Certified Ophthalmic Assistant Practice Exam

What should be done to the axis of the cylinder when following the rules for transposition?

1. Add 45 degrees

2. Subtract 90 degrees

3. Change it by 90 degrees

4. Add 90 degrees

The correct answer is: Change it by 90 degrees

When transposing a cylindrical prescription in ophthalmology, the axis of the cylinder is adjusted by changing it by 90 degrees. This transposition process involves converting the prescription from its cylinder form to its spherical equivalent, which requires a systematic alteration of both the sphere and cylinder values along with the axis. In the case of cylinder lenses, the axis represents the orientation of the cylindrical power on the lens. When converting a prescription, the cylindrical power is added to the sphere power to determine the net power of the lens, but the axis must be adjusted to reflect the orientation changes. Specifically, adding or subtracting 90 degrees is necessary to maintain the correct visual effect while ensuring the adjustments align with the optical principles governing astigmatism correction. This means if the original axis is at a certain degree position, a transposition will not keep it the same but instead rotate it to the appropriate new position, which is achieved through the 90-degree adjustment. Understanding this is crucial for effective vision correction and ensuring proper lens orientation based on the individual patient's refractive error.