A basketball versus a rugby ball
Most descriptions of the eye assume the cornea — the clear dome at the front of the eye — is shaped like a section of a basketball: evenly curved in every direction. In an eye with astigmatism, the cornea is shaped more like a rugby ball or American football instead. It's steeper in one direction and flatter in the direction perpendicular to it, rather than being uniformly curved all the way around.
This uneven curvature is almost always the cause of astigmatism. In rare cases, the natural lens inside the eye can also contribute to astigmatism, but the cornea is responsible for the great majority of cases.
Why an oval shape blurs the image
Because a basketball-shaped cornea curves the same amount in every direction, light passing through it converges to a single focus point. A rugby-ball-shaped cornea bends light differently depending on which direction it's traveling through the cornea — more steeply along the steep axis, less steeply along the flat axis. Instead of one focus point, you end up with two, at different distances from the cornea.
The practical effect is blur, and often a sense of streaking or slight doubling, that shows up at every distance — near, intermediate, and far — rather than just at one. This is a useful clue: pure myopia or hyperopia (see our myopia and hyperopia page) tends to blur one distance range more than another, while astigmatism tends to affect clarity fairly evenly across distances, because the underlying problem is the shape of the focusing surface itself, not simply how strong or weak it is.
It's common, and it's a spectrum
Astigmatism is not an all-or-nothing condition. Nearly every eye has at least a small, often unnoticeable, degree of corneal irregularity. It becomes clinically significant, and something an eye care provider will actively correct, when it's pronounced enough to measurably affect the sharpness of your vision. It can occur entirely on its own, or alongside myopia or hyperopia — plenty of people are, for example, both myopic and astigmatic in the same eye.
Correcting astigmatism
Because the underlying issue is the shape of the light-bending surface rather than its overall power, astigmatism can't be corrected with an ordinary "spherical" lens the way myopia or hyperopia can. It requires a cylindrical, or "toric," correction — a lens with different focusing power along different axes, matched to the specific orientation and amount of the eye's astigmatism. This is available in glasses, contact lenses, and, relevantly for cataract patients, in toric intraocular lenses.
What this means for cataract surgery
If you have meaningful corneal astigmatism and you're heading toward cataract surgery, it's worth discussing directly with your surgeon, because it affects which replacement lens will give you the sharpest possible vision. A standard, non-toric IOL will not correct astigmatism — residual blur from uncorrected astigmatism can remain even with an otherwise well-chosen lens. A toric IOL is designed specifically to neutralize it during the same surgery that removes the cataract. See how astigmatism factors into choosing an IOL → or take our IOL questionnaire →