In image tracing we come across object distance, image distance, focal length, radius of curvature. a system of signs for these quantities is necessary to derive relations connecting them which are consistant in all types of physical situations. that system is called sign convention. coordinate sign convention is widely used now.
1. pole is taken as origin and principle axis as X-axis.
2. all distances are measured from pole.
3. distances measured in the direction of incident rays are taken as positive.
4. distances measured in the direction opposite to incident rays are taken as negative.
5. distances above the principal axis are taken as positive and below the principal axis as negative.