The sign conventions used for spherical lenses if the object is always placed to the left of the lens so that the light from the object falls to the lens from the left hand side

Oldest census parallel to the principal axis usually called x-axis  are measured from optical lens is taken as the origin point 

All distance is measured to the right of the origin along x axis are taken as positive but distance is measured to the left of the origin are taken as negative

Along the direction perpendicular to the principal axis usually called y- axis heights measured above the principal axis are taken as positive but height measured below the principal axis are taken as negative.