In a spherocylinder lens, which surface is toric?

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

Answer: (B)

In a spherocylinder lens, one surface provides a uniform spherical power while the other introduces the cylindrical correction by having different curvatures in perpendicular meridians. The usual arrangement places the toric surface on the back (posterior) side. The light first passes through the spherical front surface, giving the base power evenly across meridians, and then traverses the toric back surface, which bends light differently along the two principal meridians to deliver the cylinder power. Keeping the front surface spherical helps maintain predictable magnification, edge thickness, and tear-film interaction, while the toricity on the back surface aligns with how astigmatic correction is implemented in practice.