Convert the plus cylinder Rx +3.00 +1.25 x 080 to a minus cylinder Rx. Which result is correct?

• A. +3.00 +1.25 x 170
• B. +4.25 -1.25 x 170
• C. +4.25 +1.25 x 170
• D. +4.25 -1.25 x 080

Answer: (B)

Converting between plus-cylinder and minus-cylinder forms keeps the same optical correction but rewrites it. You move the cylinder’s power into the sphere, change the sign of the cylinder to negative, and rotate the axis by 90 degrees. Here, the sphere is +3.00 and the cylinder is +1.25 at axis 080. Add the cylinder to the sphere to get the new sphere: 3.00 + 1.25 = +4.25. The new cylinder power is -1.25, and the axis shifts by 90 degrees: 080 + 90 = 170. So the minus-cylinder form is +4.25 -1.25 x 170. The other forms either keep a positive cylinder, keep the axis wrong, or don’t adjust the sphere and axis correctly, so they don’t represent the same prescription.