Which of these lenses +2.00 -0.50 X180 OR +1.50 -1.00 X180 has the least amount of power in the vertical meridian?

• A. +2.00 -0.50 X180
• B. +1.50 -1.00 X180
• C. +1.75 -0.25 X090
• D. +2.25 -0.75 X180

Answer: (B)

The vertical meridian power in a spherocyl lens depends on where the cylinder acts. The cylinder adds its power in the meridian perpendicular to the axis, while along the axis itself the cylinder contributes zero power. Since the vertical meridian is 90° from a horizontal reference, for lenses with axis at 180°, the vertical meridian carries the cylinder’s power in addition to the sphere: P_vertical = S + C.

Compute for each lens:

• +2.00 sphere, -0.50 cylinder, axis 180: P_vertical = 2.00 + (-0.50) = 1.50

• +1.50 sphere, -1.00 cylinder, axis 180: P_vertical = 1.50 + (-1.00) = 0.50

• +1.75 sphere, -0.25 cylinder, axis 090: vertical is along the axis, so P_vertical = 1.75

• +2.25 sphere, -0.75 cylinder, axis 180: P_vertical = 2.25 + (-0.75) = 1.50

The smallest vertical power is 0.50, which occurs with the lens that has +1.50 sphere and -1.00 cylinder on axis 180.