Astigmatism (often misheard as “a stigmatism”) is a common vision condition where the front of your eye, or sometimes the lens inside it, is curved unevenly. Instead of being round like a basketball, the surface is shaped more like the back of a spoon or a football. This irregular shape prevents light from focusing to a single point on the retina, causing blurry or distorted vision at all distances. Roughly 15% of children worldwide have a measurable degree of astigmatism, and it’s just as common in adults.
How Astigmatism Affects Your Vision
In a normally shaped eye, the cornea (the clear front surface) curves evenly in every direction, bending light so it lands on one precise spot on the retina. With astigmatism, the cornea has two different curves, one steeper than the other. Light passing through this uneven surface can’t converge to a single focal point. Instead, it creates two focal lines at different depths inside the eye. The gap between those two lines is what produces blurry, smeared, or doubled images.
This is different from nearsightedness and farsightedness, which involve the overall length of the eyeball rather than the shape of its surface. Nearsightedness focuses light in front of the retina, making distant objects blurry. Farsightedness focuses light behind the retina, making close objects blurry. Astigmatism blurs vision at every distance because light never reaches a clean focal point at all. You can have astigmatism alongside either condition, which is actually very common.
What Causes It
Most astigmatism is simply the shape you were born with. Perfectly round corneas are actually the exception, not the rule. Almost everyone has some degree of uneven curvature, though in most people it’s too slight to notice or need correction.
Beyond genetics, a few things can cause or worsen the condition. Keratoconus, a progressive thinning of the cornea into a cone shape, produces increasingly irregular astigmatism over time. Both genetic and environmental factors contribute to keratoconus, including chronic eye rubbing, allergies, and UV exposure. Eye injuries, certain surgeries, and poorly fitted rigid contact lenses can also reshape the cornea enough to create new astigmatism. There are two main types: corneal astigmatism, from the surface of the eye, and lenticular astigmatism, from an unevenly shaped lens inside the eye. In many people with cataracts, the corneal and internal astigmatism actually partially cancel each other out because they curve in opposite directions.
Symptoms, Especially at Night
The hallmark symptom is blurry or slightly warped vision, both up close and far away. You might find yourself squinting frequently or getting headaches after reading or screen time. Eye strain and fatigue are common, particularly during tasks that require sustained focus.
Nighttime is where astigmatism becomes most noticeable. In low light, your pupils dilate to let in more light, which allows more peripheral rays to pass through the uneven parts of your cornea. This amplifies the blur and introduces visual effects that can make driving uncomfortable. Oncoming headlights may appear to have starbursts or streaks radiating outward. Streetlights can look like they’re surrounded by fuzzy halos. These effects are not just annoying; they can genuinely make it harder to judge distance and read road signs at night. If you’ve noticed these symptoms, they’re a strong signal that you have uncorrected or undercorrected astigmatism.
How It’s Diagnosed and Measured
Astigmatism is detected during a standard comprehensive eye exam. Your eye care provider will use a series of tests, often including a device that shines light into your eye and measures how it reflects off the cornea. For young children, the American Optometric Association recommends a comprehensive eye exam at least once between ages 3 and 5, and again before starting school. These exams catch astigmatism early enough to prevent it from interfering with learning and visual development.
If you have astigmatism, your prescription will include two extra numbers that people with only nearsightedness or farsightedness won’t see. The first is “CYL” (cylinder), which measures the severity of the astigmatism in diopters, the standard unit for lens power. The second is “axis,” a number between 0 and 180 that describes the angle of the uneven curvature on your cornea. Together, these tell a lens maker exactly how to grind or mold a lens that compensates for the specific tilt and degree of your eye’s irregular shape.
Correction With Glasses and Contacts
Glasses are the simplest fix. The lenses are ground to counteract the uneven curvature, bending light so it reaches a single focal point on your retina. Your prescription’s cylinder and axis values ensure the correction is oriented in exactly the right direction.
Contact lenses for astigmatism are called toric lenses. Unlike standard spherical contacts, which have the same power across the entire surface, toric lenses have different corrective powers built into different zones. Because the orientation matters, these lenses are designed with stabilizing features, like a slightly thicker bottom edge, that use gravity and your blinking motion to keep them from rotating out of position. They take a little more fitting than standard contacts, and some people find they shift slightly during vigorous activity, but modern designs are reliable for most wearers.
Surgical Options
If you’d rather not depend on glasses or contacts, laser eye surgery can permanently reshape the cornea. LASIK and PRK both use the same type of excimer laser to remove microscopic amounts of corneal tissue and even out the curvature. Current laser systems are approved to correct up to 6.0 diopters of astigmatism, which covers the vast majority of cases. For people with mixed astigmatism (a combination of nearsighted and farsighted curves in the same eye), the approved limit is also 6.0 diopters. You need to be at least 18 for most procedures, 21 for mixed astigmatism, and your prescription must have been stable for at least a year before surgery.
The difference between LASIK and PRK is mainly recovery time. LASIK creates a thin flap in the cornea, reshapes the tissue underneath, and replaces the flap. Most people see clearly within a day or two. PRK removes the outermost layer of the cornea entirely and lets it regrow, which means a longer healing period of several days to a couple of weeks before vision stabilizes. Both achieve similar outcomes once fully healed.
For people who aren’t candidates for laser surgery, implantable lenses or toric lens implants placed during cataract surgery are additional options. These are especially useful in older adults whose astigmatism has been partially masked by lenticular astigmatism from an aging lens, since removing the natural lens during cataract surgery can unmask the full corneal astigmatism.
Living With Astigmatism
Mild astigmatism often doesn’t need correction at all. Many people have a small amount and never notice it. When correction is needed, the condition is entirely manageable and doesn’t worsen from reading in dim light, sitting close to screens, or any of the other activities people commonly worry about. It can, however, change gradually over time, which is why periodic eye exams matter even after you’ve gotten a prescription that works. Children especially should be monitored, since uncorrected astigmatism during early development can contribute to amblyopia (lazy eye) if one eye is significantly more affected than the other.