What is the prescription represented by the total optical cross?

• A. +0.50 -1 X 180
• B. +2.00 -1 X 180
• C. -2.00 +1 X 180
• D. +2.50 -0.50 X 180

Answer: (B)

The total optical cross shows the powers in the two principal meridians: horizontal (0/180) and vertical (90). With a cylinder axis at 180, the cylinder’s power is applied in the 90-degree meridian, so the horizontal power equals the sphere, and the vertical power equals sphere plus cylinder.

For the given prescription, the horizontal power is +2.00, and the vertical power is +2.00 + (-1.00) = +1.00. So the total optical cross corresponds to +2.00 in the 0/180 meridian and +1.00 in the 90 meridian, i.e., +2.00 -1.00 x 180. The other options would yield different horizontal/vertical powers and thus different crosses.