Exit pupil is equal to the objective diameter divided by magnification on the maximum magnification. It is also often the case in the upper half (approximately) of the magnification range. On the lowest magnification, I am not aware of any currently-made variable riflescope (maybe there some target type ones, but I have not tested those in a while) where the entire objective lens is used on the lowest magnification. For example, taking the Trijicon 2.5-10x56 mentioned earlier in this thread, it has exit pupil of approximately 16mm on 2.5x. That means on low power it only uses about 40 millimeters of a 56mm objective.
As far as how much exit pupil you need for low light use is somewhat individual, but you generally want it to be a bit more than your eye pupil. There are several good reasons for that. I hope Rick does not mind if I post a link to my website, but I did a livestream on that exact topic about a month ago:
https://darklordofoptics.locals.com/post/2946143/exit-pupil-and-how-the-eye-worksI travel a good bit for work and these are the topics that are easy to discuss without props, so I can cover them when I am on the road.
Lastly, when it is truly pitch black, thermal is the way to go. An hour after sunset is doable with some environments with conventional scopes, but that really depends on the terrain, where the moon is and at what phase, the cloud cover, etc. There is a reason why you want an exit pupil larger than your eye pupil in low light.
One of the things that happens is that your visual acuity goes down when the eye pupil dilates, so this is something you want to do some experimentation with. On top of that, your eyes depth of field goes to hell in a handbasket. The way you perceive the reticle, for example, can change substantially, especially if you have not checked eyepiece focus with dilated eye pupil.
ILya
