State the sign convention for lenses regarding object and image distances.

(N/A) According to the Cartesian sign convention for spherical lenses:
$1$. The optical center $(O)$ of the lens is considered as the origin $(0, 0)$.
$2$. The principal axis of the lens is taken as the $x$-axis $(X'X)$ of the coordinate system.
$3$. All distances measured in the direction of the incident light are taken as positive $(+)$.
$4$. All distances measured against the direction of the incident light are taken as negative $(-)$.
$5$. Distances measured perpendicular to and above the principal axis are taken as positive $(+)$.
$6$. Distances measured perpendicular to and below the principal axis are taken as negative $(-)$.
Therefore,the object distance $(u)$ is always negative,while the image distance $(v)$ depends on the nature of the image formed.