Sign Conventions for Lens on Focal Length

The sign convention for a thin lens's focal length differs from that of a spherical mirror. Because a lens genuinely has two focal points (one on each side), the sign of its focal length is not fixed by which side of the pole it happens to lie on. Instead, the focal length of a thin lens is taken as positive for a converging (convex) lens and negative for a diverging (concave) lens. Every other sign rule -- for object distance, image distance, radius of curvature, object height a …