If there’s one thing teachers like more than tenure it’s bright pupils. Hunters like bright pupils, too—in their scopes.
The two photos below are class pictures of sorts. Both show the exit pupil in the same variable-power scope. This pupil—the bright circle you see in the center of the eyepiece—is giving you a free show-and-tell lesson in optical brightness.
Pay attention, class.
4mm exit pupil.
12mm exit pupil.
Most scope shooters know that a big objective lens—the one out front pointing toward the target—lets light into the scope. The bigger this window, the more light that gets in. Fewer know that magnification also changes brightness in a scope. The higher the magnification, the less light that gets out. And it gets out through that exit pupil.
The pictures here were taken from about 18 inches behind the eyepiece of the scope, which is about where you should stand to see the exit pupil in your own scope. The circle of light is the actual diameter of the beam of light—the exit pupil—carrying the image of your target. In the picture above showing a large exit pupil, the scope, which has a 36mm objective lens, was dialed down to 3X (three power). The diameter of this exit pupil is 12 millimeters (36mm divided by 3=12mm.) I’ll tell you why that’s important in a few more sentences.
The picture showing the smaller exit pupil was taken with the same scope dialed up to 9X (nine power). The diameter of this exit pupil is just 4mm (36mm divided by 9=4mm.) So what?
So the smaller hole could provide a dimmer image. But not always. Here’s why: The exit pupil works in concert with your own eyeball’s pupil, which can dilate from about 2.5mm in bright sunlight to about 7mm in near darkness. This is how your body attempts to “see in the dark,” or see better in low light. Obviously no one can “see” in true darkness because what our eyes see is reflected light, or the reflected part of the electromagnetic spectrum between about 400 nanometers (violet) and 700 nanometers (red.) This is another way of saying my “simple” show-and-tell of scope brightness is trending toward college physics, so I’d better back off.
The point is this, if you’re aiming at a target in full daylight when your pupils are constricted to 2.5mm, a 4mm exit pupil is excessive. But if you’re trying to see a target near the end of legal shooting light and your pupils are dilated to 5mm but your scope’s exit pupil at 9X is just 4mm across, you’re not taking in as much light as your pupils will allow. Oh dear! Or is it, “No deer?” How do you increase brightness? One way is to run and buy a scope with a larger objective. Kind of slow. A faster way is to dial the power down to 8X. Suddenly, 36mm divided by 7=5.1mm. Dial down to 6X and the exit pupil opens to 6mm. Aha! Your 5mm pupils are suddenly receiving all the light they can suck in. In fact, they’re losing some because that extra 1mm edge of light bounces off your iris and never enters your pupils.
This might suggest that dialing down scope power always makes it easier to see and hit targets, but that’s obviously not quite true or the best scope would be a 1X—maximum brightness. But something’s missing. Magnification. A larger image is easier to see, even if it’s a lot dimmer, because it fills more surface area on your retina. This is why you walk closer to unidentifiable objects like a black tree stump that looks like a bear from 50 yards, but from 5 hards is clearly (whew!) a stump. The light didn’t get any brighter while you walked over, but the dim, reflected light off the stump filled more of your retina. Similar things happen as you dial up power on your scope.
So this makes it easy. Just buy a 2-25X scope with a 100mm objective lens. Oh, wait … that would get a bit unwieldy. Besides, no one makes one. But they do make 56mm objectives. Why not get one? Well, perhaps because it costs more than my last used truck and is about the same size. I’m kidding, but bigger scopes do add bulk and weight to rifles. If you’re just sitting and waiting for game, no huge deal, but if you’re actually hiking and hunting or stalking through brush it’ll be a monkey on your back. The scope could be more bother than worth because you only need that super big exit pupil at super high magnification when you see the buck of your dreams at the last second of legal shooting light 300 or more yards away. How often does that happen in a lifetime?
We can go round and round about this, but the easiest answer is to compromise. There aren’t many game animals within reasonable shooting range (500 yards or less) you can’t put crosshairs on at 10X. And scopes with fully multi-coated lenses transmit 90 to 95 percent of light that enters them. This means that, with an exit pupil of 4mm to 5mm, you’ll have sufficient light to see black reticles on a gray or brown animal a half-hour after sunset, often 45 minutes to an hour after sunset. A 2.5-10X scope with 50mm objective is going to suffice. And the same power at 44mm, 42mm or even 40mm will probably work just fine, too. Decide how much extra weight and bulk you’re willing to pack around for 10 hours a day in case you need what they provide for 2 or 3 minutes each day.