What is the sphericity of the front and back curves of an aspheric spherical lens?

• A. Front spherical, back aspheric
• B. Both surfaces spherical
• C. Front aspheric, back spheric
• D. Both surfaces aspheric

Answer: (C)

Sphericity refers to whether a surface is a true sphere or deviates from it. An aspheric surface is shaped to reduce aberrations, especially spherical aberration, by changing curvature as you move from the center to the edge.

When a lens is described as “aspheric spherical,” it means one surface is aspheric and the other is spherical. In practice, the front surface is made aspheric to better match the eye’s optical needs and to help produce thinner edges and sharper off-axis image quality. The back surface is kept spherical because it provides a simple, predictable interface with the eye and makes manufacturing, centering, and maintaining the prescribed power easier and more reliable.

So, the front surface is aspheric while the back surface remains spherical. If both surfaces were aspheric, the lens would be described as fully aspheric; if both were spherical, it would not be an aspheric spherical design.