In a soft lens prescription, what do cylinder power and axis represent, and when are they required?

• A. Cylinder power corrects astigmatism; axis indicates orientation; required when refractive cylinder is present beyond a minimal threshold (toric lenses)
• B. Cylinder power is used for all prescriptions; axis is always 0 degrees
• C. Cylinder power has no clinical meaning; axis is only a cosmetic parameter
• D. Cylinder power and axis are only used in specialty lenses, never soft torics

Answer: (A)

Astigmatism correction uses two pieces of data: how much cylindrical power is needed and the orientation of that correction. The cylinder power tells you how much extra power is needed to balance the uneven curvature of the eye, addressing the different refractive strengths in meridians. The axis specifies the direction of that cylinder—what meridian is the reference for applying the cylindrical correction—measured in degrees from 0 to 180.

In soft contact lenses, this is provided only when there is a meaningful refractive cylinder. If astigmatism is minimal, a spherical soft lens suffices and no cylinder or axis is prescribed. When the cylinder value is present beyond a small threshold, a toric soft lens is chosen, delivering both the spherical correction and the cylindrical correction aligned at the specified axis. If the axis is off or the lens rotates, the cylinder won’t align properly, leading to blurred vision, so axis information is essential.

So, cylinder power corrects astigmatism and axis indicates its orientation, and they’re required only when astigmatism is significant enough to warrant toric soft lenses.