* Radius of curvature* (

The sign convention for the optical radius of curvature is as follows:

- If the vertex lies to the left of the center of curvature, the radius of curvature is positive.
- If the vertex lies to the right of the center of curvature, the radius of curvature is negative.

Thus when viewing a biconvex lens from the side, the left surface radius of curvature is positive, and the right radius of curvature is negative.

Note however that *in areas of optics other than design*, other sign conventions are sometimes used. In particular, many undergraduate physics textbooks use the Gaussian sign convention in which convex surfaces of lenses are always positive.^{[5]} Care should be taken when using formulas taken from different sources.

Optical surfaces with non-spherical profiles, such as the surfaces of aspheric lenses, also have a radius of curvature. These surfaces are typically designed such that their profile is described by the equation

where the optic axis is presumed to lie in the **z** direction, and is the *sag*—the z-component of the displacement of the surface from the vertex, at distance from the axis. If and are zero, then is the *radius of curvature* and is the conic constant, as measured at the vertex (where ). The coefficients describe the deviation of the surface from the axially symmetric quadric surface specified by and .^{[4]}

- Radius of curvature (applications)
- Radius
- Base curve radius
- Cardinal point (optics)
- Vergence (optics)

**^**Ernst, Steve. "The Lensmaker's Equation" (HTML).*Boundless*. Retrieved 8 August 2017.The best reference.

**^**https://physics.stackexchange.com/questions/168750/radius-of-curvature-of-a-lens**^**"Optics Theory" (in Russian).- ^
^{a}^{b}Barbastathis, George; Sheppard, Colin. "Real and Virtual Images" (Adobe Portable Document Format).*MIT OpenCourseWare*. Massachusetts Institute of Technology. p. 4. Retrieved 8 August 2017.