- All figures are drawn with light traveling from left to right.
- All distances are measured from a reference surface, such as a wavefront or a refracting surface. Distances to the left of the surface are negative.
- The refractive power of a surface that makes light rays more convergent is positive. The focal length of such a surface is positive.
- The distance of a real object is negative.
- The distance of a real image is positive.
- Heights above the optic axis are positive.
- Angles measured clockwise from the optic axis are negative.
Because the direction of light travel is consistent and there is a consistent convention to determine the sign of all distances in a calculation, this sign convention is used in many texts. It has some advantages when dealing with multilens systems and more complex optical instruments.