omnivorr, Hi again, yes at F:40 the rays have a very small included angle, but if the reduction lens at half the focus, the aperture of the red lens will become the same as the OG, in fact, doubled. The focal length of the reduction lens will be increased to as the Effective Focal Length comes into play. Because the objective lens has a very shallow curve on the front surface, the spherical aberration is very small and will not matter in this case, more important, the red lens does not like converging rays coming at the front surface, and some coma will result, this can be corrected by careful design.