I had an interesting discussion with an acquaintance recently concerning binoculars, telescopic sights for rifles, and how his (and my) aging eyes affected our ability to make the best use of our optics. He'd never gone into the notion of "exit pupil" size, and wasn't aware of how it should condition our optics choices: so I thought some of my readers might benefit from a brief discussion.
I'm not going to try to define the "exit pupil" here, because there's an article that's done a far better job of it than I can. Please click over to "Exit Pupil – The Complete Guide" and read it. When you're done, come back here and we'll continue.
The human eye's pupil will widen or contract depending on light conditions. The eye's ability to do this will also be affected by age. Wikipedia provides this chart of how age affects our pupil size:
For a more in-depth discussion of age and pupil size, see here.
If one chooses an optical instrument (binoculars, telescope, telescopic sight, etc.) that provides an exit pupil as wide as one's own pupil, that'll give the best light perception and focus for our eyes. Wider than that, and we won't be able to take advantage of it; narrower than that, and we'll not be able to use our eyes' full capability to see whatever it is that we're looking at.
I'm in my sixties, so my vision at dusk and dawn is less "bright" than it should be. For that reason, I prefer lower-magnification optics with an exit pupil that maximizes my ability to see in low light. My current favorite pair of binoculars is this Vanguard 8x42 model:
Its exit pupil of 5.25 mean that my older eyes are getting as much light as they can profitably use, and the very sharp glass provides clarity and definition. If I go to a 10x42 model, the exit pupil would drop to 4.2, a reduction of almost one-fifth. I've done that comparison outdoors in low light, and I can notice the difference in light transmission to my eyes with the higher-magnification model. By selecting the 8x42 unit, I've optimized the exit pupil size of my binoculars to the physical capabilities of my eyes at my age.
In the same way, I'm currently re-evaluating my choice of telescopic sights on rifles equipped with them. Here, exit pupil isn't the only criterion. For example, I used to like Weaver Classic 1-3x scopes (shown below) for short- to medium-range brush hunting.
Their exit pupil is fine: at 3x magnification, it's 6.67. However, their tube doesn't have an expanded "bell" at the front to gather the maximum possible light, as most larger scopes do. As my eyes deteriorate with age, I want as much light as possible in the field. A good-quality 2-7x32 or 3-9x40 scope offers a much wider front lens (32mm or 40mm respectively, instead of 20mm), admitting more light. If I adjust the power to maximize the usable exit pupil for my eyes (respectively, up to about 6x or 8x), they're a lot brighter for me. I usually use lower power settings than that, because in shorter-range brush hunting you don't need high magnification. One could also go to a bigger scope tube; 30mm diameter instead of the conventional 1", or even wider. That's become the norm in so-called "sniper scopes".
I hope this helps those of you thinking about buying binoculars or telescopic sights.