Every hunter wants a bright binocular, but no binocular is really bright. Some laser rangefinding binoculars are good at math and trigonometry, but ask them the simplest question about biology or chemistry and they hang there, mute as a stone. Stupid, really.
On the bright side, however, many binoculars excel at transmitting light, and I think that’s what we hunters really care about.
Binocular choices come down to a compromise between price, performance, brightness and size.
I opened with this silliness to emphasize an important reality about binoculars: They don’t “gather light,” if by that you mean “collect it into a larger quantity.” A binocular can’t make a scene “brighter.” The reason a deer in a field at dusk suddenly looks brighter through an 8X binocular is because it’s effectively eight times closer. Walk eight times closer to anything in dim light and it you can see it more clearly, as if it got brighter. But no one turned up the light, so the scene couldn’t have gotten brighter.
What binoculars really do is make things darker—guaranteed, every time, every binocular. It’s basic physics. When light strikes a lens, some is lost to reflection. That’s why you see yourself in a store window. Raw optical glass reflects about 4 percent of light that strikes it, another 4 percent that leaves it. It’s those air-to-glass transitions that cause the reflection loss.
Now, here’s the really bad news: Each binocular has about 10 air-to-glass transitions. They’re losing 40 percent of the light. Yikes.
A huge objective lens lets more light into a binocular, but the higher the magnification, the less that gets out. In this picture, the 10x56mm below yields a 5.56mm EP. The 8×20 above it yields a 2.5mm EP. The Drawback to the 10x56mm is its larger carry size and weight. In full daylight, your pupil shrinks to 2.5mm, so the smaller binocular above would transmit as bright an image as the big one below—until the sun begins to sink.
OK, what about bigger objective lenses? Those are supposed to brighten things up, right? Well, sort of. Sure, a bigger window will let more light in, but no matter how large the lens, it still loses 4 percent of the light coming in and 4 percent going out. And when that light bounces off the prism mirror, another 2-5 percent is lost. That brings us to the final bad news (after which things get better): The higher the binocular’s magnification, the less light that gets out. Ouch.
Let’s review the sad truths about binoculars:
1. Lenses lose a lot of light.
2. Big objectives let more light in, but still lose about 8 percent to reflection.
3. Prism mirrors lose 2 to 5 percent of the light that strikes them.
4. Magnification reduces the amount of light that gets out, so the more powerful the binocular, the darker the image.
Put away your crying towels. We’re moving on.
THREE KEY TIPS FOR BUYING HUNTING BINOCULARS
Binocular Buying Tip No. 1: Anti-reflection coatings can knock reflection loss from 4 percent per surface to less than .02 percent per surface. If this sounds huge, it’s because it is huge. The trick has to do with wave interference. It’s rocket science, but thankfully someone figured it out. A single-layer coating cuts reflection loss in half. Multi-layer coating knocks it clear down toward that .02 percent range. Each company has its secret coating recipe, so the best we can do is insist on our binoculars being what the industry calls “fully multi-coated.”
Binocular Buying Tip No. 2: Porro-prism binoculars—the old-fashioned style with the two-stage or dog-legged barrels—don’t require a mirror, so no light is lost there. Abbe-Koenig prisms in some roof prism binoculars (straight barrels) don’t need mirrors either. But most roof-prism binoculars use the shorter, compact Schmidt Pechan prisms, which require one mirrored surface. The brightest (most efficient) is dielectric. Second best is silver, worst is aluminum.
Binocular Buying Tip No. 3: You can balance objective lens size and magnification for maximum brightness. The two work in concert to create the exit pupils (EP). The exit pupils are the small circles of light you see in a binocular’s eyepieces if you hold them at arm’s length and point them toward a bright sky or wall. Those little circles are the windows that let light out of the binocular. You can see several together in this video. Ideally, EP should match the diameter of your own pupils. If they do, you’re taking in all the light you possibly can. If they aren’t, you aren’t. And if they’re larger, the extra rim of light just spills off your iris without ever getting inside to your retina.
The little circles of light you see in eyepieces of binoculars held at arm’s length are exit pupils (EP). The top binocular shown in this image have a 2.5mm EP. The bottom binocular has a 5.56mm EP.
Here’s where our math skills come into play. Divide objective diameter in millimeters (mm) by magnification and you get EP diameter in mm. A 50mm by 10X binocular would create 5mm EP. The same 50mm with 8X power would give 6.25mm EP, and a 6X would create a huge 8.33mm EP, which is larger than you can use.
The human eye dilates to about 7mm in the dark, so that’s really all the larger any EP needs to be. In full daylight, your pupil shrinks to 2.5mm, so a binocular EP that small would be sufficient. In practice, a binocular that is fully multi-coated with no mirror or a silver mirror or dielectric mirror, and EP of 4mm or larger, will transmit a bright enough image 45 minutes after sunset to let you clearly see a deer in an open field. In other words, through legal shooting light. This means you don’t need to haul around a 10x70mm monster binocular with its 7mm EP. It would transmit a brighter image, but if you’re seeing deer with 4mm EP, do you really want to haul around a huge 10x70mm for a few minutes of extra viewing time?
A binocular’s exit pupil (EP) must be as wide as your own pupils to let in maximum light, but in real world conditions, a top-quality binocular with 4.5mm EP will see you through legal hunting hours.
Now, for the grand finale: If you want an eminently useable binocular that will transmit images bright enough for clearly seeing through legal hunting hours, balance magnification and objective lens size to get at least a 4.5mm EP. Make sure it has fully multi-coated lenses—meaning all air-to-glass surfaces are multi-layer coated. Get a Porro-prism unit or an Abbe-Koenig roof prism for maximum light transmission. If you buy a Schmidt-Pechan roof prism, get a dielectric mirror to maximize transmission. A silver mirror is the next best. Avoid aluminum mirrors.
Those simple factors will help your binocular be as bright as any inanimate, non-computerized object can be.