Of optics, exit pupil size and the real world


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 8×42 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 10×42 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 8×42 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 2-7×32 or 3-9×40 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.



  1. Thanks for an excellent article, especially now that I am in the older age group (75+). I have noticed when shooting that things just are not as clear. I also appreciate your comments regarding hunting scopes. For years I used a Weaver 1.5 to 4.5 variable and for the most part kept it set at 1.5, which always worked, especially for hunting in the woods, but then it seemed that everybody got power hungry. I know one guy who put a 4X12 on a Winchester Model 94 in .30-30. To each his own I guess. My Weaver was on a Ruger Model 77 in .308. One that I still kick my self for selling. Thanks again for the articles.

  2. As I have been recently looking for a newer pair of sporting glads, I followed the link for the Vanguard 8×42 Endeavor ED. They arrived this morning, 8 days earlier than estimated. So far, I am very pleased with my purchase. The optical clarity is superb, including swift and sure focusing. They are a great pair of light weight field glasses. Thank you for your report which pushed me into the actual purchase.

    BTW: I am not easy to please when it comes to quality of equipment. My expectation of best performance is as demanding as my use in the field. So, for me to gush over these binoculars is really saying something. This is a top value considering the purchase price and performance of the binoculars.

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