Interesting. So a 3-9x40mm scope will give you an exit pupil diameter of 4mm. Which the eye, when fully dilated, will see everything in the lense. (Nothing wasted) Is that correct?
No a 3-9X40 scope will give you an exit pupil size of 13.3mm at 3X to 4.4mm at 9X.
With an exit pupil size the same size as your eyes pupil you will get the maximum ammount of light. If the light is smaller than your eyes pupil then as the ambient light gets dim the less light your eye will have to use and your sighe picture will be dim. If the light is larger than your pupil size then some of the available light is not being used.