Astigmatism is caused by an uneven or irregular shape of your cornea, the front surface of your eye, or your lens, the part of the eye that focuses light onto your retina. If you have astigmatism, your vision may be blurred or distorted at all distances, near or far.1
Normal Eye vs. Astigmatic Eye
With a healthy, normal eye, the shape of the cornea is spherical, like a basketball. This allows light to pass through the lens without distortion. Light rays focus properly on your retina to provide a sharp image of what you are seeing.
With astigmatism, the cornea has more of an oblong shape, like a football, causing light to bend and distort as it passes through the lens. This makes objects appear blurry or unfocused because light rays are not focused on one spot to provide clear vision.
Though astigmatism is most often hereditary, it can also be a result of surgery or trauma, injury, infection or certain rare conditions that cause the shape of the cornea to change over time.
Another type of astigmatism is mixed astigmatism. If you have mixed astigmatism, symptoms of nearsightedness and farsightedness occur at the same time. This combination of vision problems makes it so that you cannot see images clearly.
What Astigmatism Means
Astigmatism is not a disease; this is a very common misconception. It's simply a variation in the shape of your cornea or lens, and it's very common in people of all ages. If you are nearsighted or farsighted, there's a good chance you'll also have a touch of astigmatism. A small degree of this refractive error is considered normal and does not require correction. But when astigmatism affects your ability to see clearly, glasses, contacts or LASIK surgery are common ways to solve the problem.
What Is the Cornea?
The cornea is the clear protective surface over the front of your eye. When you look at something, light reflects off that object and enters your eye through the cornea. The light then passes through the pupil into the lens, forming an image that is focused on the retina. The retina contains millions of tiny light-sensing nerve cells called rods and cones that send information about that image to your brain.
The cornea is an important part of your ability to see clearly because it refracts, or bends, light as it enters the eye and helps focus it toward the retina.4 A correctly shaped cornea is round and smooth like a basketball. Light can hit it from any direction and be focused sharply on a single point on the retina. But if your cornea has a steeper curve in one direction—more like a football than a basketball—light will bend in slightly different directions as it enters the eye. As a result, rather than coming to a simple point of focus, the focus is spread out, making images look blurry, fuzzy, or slightly distorted. This spreading out of the focus is called astigmatism.2