Well m not very much sure about dis...par still i hope dis helps u...

For a spherical mirror having small aperture, the principal axis(F) lies exactly midway b/w the pole(P) and centre of curvature(C). So the focal length of spherical mirror (concave or convex) is equal to half of its radius of curvature.If f is the focal length of a spherical mirror and R is its radius of curvature,then,

f=R/2