By how many degrees does the axis change during transposition?

In the process of transposition, the axis of a cylindrical lens is typically changed by 90 degrees. Transposition refers to the conversion between a prescription in one form to another, changing a lens prescription from the cylindrical format to a plus or minus format.

When you transpose a prescription, you effectively switch the sphere and cylinder values of the prescription while simultaneously altering the axis by 90 degrees. This is due to the relationship between the cylindrical power and the orientation of the axis. For example, if you have a cylinder with an axis of 180 degrees, after transposition, the new cylinder will end up at 90 degrees. The mathematical manipulation during the transposition operation mandates this 90-degree change in axis.

Understanding this principle is crucial for ophthalmic assistants, as accurate transposition is necessary for proper lens fitting and correcting refractive errors in patients. The other degree options do not reflect the standard practice involved in this process, as only a 90-degree change accurately captures the relationship between the cylinder adjustment and the axis realignment.