When drinking out of a gushing firehose, you can consume just so much. The rest spills out of your mouth and is wasted. The same principle applies when looking through an exit pupil: you can consume just so much. The rest “spills out” of your eye and is wasted.
The exit pupil of an optic is the column of light transmitted by the ocular lens, appearing as a circle of light visible in the center of the eyepiece when viewed from half an arm’s length away from your face. This exit pupil is measured in millimeters.
… if you can match the amount of light exiting a riflescope or binocular eyepiece to the maximum amount your eye can “swallow,” this is the most efficient, and nothing is wasted.
The formula to determine exit pupil of an optic is simple:
Objective lens diameter in millimeters divided by magnification
Example: With a 3.5-10×50 scope set at 10 power, divide the 50mm objective lens size by the magnification of 10; the answer is 5mm. Now, lower the magnification to 5x. Using the same formula, the exit pupil is now 10 (50mm objective lens divided by 5x = 10mm).
If you can match the amount of water exiting from a firehose to the maximum amount you can drink, this is the most efficient, and nothing is wasted. You also don’t have to manhandle the impressive power that a fully charged firehose wields.
Likewise, if you can match the amount of light exiting a riflescope or binocular eyepiece to the maximum amount your eye can “swallow,” this is the most efficient, and nothing is wasted. You also don’t have to manhandle the extra weight, size and expense that an optic with a giant exit pupil might wield.
Young pupils dilate (open) a lot larger than older pupils. Various scientific studies reflect that an average maximum pupil size when dark-adjusted is about 7mm for people in their 20s to a maximum of 2mm for someone in their 80s.
Other factors—including stimuli such as sexual excitement, a math problem, buck fever, medicines and fight-or-flight responses under stress—also affect dilation size.
Young Pupils vs. Old Pupils
I corresponded with a Ph.D. from MIT who was involved in such published studies. He cautioned me to point out that the variations of maximum dilation among individuals (AMY: ital. “among individuals”) is larger than the effect of age degradation of dilation from 10 to 80 years old and that the general trend is that young eyes can expand approximately 40 percent more than very old eyes. Keep this in mind when viewing any charts showing the pupil size for a given age. Maximum pupil dilation is very much dependent on individual. Other factors—including stimuli such as sexual excitement, a math problem, buck fever, medicines and fight-or-flight responses under stress—also affect dilation size.
Exit pupil is one reason the 7×50 binocular is prevalent for marine use. Dividing the 50mm objective lens size by the 7x magnification results in a 7.1mm exit pupil—enough to match the dilation of most users and enabling a view that is as bright as an eye allows. The human eye adjusts to darkness in a few minutes but takes hours to fully acclimate to no light, gaining pupil diameter slowly with time. That’s why fighter pilots on an aircraft carrier wear dark goggles long before night operations.
However, an exit pupil that is a bit larger than the amount of your pupil dilation is useable—just not any brighter. It gives you a bit more room to move your head around behind the optic and still have a full field of view. You move your head position more than you might think with both binoculars and riflescopes and quite a lot with spotting scopes.
Most mid-range variable-power riflescopes such as a 3-9×40 set at medium magnification will offer an exit pupil with enough size and light for just about any conditions and any eye.
Think you could get a brighter picture from an EOTech or SureFire red dot than an issue 4×32 ACOG? Think again. The exit pupil on that ACOG is a whopping 8mm, more than just about any eye can open. And it’s almost never complete darkness, either, so rarely are your pupils dilated as much as possible. For soldiers, there might be lighted rooms, street lights and headlamps, and for an early-morning tree stand hunter, there is maybe a flashlight or an auto dashboard or the moon and stars.
In daylight, the game changes. The view from my little 8×20 Swarovski pocket binocular has only a 2.5mm exit pupil, but because conditions are relatively bright when I use them, and my pupil is not dilated, I have all the light I need. It’s the same with a high-magnification scope at the range. When set at 25x, the small, 2mm exit pupil of my 8.5-25×50 Leupold Mark 4 is just fine in the daytime. But in darkness, I can turn it down to a lower power, increasing the size of my scope’s exit pupil, thereby allowing my dilated pupil to accept more light.
Exit pupil is simply a math equation, though. It does not take into account lens quality or coatings. So, in some cases, a slightly smaller exit pupil from a quality optic might result in a brighter overall picture than a larger exit pupil from a lesser-quality product.
Enough Size and Light
Matching your optic’s exit pupil to the maximum amount your pupil can dilate is extremely useful and the most efficient in low-light situations— particularly with binoculars, where the exit pupil is not adjustable by varying the magnification. In daylight, the exit pupil has much smaller importance regarding light and more importance in giving comfort and ease of use in viewing. Most mid-range variable-power riflescopes such as a 3-9×40 set at medium magnification will offer an exit pupil with enough size and light for just about any conditions and any eye.
Editor’s note: A version of this article first appeared in the August 2017 print issue of Gun World Magazine.