Cylinder (CYL)

Optical glossary/Reading a prescription

The correction for astigmatism, where the eye's optics focus light along two different lines instead of one point. Always paired with an axis.

In more depth

A cylinder adds power in one direction across the lens and none at right angles to it — the shape an eye with astigmatism needs, because its own optics focus more strongly in one direction than the other. It always travels with an axis, which says which way that direction lies.

The same correction can be written in two ways. Optometrists usually write minus cylinders and some ophthalmologists plus cylinders, and both describe an identical lens: add the cylinder to the sphere, flip the cylinder's sign, and turn the axis through 90 degrees. A prescription that looks different from your last one may not be different at all.