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.
We first have to understand some very basic concepts in visual optics. Stay with me here, it’s not as bad as you think!
- We see because light from the outside world enters our eye and stimulates the retina.
- We see clearly when our eyes focus said light onto the retina.
- 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.
- 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.
- 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.
- 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.
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.