By how many degrees does the axis change in transposition?

The correct answer reflects the concept of axis transposition in the context of optics and the correction of astigmatism. In this context, the axis refers to the orientation of the cylinder lens that is used in prescription eyewear to correct astigmatism.

When transposing the prescription from one form to another, the axis of the cylinder changes by 90 degrees. This is essential to maintaining the proper alignment of the lens to effectively correct the visual aberration caused by astigmatism. Understanding this principle is crucial for accurate lens crafting and ensuring that patients receive the correct optical correction.

Hence, in the transposition process, adjusting the axis by 90 degrees ensures that the combined powers of the lenses remain consistent while effectively addressing the specific visual needs dictated by the astigmatic error.