Fig. 6. A sign convention for lens surface power and radius of curvature. C1 and C2 are the centers of curvature of the front and back surfaces, respectively, of this meniscus lens. The points A1 and A2 mark the position of each surface. The distances measured from A1 to C1 and from A2 to C2 are both positive because both are measured in the direction of the incident light.