In a spherocylinder lens, which surface is toric?

• A. Back surface is toric
• B. Front surface is toric
• C. Front surface is spherical
• D. Back surface is spherical

Answer: (A)

In a spherocylinder lens, astigmatic correction comes from having one toric (cylindrical) surface and one spherical surface. Placing the toricity on the back surface while keeping the front surface spherical is the common design because the back surface primarily determines the cylindrical power along the two principal meridians without introducing extra distortion on the eye-facing front surface. This arrangement preserves a smooth, high-quality front surface for comfortable wear, reduces manufacturing complexity, and keeps the cylinder axis stable with respect to the wearer’s gaze. If the front surface were toric, it would increase variability with eye movement and complicate production, while making the back surface spherical would fail to provide the necessary astigmatic correction.