What is the term used to describe converting a prescription to the power of the meridian perpendicular to the given meridian?

• A. Transposition
• B. Refracting
• C. Prism
• D. Rotation

Answer: (A)

This question tests transposition in eyewear prescriptions. Transposition is the process of rewriting a prescription so the cylinder power is described for the meridian perpendicular to the one given, with the axis shifted by 90 degrees and the sphere adjusted accordingly. In a sphero-cylindrical prescription, the power in the meridian along the axis equals the sphere, while the power in the perpendicular meridian equals the sphere plus the cylinder. When you transpose, you move the cylinder to the perpendicular meridian, flip its sign, and adjust the sphere by the cylinder amount, with the axis changing by 90 degrees. For example, a prescription written as -1.25 sphere with -0.75 cylinder at 180 can be expressed as -2.00 sphere with +0.75 cylinder at 90. The powers in the two principal meridians stay the same: the 180-degree meridian remains -1.25, and the 90-degree meridian becomes -2.00. This is why the term describing the action is transposition. Refracting is measuring, prism adds a prismatic effect, and rotation isn’t the term used for this process.