According to the textbook, for a spherical mirror of small aperture, the principal focus F lies mid-way between the pole P and the centre of curvature C. This relationship is expressed as f = R/2. The context from Example 9.1 demonstrates this formula for a convex mirror, showing that focal length equals half the radius of curvature. Since the principal focus of a convex mirror is behind the mirror, the focal length is taken as positive.