What is the purpose of 'Axis' in relation to astigmatism?

The purpose of 'Axis' in relation to astigmatism is to indicate where the astigmatism power lies. In astigmatism, the cornea or lens is not perfectly spherical, leading to blurred or distorted vision. The 'Axis' is expressed in degrees and refers to the orientation of the cylindrical power needed to correct this distortion.

For instance, when a prescription is written for glasses or contacts for someone with astigmatism, it will include not just the power of the lens, but also the axis to ensure that the corrective lens is positioned correctly. This is crucial because the cylinder power could be positioned in various directions, and the axis tells the optician where to align that power to provide the best visual acuity.

In contrast, the other options address different concepts related to vision correction. Correcting double vision relates to different forms of prism correction rather than astigmatism directly. The curvature of contact lenses pertains to their fit and design, not specifically to the axis of astigmatism. Similarly, defining the circumference of the lens does not pertain to astigmatism, but rather to lens shape and size in general. Thus, the role of 'Axis' is specific and critical in the