A practical application of lenses is in the correction of the defects of vision. The power of a lens is defined as the reciprocal of its focal length in metre.

**P = 1/f**

The S1 unit of power of a lens is m^{–1}. **Dioptre** is only a commercial unit generally used by opticians.

The power of a convex lens is positive and that of a concave lens is negative. Greater power implies smaller focal length. Using lens maker’s formula, you can relate power of a lens to its radii of curvature.