Standard wisdom says a hunter should own a binocular. I say he and she should own several! But how to choose hunting binoculars?
A good binocular pushes a hunter toward Superman status. You won’t see through buildings, but you’ll see antlers and horns thousands of yards farther than you can with naked eyes alone. But with roughly 14,713 binoculars on the market in 6,000 assorted flavors at 10,000 different prices, how do you find the right one? Should it be a 6X, 8X, 10X or 15X? Will it require a 56mm, 50mm, or 30mm objective? Let’s dive into the mechanics and sort this out.
Most of us are dazzled by power. Why settle for a 6X magnification when we can get 12X? Because the 6X might reveal more game, that’s why! Magnification involves compromises. The higher the magnification, the narrower the field of view. While you concentrate on that shadow behind that tree with your 12X, the world’s record buck could walk out from behind another tree just outside your field of view. With the wider field of view in an 8X or 6X, you would have seen it.
Power also magnifies shake. Subtle hand tremors and twitches make the scene jump and swim so you can’t get a fixed look, the steady hold for studying those antlers. Most of us find that any power above 10X requires a tripod mount in order to maximize performance. High power also magnifies any optical defects within the system.
The final high-power bugaboos involve weight and bulk. What’s convenient lying on your truck seat might not be when hanging around your neck. High-power instruments aren’t heavy due to the power so much as the increase in objective lens diameter to match up to it. As power increases, objective lens diameter must increase or the instrument loses too much light.
Most of us think binoculars gather light, but they don’t. They transmit it, but reduce it as they do. No binocular can turn up the intensity of ambient light. The sun does that. Thus, photons reflecting off objects (like bucks and bulls) travel 186,000 feet per second to hit your binocular’s objective lens. There, some are lost to reflection. What remains passes through to the prisms where a bit more is lost to reflection before hitting any mirrors (more lost). Next, that remaining light passes through the eyepiece lenses, where there’s more reflection loss. Finally, the remaining light exits to hit the retinas at the back of your eyes.
So if a binocular is always losing light, why do objects appear brighter when seen at magnification? Because a greater percentage of your retina is filled with the light reflected off them. Seeing a black bear “brighter” at 8X after sunset, is akin to seeing it better if you walked eight times closer to it.
Now, it is true that a larger front window (objective lens) lets in more light. But if that light isn’t properly controlled, your magnified view can be severely compromised due to reflection chaos. Raw glass reflects about 4 percent of light that hits it. More is lost to internal reflection as it exits. Depending on how many air-glass surfaces there are, a binocular could lose some 32 percent of the light that first reaches it, unless…
…unless you hire anti-reflection coatings to save the day. Or daylight. Micro-thin layers of certain rare-earth elements like magnesium fluoride can reduce reflection loss to just 1 percent. Additional layers and materials reduce it even more. This is your “best buy” to get brightness in a binocular. But check carefully. Ads claiming “coated optics” or “coated lenses” could mean just one coating on one or two lenses. “Multi-coated” could mean two or more layers on just one lens. Fully multi-coated should mean two or more layers on every air-to-glass surface. This is well worth the price. Maximum light transmission at no increase in size or weight.
The other key to brightness is the power/objective relationship. A bigger objective lets more light in. A higher magnification lets less light out. You can see this as the exit pupil in each eyepiece, with the binocular held at arm’s length. You can measure this circle of light by dividing objective diameter by magnification. The answer will be the exit pupil in millimeters. This little circle of light must equal the diameter or your pupil in order to funnel all the light onto your retina. If it’s smaller, the image appears darker. If it’s larger, the extra rim of light just bounces off your iris. So, at 10X a 50mm objective sends a 5mm circle of light through the eyepiece. A 30mm objective yields a 3mm exit pupil. In bright sunlight your pupil will likely shrink to about 2.5mm, so a 3mm exit pupil is more than enough. But at dusk your pupil should dilate to 5mm. The 30mm objective won’t match up.
This exit pupil relationship suggests you buy a binocular with a 7mm exit pupil, the maximum dilation of your pupil at dark. But the resultant bulk and weight argue against this. Do you really want a 10x70mm binocular tugging at your neck all day in order to perhaps clearly see a buck a half hour after legal shooting light? Or even an 8x56mm? And you don’t have to. In the real world, a top quality, fully multi-coated 8×42 binocular—or any power/objective combination that yields a 4mm to 5mm exit pupil—should clearly transmit useable images through legal shooting light and often a half hour after. On moonlit nights you can usually see deer in fields quite clearly.
Anti-reflection coatings don’t just maximize brightness; they also minimize flare and glare. Because they reduce reflections, there is
less light bouncing around inside the barrel. You see this uncontrolled, reflected light as an orange haze and flare when the sun strikes the objective lenses. By minimizing this flare, coatings also increase contrast. This makes it easier to detect subtle differences in the color tones between brush and fur and antlers.
ED and HD Glass
Extra-low dispersion or High Definition lenses are nice, but not a major contributor to useable performance at powers less than 20X. What they do is minimize color fringing around objects, making them sharper. However, this fringing doesn’t usually become apparent until 20X. So don’t pay extra to get ED glass in a binocular of less than 15X. It doesn’t hurt and might help, but it’s not a big deal.
“Don’t make the mistake of choosing a 10X for woods hunting. There, the wider field of view of a 6X is almost magical.”
Virtually all binoculars are waterproof, dustproof, and fogproof these days. But to what degree? Some can be submerged to 10 feet, others just resist rain and snow. Check the warranties. Nitrogen and argon gas purging are done to push every molecule of moisture out of the instrument before sealing. This should prevent internal fogging when taking a frozen binocular into a warm room or truck.
Roof or Porro Prism?
A prism is a big, multifaceted chunk of glass in a binocular barrel. Its job is to bend light (the magnified image) to increase magnification without needing to lengthen the barrel excessively. Prisms also reorient the backwards and upside-down image that emerges from the objective lenses. Without prisms, the game you glass would be standing on its head and facing the wrong direction.
Prism shape determines binocular shape and optical efficiency. The Porro prism includes two chunks of glass that are offset, giving the binocular an old fashioned look, the two-stage, dog-leg offset between eyepiece and objective barrels. A bit clunky, but delightfully inexpensive and surprisingly sharp and bright because Porro prisms require no mirrored surfaces that suck up some light. The wide spread of the objective lenses also increases depth perception via enhanced triangulation. Alas, this spread makes the instrument more susceptible to knockabout damage, i.e. knocked out of alignment.
Roof prisms are stacked one atop the other, so fit into a more-compact barrel with no bend or offset required. This makes for a more compact and rugged package, the prism unlikely to get bumped out of collimation. The downside to Schmidt-Pechan roof prism units is light lost to the mirrors required on one surface of the glass. An Abbe/Koenig roof prism doesn’t need mirrors, but it makes a binocular longer, so isn’t often used. The brightest mirrors are dielectric. Second best are silver. Poorest are aluminum. You usually have to spend about twice as much for a roof prism to match the optical quality of a Porro. That said, there are some surprisingly excellent roof prism binoculars in the $300 price range.
Armor coating is nearly standard these days and well worth it, protecting barrels and housing from dents and scratches. Water-resistant external lens coatings minimize frustrating fogging, fingerprints, and light scratches. Check eye relief. It must be long enough so that your eyelashes don’t smear the eyepiece glass. Adjustable-length rubber eye cups increase versatility. The pull- or twist-up styles last longer than the flexible rubber.
High-magnification binoculars should include a threaded attachment point for aftermarket adapters, for securing to a tripod. A good warranty is worth having. One that guarantees the instrument against all defects for life suggests the maker is confident the unit will last.
Your Final Assessment
Weight, bulk, optical quality, durability, “feel,” and price must all be balanced. For hiking, especially backpacking in mountains, small is beautiful. Ditto travel. For last-light discovery from a blind, big is beautiful. For finding bedded animals at distance, magnification and more magnification are paramount. Excellent sharpness/resolution is critical. The 8×42 roof prisms are perhaps the most-popular size for a do-it-all hunting binocular. Many western and longer-range mountain hunters prefer a 10X. Don’t make the mistake of choosing a 10X for woods hunting. There, the wider field of view of a 6X is almost magical. Yes, 6X is more than powerful enough for picking eyes, ears, tines, noses, and legs out of forest clutter to 200 yards. Everything will look 6 times closer, so a whitetail at 100 yards will appear as if only 17 yards away. One at 200 yards will look like 34 yards.
To check for optical quality, compare one binocular against another in poor light, ideally low light at dusk and dawn. What looks best to you? Also, check at sundown or sunrise with the rays striking the objective. This tests flare control. Look for detail in a shadowed area toward the sun, but don’t look right at the sun. The less flare and glare the better. To compare sharpness, it’s best to rest binoculars on a solid surface. This lets you study small details without shake. Black ink on a standard fiberboard shipping box at 25 to 50 yards is a good test object. Which binocular reveals the smallest letters? That’s the one with the best resolution. Your “best buy” might well be a waterproof, internally focused, fully multi-coated, top-line Porro prism for less than $200.
Congratulations; now you know how to choose hunting binoculars. Happy shopping. Once you’ve bagged your ideal binocular, it will help you fill a lifetime of tags.