The following set of sign conventions will be adhered to throughout the following
chapters on geometrical optics, and it would be well to have them firmly in mind:
1 All figures are drawn with the light traveling from left to right.
2 All object distances (s) are considered positive when they are measured to the
left of the vertex and negative when they are measured to the right.
3 All image distances (s') are positive when they are measured to the right of the
vertex and negative when to the left.
4 Both focal lengths are positive for a converging system and negative for a
diverging system.
5 Object and image dimensions are positive when measured upward from the
axis and negative when measured downward.
6 All convex surfaces are taken as having a positive radius, and all concave
surfaces are taken as having a negatfve radius.