Originally Posted by koshkin
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-works

I 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

Good post and paragraphs.

Thanks for taking to time.
Originally Posted by BobbyTomek
Originally Posted by FTR_Shooter
...but you would probably not be using high magnification at night.

Actually, that's a common misconception. Most of my moonlight shots are at 8-10x and sometimes higher. You need magnification to discern detail at night. For example, a 150 or 175 yard shot on a hog in daylight is a piece of cake with 3x or 4x. But at night, you'd have an extremely difficult time at those lower magnifications and likely couldn't tell for certain if the animal was broadside, quartering, etc. -- or even if it's a hog at all and not a dog or calf.

So few actually use an optic in low light.

Magnification increases the "brightness" of the image.

On 10X the hunter is "sort of" 10 times closer.

The best image will be at the max zoom that offers a bigger or equal exit appature to you eyes pupil.

Add in the better resolution of the large objective and a significant gain can be had in low light.

John Burns

I have all the sources.
They can't stop the signal.