1. In case of mirrors, all the distances are measured from the pole of the mirror and all ray diagrams are drawn with the incident light travelling from left to right.

So, all distances measured in the direction of incident light are taken to be positive and distances measured in the opposite direction of incident light are taken to be negative.

2. In the case of lenses, all the distances are measured from the optical centre of the lens.

The distances measured in the same direction as the incident light are taken as positive and distances measured in the opposite direction of the incident light are taken negative.