Following sign convention is used for measuring various distances during the formation of images by lenses:

(ii) The distances measured in the same direction as that of incident light are taken as positive.

(iii) The distances measured against the direction of incident light are taken as negative.

● All distances on the principal axis are measured from the optical center.

● The distances measured in the direction of incident rays are positive and all the distances measured in the direction opposite to that of the incident rays are negative.

● All distances measured above the principal axis are positive. Thus, height of an object and that of an erect image are positive and all distances measured below the principal axis are negative.