In geometry, the conic constant (or Schwarzschild constant, after Karl Schwarzschild) is a quantity describing conic sections, and is represented by the letterK where is the eccentricity of the conic section.

The equation for a conic section with apex at the origin

where R is the radius of curvature at.

This formulation is used in geometric optics to specify oblate elliptical (), spherical (), prolate elliptical (), parabolic (), and hyperbolic () lens and mirror surfaces. When the paraxial approximation is valid, the optical surface can be treated as a spherical surface with the same radius.

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