What are the sign conventions followed for lenses?

Explanation

The sign conventions for thin lenses differ only in the signs followed for focal lenghts.
(a) The sign of foal length is not decided on the direction of measurement of the focal length from pole of the lens as they have two focal lenghts, one to the left and another to the right (primary and secondary focal lenghts on either side of the lens).
(b) The focal lenght of the thin lens is taken as positive for a converging lens and negative for a deverging lens.