But back to explaining astigmatism. Just what is different about astigmatic eyes?

Basketballs vs. rugby balls.

Astigmatism occurs when the front surface of the eye, called the cornea, is not perfectly round. Normal corneas are equally curved in all directions, like the shape of a basketball. But an astigmatic cornea has unequal curvatures. It more closely resembles the side of an American football, or a rugby ball.

So what does the curve of the cornea have to do with our ability to see?

Quite a lot. When light enters the front surface of the eye, the cornea does most of the work needed for clear vision. It bends rays toward each other, ideally making them fall on a single plane where the retina lies. The brain receives information from the retina and produces vision. If all the rays do not meet on the retina, blurred vision occurs.

The curvature of the cornea affects its ability to bend or focus light.

Since astigmatic corneas are not equally curved, they focus light rays on many different planes, making it impossible for all rays to meet on the retina. The rays of light may actually meet in front of or behind the retina, or both. In the example illustrated, the rays of light fall behind the retina, creating a condition known as hypermetropia.
An astigmatic cornea may direct light rays to two separate planes. As shown in the illustration, one of the cornea's curvatures is directing rays of light to focal plane "h," while another curvature is causing them to intersect at a further plane, "v." These planes, "h" and "v," may fall in front of or behind the retina, or both. Some rays of light may even fall on the plane of the retina itself.

In any case, a blurred image forms because all rays fail to focus on the retina at the same time. If it occurs below a certain level and causes no visual difficulties, astigmatism may be left uncorrected. But when it causes headaches, fatigue, squinting, or eye discomfort and irritation, corrective lenses can usually solve the problem.