What are the sign conventions followed for lenses?

Explanation

(i). The sign of focal length is not decided on the direction of measurement of the focal length from the pole of the lens as they have two focal lengths, one to the left and another to the right (primary and secondary focal lengths on either side of the lens).
(ii) The focal length of the thin lens is taken as positive for a converging lens and negative for a diverging lens.
(iii) The Incident light is taken from left to right (i.e. object on the left of mirror).
(iv) All the distances are measured from the pole of the mirror (pole is taken as origin).
(v) The distances measured to the right of pole along the principal axis are taken as positive.
(vi)The distances measured to the left of pole along the principal axis are taken as negative
(vii) Heights measured in the upward perpendicular direction to the principal axis are taken as positive.
(viii) Heights measured in the downward perpendicular direction to the principal axis, are taken as negative.