One use of transposition is to determine the power in the meridian 90 degrees away.

• A. To calculate prism power
• B. To determine the axis
• C. To measure center thickness
• D. To determine the power in the meridian 90 degrees away

Answer: (D)

Transposition helps you rewrite a sphero-cylindrical prescription so you can see the power in a specific meridian, especially the one 90 degrees away from the axis. In a Sph-Cyl-Ax prescription, the sphere power applies along the axis, while the cylinder power adds additional power in the meridian perpendicular to that axis. Therefore, the power in the meridian 90 degrees from the axis is the sphere value plus the cylinder value. Transposing the prescription rearranges the numbers so that this perpendicular-meridian power can be read or calculated directly, often involving the axis shifting by 90 degrees in the process. For example, the perpendicular-meridian power equals sphere plus cylinder, which is exactly what transposition facilitates.