The Ideal Lens feature can be used to treat a flat surface (in 3D) or an edge (in 2D) as if it were a lens, allowing it to focus or defocus rays.

The optical power K (SI unit: 1/m) of a lens is

where feff is the effective focal length (SI unit: m). For a thin lens, we have

with n1 and n2 being the refractive index before and after the lens respectively, and where n is the refractive index of the lens. The radii of curvature of the first and second lens surfaces is R1 and R2 (SI unit: m) respectively.

For a lens with thickness Tc (SI unit: m), the optical power is instead

For each point r (SI unit: m) on the lens surface, define an orthonormal coordinate system with unit vectors er, et, and ns (the surface normal), where

where the subscripts r and t refer to the radial and tangential directions respectively. The distance δr is defined with respect to a reference point on the lens surface as δr = r - rref.

where ni is the incident ray direction. The ideal lens will only affect the radial component of the ray directions; that is