The process to convert a plus cylinder into a minus cylinder in an Rx is called what?

• A. Inverse
• B. Switch
• C. Transpose
• D. Reframe

Answer: (C)

Transposition is the process used to convert a plus-cylinder prescription into its minus-cylinder form (and vice versa) while preserving the same optical effect. The trick is to change the cylinder sign, rotate the axis by 90 degrees, and adjust the sphere accordingly. In practice, new sphere equals the old sphere plus the cylinder, the new cylinder is the negative of the old cylinder, and the new axis is the old axis plus 90 degrees (mod 180). For example, a plus-cylinder form of +0.50 sphere with +1.00 cylinder at axis 60 converts to a minus-cylinder form with +1.50 sphere, -1.00 cylinder, axis 150. This is the standard term used for this conversion.