Which of the following lenses has the least amount of power in the vertical meridian? HINT: +__________ - _________ X______

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

Answer: (A)

In a spherocylindrical lens, the cylinder adds power only in the meridian 90 degrees away from the axis; along the axis itself there is no cylinder power. So to find the power in the vertical meridian, compare how far the vertical line is from the axis and add the cylinder accordingly.

• The prescription with sphere +1.50 and cylinder -1.00 at axis 180 has the vertical meridian 90 degrees from the axis, so its vertical power is 1.50 + (-1.00) = +0.50 D.

• The prescription with sphere +2.00 and cylinder -0.50 at axis 180 gives vertical power 2.00 + (-0.50) = +1.50 D.

• The prescription with sphere +1.75 and cylinder -0.75 at axis 090 has the vertical meridian aligned with the axis, so the cylinder adds nothing there; vertical power is +1.75 D.

• The prescription with sphere +2.25 and cylinder -0.50 at axis 180 yields vertical power 2.25 + (-0.50) = +1.75 D.

The smallest vertical power is +0.50 D in the first prescription.