As part of a typical lens optimization process, a grid of rays is traced each optimization cycle to evaluate the error function and variable derivative increments. Smooth optimization convergence relies on a consistent set of rays impacting the error function. The default settings for CODE V’s Automatic Design aims rays at the vignetted entrance pupil, which has flexibility and speed benefits for optimization, and works effectively for most imaging systems.
Some systems specify no vignetting. In these systems, the light bundle is controlled by the stop and will not be clipped at other surfaces. In these systems, CODE V vignetting factors should be set so the vignetted pupil corresponds with filling the stop initially. (Note that you can quickly set the CODE V vignetting factors to match the lens apertures by selecting the menu Lens > System Data > Fields/Vignetting and clicking the Set Vignetting button, or by running the SETVIG supplied macro: “IN CV_MACRO:SETVIG”.)
However, as the optimizer changes the system, the vignetted pupil may drift from filling the stop. In many scenarios the impact on converging to a solution is minimal, but there may be efficiency to be gained when optimizing systems where the stop is near the back of the optical train and closer to the image (see figure below).