Consider a point source placed at the focal point of a concave mirror. The initial vergence is then Vi = -1/f. The power of the surface is P = +1/f. The final vergence is thus zero. When an image is formed in front of the mirror, the distance of image is taken as – (negative) and when the image is formed behind the mirror, the distance of image is taken as + (positive). In the case of a convex mirror, the image always formed behind the mirror, thus the distance of image is taken as positive or will be always an erect image is formed, thus the height of the image is taken as positive.