Different power in different directions
The cornea is the clear focusing surface at the front of the eye. Without astigmatism, its central curvature is similar in different directions. With regular astigmatism, one main direction is steeper and the direction across it is flatter.
The cornea is the most common source of regular astigmatism, but the natural lens can also contribute. Cataract surgery removes the lens component; the corneal component remains unless the surgical plan corrects it.
Why different meridians blur the image
Light passing through those two curvatures is not brought to one clean focus. The steep and flat directions focus differently, producing two principal focal lines rather than one point.
Astigmatism can blur or distort objects both near and far. How much you notice depends on its amount and the rest of your eye’s focusing error. It may occur together with myopia or hyperopia.
It's common, and it's a spectrum
Astigmatism is not all-or-nothing. Many eyes have a small measurable amount that causes little or no noticeable blur. It becomes clinically important when its size and pattern affect vision or the surgical target. It can occur alone or together with myopia or hyperopia.
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 →