Why does the focal length keep increasing while the optimization proceeds?
You must either constrain the effective focal length using the EFFL operand, or by using a marginal ray angle curvature solve on the last lens surface. Alternatively, you can simply fix the back focal distance (rather than vary it). The reason the effective focal length increases is because slow lenses perform better, and unless specifically constrained, the optimizer will gravitate to slower F/# systems. This problem may also be circumvented by using image space F/# instead of entrance pupil diameter for the system aperture.
