What is the total optical power in the horizontal meridian?

• A. -2.00
• B. 0.50
• C. 0.00
• D. +2.00

Answer: (D)

Total optical power in a meridian is the sphere power plus the cylinder power that applies to that meridian. The cylinder affects meridians at 90 degrees to its axis, while the axis itself has no cylinder power. The horizontal meridian is 90 degrees away from a vertical cylinder axis, so it gains the full cylinder power. If there is no spherical error and the cylinder magnitude is 2.00 diopters, the horizontal meridian power is +2.00 diopters. That’s why the correct result is +2.00. If there were a spherical component, you would add it to the cylinder contribution in that meridian; and changing the axis would shift which meridian receives the full cylinder power.