How is the focal length of a spherical mirror related to its radius of curvature?

1. Using the law of reflection and some trigonometry, the focal length of a spherical mirror is half its radius of curvature. 2. In symbols this is f = R divided by 2. 3. The result holds only when the mirror is small compared with its radius of curvature, which is the mirror version of the thin lens approximation. 4. It follows that the smaller the radius of curvature, the smaller the focal length and the more powerful the mirror. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.7 Image Formation by Mirrors_