In this blog post, we’ll take a simple look at what astigmatism is, how it occurs, and how it’s corrected.
Astigmatism is a condition in which the eye’s refractive power isn’t uniform across all directions of the eyeball, preventing light from focusing precisely on a single point. In the past, many people had vision of 1.0 or better without surgery, but today it is common for people to wear glasses or contact lenses, or to undergo vision correction surgeries such as LASIK or LASEK. As a result, terms like myopia, astigmatism, and hyperopia are familiar to most people. Additionally, students in science and engineering fields are relatively well-versed in the principles of myopia and hyperopia through their studies of optics. However, they rarely have the opportunity to learn exactly what astigmatism is. In this article, we will take a closer look at what astigmatism is.
Astigmatism refers to a condition in which the eye cannot focus on a single point because its refractive power is not the same in all directions. In other words, the eye’s refractive power varies depending on the direction of light, meaning that light entering from different directions cannot converge at the same point. For example, assuming there are vertical and horizontal axes, in a normal eye (emmetropia), points A and B—which should converge at a single point—form focal points at different locations in an astigmatic eye. As a result, people with astigmatism may see objects as blurred or overlapping, or they may feel as if the images are wobbling. They may also experience severe eye strain, eye pain, or headaches.
Regular astigmatism, the most common type, occurs because the curvature of the cornea varies depending on the direction. However, irregular astigmatism, which is caused by certain conditions such as corneal scarring, keratoconus, early-stage cataracts, or a conical lens, may have different underlying causes. In regular astigmatism, the astigmatic axes—that is, the two axes with different refractive powers—generally form a right angle with each other; this characteristic can be used to diagnose and correct astigmatism.
In a normal eye, lines in all directions can be seen clearly. In contrast, a person with astigmatism sees lines in a specific direction much more clearly than those in other directions, while lines perpendicular to that direction appear relatively blurry. To verify this, you can zoom in on a screen or image, or compare the view while wearing and without your glasses; this makes it a little easier to notice the difference.
The direction that appears sharp and the direction perpendicular to it constitute the astigmatism axis. When correcting astigmatism with glasses, cylindrical lenses are used, which have different refractive powers depending on the direction. For example, in astigmatism with horizontal and vertical axes where the refractive power in the vertical direction is greater, the glasses appropriately correct the horizontal image so that light from both the horizontal and vertical directions focuses at the same point. Therefore, when you first wear astigmatism-correcting glasses, the proportions of objects may feel different than before—for instance, a square might appear slightly rectangular.
When wearing glasses or contact lenses for astigmatism, the proportions of real objects may feel different. This adjustment period is also why many people experience dizziness when they first start wearing glasses with a prescription for astigmatism or when they change their prescription. However, since astigmatism is a refractive error that significantly causes eye strain, wearing glasses or lenses that match your vision and the degree of your astigmatism is beneficial for your eye health, even if you feel slightly dizzy.
