The focal length of a spherical mirror gives the distance from the mirror to a point at which parallel incoming rays converge. For a spherical mirror that is small with respect to its radius of curvature, the focal length is equal to half the radius of curvature:

    f = r/2

By convention,  focal length and radius of curvature are positive for a concave mirror, negative for a convex mirror. A positive focal length means that parallel incident rays are focussed to a point in front of the mirror. A negative focal length means that parallel incident rays are reflected so as to form a virtual image that appears to be behind the mirror.