In the prescription example +2.00 -1.00 X 100, what is the axis value?

• A. 50
• B. 100
• C. 0
• D. 180

Answer: (B)

Axis tells the orientation of the cylindrical correction in astigmatic prescriptions. The cylinder power is applied 90 degrees away from the axis, so the axis value indicates where there is no cylindrical power. In +2.00 -1.00 X 100, the -1.00 cylinder is oriented at 100 degrees, meaning the corrective power acts along meridians perpendicular to 100 degrees (roughly 010 and 190 degrees), while the 100-degree meridian has no cylinder power. Axis values run from 0 to 180, with 180 equivalent to 0, so the axis here is 100 degrees. The other numbers describe the spherical power and cylinder strength, not the axis.