NEW CARTESIAN CONVENTION
All distances are measured from the optical centre, being positive if in the same direction as the incident light and negative if against it.
This convention is more easily applied if the object is always placed to the left of the lens, in which case all distances to the left of the lens are negative and all those to the right are positive. This automatically brings all signs into agreement with the ordinary Cartesian graphical convention used in mathematics; hence its name (Fig. 24.26). The focal length of a converging lens is +ve; a diverging lens – ve.