Surface definition
z = cr² / (1 + √(1 − (1 + k)c²r²)) + A₄r⁴ + A₆r⁶ + A₈r⁸ + A₁₀r¹⁰
Use c = 1/R, or c = 0 for a flat base. Positive z runs from the vertex toward the center of curvature of a positive-radius surface. The radial height r is measured from the axis in millimeters. The coefficients multiply physical powers of r, with no normalizing radius.
Check the default surface
At R = +50 mm and k = −1 the conic is a paraboloid. At r = 5 mm its sag is 0.25 mm. A₄ = 1 × 10⁻⁵ mm⁻³ adds 0.00625 mm, giving 0.25625 mm in total. A₆, A₈ and A₁₀ are zero.