One use of transposition is to?

• A. Determine the center thickness
• B. Determine the axis
• C. Determine the prism power
• D. Determine the power in the meridian 90 degrees away

Answer: (D)

Transposition in optics allows you to convert between minus cylinder and plus cylinder prescriptions and to read off the lens powers in each principal meridian. The axis marks the meridian with the cylinder effect canceled, while the meridian 90 degrees away carries the full cylindrical power in addition to the spherical power. So, using transposition to find the power in that perpendicular meridian is a natural consequence: you add the sphere and cylinder powers to get the total power in the meridian 90 degrees from the axis. That’s why this option is the best choice. The other items aren’t determined by transposition: center thickness isn’t read from the prescription directly, axis is a fixed parameter from the notation rather than something you derive from transposition, and prism power involves a different concept entirely.