Originally Posted by BRvalley
very interesting, I'm 29 and my hair started thinning this year, better snack on more carrots...
You're still young enough that your pupils dilate to at least 7 mm in low light. I'm 56 and mine stop at about 6 mm. People in their 60s limit out closer to 5 mm.
As long as the exit pupil of the scope is at least as large as your eye pupil, the image will appear as bright as with the naked eye. If the exit pupil is smaller, the image will appear to loose brightness. It's just the physics of optics.
As you increase the magnification the scope exit pupil decreases. For a 50 mm scope the exit pupil drops below 7 mm at just over 7x magnification. To have the brightest image in low light, you want to set the scope magnification no higher than 7x. Either a 4-16x or 6-20x can give about the same brightness when set to 7x or less.
Obviously, this assumes your eye is centered in the exit pupil, which can be difficult to do in very low light. The 4-16x model may have an advantage here because of the larger exit pupils available below 6x, which make eye alignment easier.