focused the same in all directions, creating a single point of focus, such as in the image below. When that is the case, there is no astigmatism present.
    No astigmatism

    Light comes to a single point of focus within the eye.

  6. However, when the cornea has two different curvatures, it losses its spherical shape and becomes more oval-shaped. This is where the football analogy comes in. When the cornea has two different curvatures, each curvature will focus light differently, and give rise to two separate points of focus within the eye, such as in the picture below. This is astigmatism.

    Light forms two separate points of focus within the eye.

Note that oval-shape of the cornea with astigmatism in the pictures above cannot be appreciated because they are 2-D pictures of a cross-section of the eye. 

When astigmatism is present, glasses or contact lenses must be placed in front of the eyes to counter balance the two separate curvatures of the cornea. This allows the light to be focused on the retina at a single point instead of two, which is what we need in order to see clearly.

Astigmatism is not a disease! It is a refractive condition, just like nearsightedness or farsightedness are. In fact, most people have some degree of astigmatism. But wearing contact lenses for astigmatism is not necessary until the astigmatism reaches a certain level. Unfortunately, if your astigmatism is too high, you may not be able to wear contact lenses because range of prescriptions available in contact lenses is limited.


Author Julie

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Join the discussion 4 Comments

  • Peggy Menke says:

    Hello, Julie. Thank you so much for this valuable information. I was told I have a slight astigmatism in my left eye and have been wearing glasses for the last 5 years. I never really understood why this happens, but because of your information I know now. Thanks again for sharing this!

    • Julie says:

      Hi Peggy!
      Thanks so much for the comment!
      I’m so happy that you read this and got something out of it. I wasn’t sure I could explain in a way that would make sense. I really appreciate your feedback 🙂

  • Niki Waldrep says:

    Hello, Julie!
    Thanks for the explanation about astigmatism. When I was 10 yrs old & was first told I needed eyeglasses due to ” near-sightedness”, my great aunt, known from her routine letters back and forth with me, explained that it meant that my “eyeball” shape was more like an egg, instead of perfectly round like a golf ball, and this caused light to not reach a focus point on my retina where it should, but more like “too early”. She even accompanied the explanation with a little illustration. (How much I miss her, and the way she kept routine correspondence with me, as well as taking the time to explain things about the world! Letter writing has become a lost art, at my current age of 51!) So when I developed an astigmatism at my last eye exam, I didn’the truly understand it, as I was already aware that my cornea was “egg-shaped”. I’m grateful for the explanation that your cornea can have uneven areas of the surface, causing light to have multiple areas of incorrect focus. Thank goodness for people such as yourself who are able to take a sort of complicated subject, and condense it down to a basic understanding of the situation!
    Wishing you the very best-
    Niki Waldrep

    • Julie says:

      Niki, thank you so much for sharing that great story about your aunt and how she explained to you what astigmatism is. She was quite knowledgeable about eyes (as well as many other things I’m sure)! It’s so nice to hear back from people who read my articles. I’m so happy that you enjoyed it 🙂

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