What does RA -0.50 x 180 represent?

• A. 0.50 D at 180
• B. -0.50 D at 180
• C. 0.50 D at 90
• D. -0.50 D at 90

Answer: (B)

The main idea is how astigmatism is written in refraction notation: a cylinder power with a specified axis. RA means the right eye. The minus sign before the cylinder power shows a minus cylinder correction, which is used for astigmatism. The magnitude, 0.50 diopters, is the amount of cylinder power. The axis, 180, tells you the orientation of that cylinder; the cylinder power is applied at a meridian perpendicular to the axis, so the power is in the 90-degree meridian. Put together, this means the right eye has 0.50 diopters of astigmatic correction oriented at 180 degrees, i.e., the cylinder acts in the 90-degree direction. The sign and the axis determine the exact correction, so -0.50 D at 180 is the correct interpretation.