Cartesian Sign Convention
A standardized coordinate rule used in geometric optics to assign consistent algebraic signs to distances, heights, and focal lengths.
Governing formula
Origin at optical center/pole; light travels left to right (+x)
SI unit
N/A
In depth
Distances measured in the direction of incident light are positive (+); against incident light are negative (-). Heights above the optical axis are positive (+); below are negative (-).
Examples in the real world
Assigning negative object distance $u = -30 cm$ for a real object placed in front of a lens.