Astigmatism causes blurred or distorted vision because the front of your eye (or the lens inside it) is curved unevenly. Instead of being round like a basketball, the surface is shaped more like a football, bending light unevenly so it doesn’t focus to a single sharp point. Most people with astigmatism notice it as general blurriness, but the signs are often more specific than that, especially in certain lighting conditions.
The Most Common Signs
Blurry vision is the hallmark, but astigmatism blurs things in a particular way. Rather than everything looking equally fuzzy (the way nearsightedness makes distant objects uniformly soft), astigmatism tends to stretch or smear images in one direction. You might notice that vertical lines look crisp while horizontal ones seem slightly doubled, or vice versa. Letters on a screen can look shadowed or ghosted rather than simply out of focus.
Other everyday signs include squinting to sharpen your view, headaches after reading or screen work, and a tired, strained feeling around your eyes by the end of the day. These symptoms often worsen when you’re doing tasks that demand sustained focus, like working on a computer, reading small print, or sewing.
Why Night Driving Feels Worse
If your vision seems fine during the day but falls apart behind the wheel at night, astigmatism is a likely culprit. In low light your pupils widen, which lets more of the unevenly curved surface contribute to the image your eye forms. The result is a set of visual effects that many people recognize immediately once they know what to look for:
- Starbursts: Points of light, like oncoming headlights, shoot rays outward in a spiky pattern.
- Halos: Glowing rings appear around streetlights and traffic signals.
- Streaks: Headlight beams seem to stretch or smear across your field of view.
- Increased glare: Bright lights feel more intense and harder to look past.
These effects also show up around screen light in a dark room. If you notice halos around icons on your phone when you check it in bed, or extra eye strain when watching TV with the lights off, the same uneven curvature is at work.
A Simple At-Home Screening
You can get a rough sense of whether astigmatism is present with a fan dial chart, which is a circle of evenly spaced lines radiating outward like the spokes of a wheel. Every line is drawn at the same width and darkness. To use it, cover one eye and look at the center of the chart from about arm’s length.
If all the lines look equally sharp and dark, your eye is focusing light fairly evenly. If certain lines appear noticeably darker, bolder, or more in focus while others look faded or blurry, your eye is focusing better along one plane than another. That difference is the signature of astigmatism. Repeat with the other eye, since the two eyes can have different amounts.
This kind of screening can hint at a problem, but it can’t tell you how much astigmatism you have or whether you need correction. It’s a useful first step, not a diagnosis.
What Causes It
Most astigmatism is simply the shape you were born with. It’s hereditary, passed down from parents, and it’s extremely common. Slight amounts are almost universal. In corneal astigmatism, the clear front surface of the eye has mismatched curves, steeper in one direction than the other. In lenticular astigmatism, the lens inside the eye is unevenly shaped instead. The cornea is responsible for the larger share in most people, while the lens contributes a relatively constant, smaller amount throughout life.
This “regular” astigmatism is predictable: the two curves sit roughly 90 degrees apart, like the wide and narrow sides of a football. Irregular astigmatism is less common and has more complex causes. An eye injury, a corneal disease called keratoconus (where the cornea thins and bulges forward), or scarring from eye surgery can all distort the surface in uneven ways that are harder to correct with standard glasses.
How an Eye Exam Confirms It
An eye care professional can measure astigmatism precisely in a few ways. Keratometry uses an instrument that looks a bit like a telescope. You rest your chin on a support, look through an eyepiece at a set of targets (usually three circles with plus and minus signs), and hold still while the examiner lines everything up. The keratometer measures the curve of your cornea along different angles to detect any mismatch.
Many offices now use corneal topography instead, which maps the entire front surface of the eye in fine detail. It’s faster and more comprehensive. Your provider may also use an autorefractor, a machine that bounces light into your eye and reads how it comes back, to estimate your overall prescription before fine-tuning it with the familiar “which is better, one or two?” lens comparison.
Together, these tools pinpoint exactly how much astigmatism you have, where it sits on the eye, and whether it’s the common regular type or something that needs closer attention.
Reading Your Prescription
Once you’re diagnosed, astigmatism shows up on your glasses or contact lens prescription in two numbers: CYL (cylinder) and Axis. CYL represents the amount of extra corrective power needed to compensate for the uneven curve. It’s measured in diopters and may have a minus sign (correcting nearsighted astigmatism) or a plus sign (correcting farsighted astigmatism). A CYL of -0.50 is mild; -2.00 or higher is more significant.
Axis is a number from 1 to 180 that tells the lab which angle to orient that correction. Think of it as the compass heading for the steepest curve on your eye. Without the axis, the cylinder power would be pointed in the wrong direction and wouldn’t help. If your prescription has a CYL value, it will always have an axis paired with it.
What Mild Astigmatism Feels Like
Many people have small amounts of astigmatism and never realize it because the eye compensates well enough during the day. You might only notice symptoms when you’re tired, when lighting is poor, or when you’ve been staring at a screen for hours. If your CYL value is -0.75 or less and you’re not bothered by your vision, correction isn’t always necessary.
Higher amounts are harder to ignore. Text may never look fully sharp, depth perception can feel slightly off, and the nighttime glare effects become pronounced enough to make driving uncomfortable. Children with uncorrected astigmatism sometimes tilt their heads or hold books at odd angles without knowing why, so pediatric eye exams are important even when a child doesn’t complain about their vision.
Correction Options
Standard glasses with a cylinder correction are the simplest fix and work well for both regular and irregular astigmatism. Toric contact lenses are designed with different powers along different angles, weighted so they stay oriented correctly on the eye. They’re available in soft, rigid gas-permeable, and hybrid designs. Rigid lenses can sometimes correct irregular astigmatism that soft lenses can’t, because the hard surface creates a smooth optical front regardless of the cornea’s shape underneath.
Laser eye surgery (LASIK or PRK) reshapes the cornea to even out the curvature and can reduce or eliminate astigmatism permanently for many people. The procedure is quick, but not everyone is a candidate, especially if the cornea is unusually thin or if keratoconus is the underlying cause. For people with keratoconus, specialty rigid or scleral lenses, and sometimes a collagen cross-linking procedure to stabilize the cornea, are the more appropriate path.