An optician's trial frame with two round lenses on a dark desk, light passing through them in two bright rings.

Reading a prescription

Axis

Optical glossaryReading a prescription

The orientation of your astigmatism correction, from 1 to 180 degrees. A cylinder value without an axis is meaningless.

The axis protractor, read as though facing the wearer: 90 is vertical and 180 horizontal. The axis marks the cylinder's line of no power; its power acts at right angles to it.

Illustrative · the shape of the principle, not measured data

In more depth

The axis is a direction, not a power: a number from 1 to 180 giving the angle, in degrees, of the cylinder's line of no power. Both eyes use the same protractor, read as though you were facing the wearer, with 90 vertical and 180 horizontal.

Its precision matters more as the cylinder grows. A few degrees of error on a small cylinder goes unnoticed; on a strong one it blurs and tilts the view. Lenses are checked for axis before they leave the lab, and a frame that sits crooked on the face can rotate an accurate lens out of position.

Related terms

Cylinder (CYL)The correction for astigmatism, where the eye's optics focus light along two different lines instead of one point. Always paired with an axis.AstigmatismAn eye whose optics are curved unequally in different directions, so light focuses along two lines rather than one point. Extremely common and fully correctable with a cylinder.DispensingThe professional work between prescription and finished glasses: frame selection, measurement, fitting, and adjustment. Where most of a lens's real-world performance is decided.

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