What do the terms sphere, cylinder, and axis refer to in optometry?

In the context of optometry, the terms sphere, cylinder, and axis specifically relate to refractive error correction prescriptions.

Sphere refers to the lens power required to correct nearsightedness (myopia) or farsightedness (hyperopia). The measurement indicates the strength of the lens needed, with positive values indicating lenses for farsightedness and negative values for nearsightedness.

Cylinder is used to describe the power needed to correct astigmatism, which occurs when the cornea or lens has an irregular shape. The cylinder value in a prescription specifies the additional lens power needed for this condition.

Axis indicates the orientation of the astigmatism correction, expressed in degrees. This measurement shows the angle at which the cylindrical power needs to be positioned to effectively address the astigmatism.

These terms are crucial in creating a lens prescription tailored to an individual’s visual needs, making the selection of proper corrective lenses possible. Thus, the correct answer clearly aligns with how these measurements are used in optometry to correct various refractive errors.