The Secondary Focal Point is an axial image point whose axial object point is at infinity. The rays associated with the Secondary Focal Point are colored blue and numbered 2 to correspond with Kirkpatrick and Wheeler. Sign conventions for algebraic manipulation. These equations are correct descriptions if a particular set of sign conventions are followed. These are illustrated in the following two diagrams.
Light is assumed to be incident on the mirror or lens from left to right in both diagrams.
For a Spherical Mirror the positive directions are as indicated below. Problem 16.12 Statement of problem. Lens to collect pinhole images.