In Chapter 3, we did a quick check to see if the initial entry of our OSDlens was correct using the command FIR. The first-order listing for OSDlens is shown again in Table 4.1. The first three numbers may be puzzling because there are three separate focal lengths (EFL, BFL, and FFL) listed, and one of them is negative. Furthermore, none of these values exactly matches the thin lens focal length value of 68.294 mm calculated in Section 3.5. So what is the correct value for the focal length?

4.1 Principal Surfaces and Planes

To help you understand why several focal lengths are computed and used for a single lens, we begin with an exaggerated example. Figure 4.1 shows a thick lens in air with a very short radius of curvature on its back surface focusing a fan of parallel axial rays. The solid red lines show the standard trace of the rays through the lens. Paraxial rays, those close to the axis, focus at the back focal point F'. A plane perpendicular to the axis at this point is defined as the back focal plane (BFP).