Toric lens

One of the lens surfaces is shaped like a "cap" from a torus (see figure at right), and the other one is usually spherical.

When R = 0, the torus degenerates into a sphere with radius r. The greatest radius of curvature of the toric lens surface, R + r, corresponds to the smallest refractive power, S, given by where n is the index of refraction of the lens material.

Consequently, in contrast with a popular assumption, the toric lens is not an ellipsoid of revolution.

At intermediate orientations, the refractive power changes gradually from the greatest to the smallest value, or reverse.

With modern computer-controlled design, grinding and polishing techniques, good vision corrections can be achieved for even wider angles of view by allowing certain deviations from the toric shape.

Toric lens surface as "cap" (top-right) from a torus (here with R = 1.2 r ).
A torus results when a circle with radius r rotates around an axis lying in the same plane as the circle (here the z axis) at a distance R from the centre of the circle.
When the major radius R approaches 0 (here from right to left), the torus becomes a sphere.