Which steps are involved in transposing a plus cylinder prescription to a minus cylinder prescription?

Which steps are involved in transposing a plus cylinder prescription to a minus cylinder prescription?

• A. Subtract sphere and cylinder powers; axis unchanged
• B. Swap axis; keep cylinder sign
• C. Add sphere and cylinder powers; change sign of cylinder; adjust axis by 90 degrees
• D. Multiply powers by -1

Answer: (C)

Transposing a plus cylinder prescription to a minus cylinder prescription involves shifting the cylinder’s effect into the spherical term and flipping the cylinder sign, while rotating the axis by 90 degrees. The plus cylinder adds power in the meridian 90 degrees away from the axis, so to keep the eye’s refractive correction the same but expressed as minus cylinder, you combine the sphere and cylinder powers and invert the cylinder’s sign, then rotate the axis accordingly. Concretely: new sphere power equals the original sphere plus the original cylinder. The new cylinder power is the negative of the original cylinder. The new axis is the original axis plus 90 degrees (and wrap around so it stays within 0–180 degrees). Example: if the original is sphere +2.50 D, cylinder +1.00 D, axis 180°, the transposed minus-cylinder form is sphere +3.50 D, cylinder −1.00 D, axis 90°.

Transposing a plus cylinder prescription to a minus cylinder prescription involves shifting the cylinder’s effect into the spherical term and flipping the cylinder sign, while rotating the axis by 90 degrees. The plus cylinder adds power in the meridian 90 degrees away from the axis, so to keep the eye’s refractive correction the same but expressed as minus cylinder, you combine the sphere and cylinder powers and invert the cylinder’s sign, then rotate the axis accordingly.

Concretely: new sphere power equals the original sphere plus the original cylinder. The new cylinder power is the negative of the original cylinder. The new axis is the original axis plus 90 degrees (and wrap around so it stays within 0–180 degrees).

Example: if the original is sphere +2.50 D, cylinder +1.00 D, axis 180°, the transposed minus-cylinder form is sphere +3.50 D, cylinder −1.00 D, axis 90°.