The trouble with buying a binocular is knowing how to detect good, better and best. Most critical parts are hidden inside, and subtle performance differences are difficult to detect. How do you choose what’s best at a good price?
Start with the numbers. An 8×42 binocular magnifies images eight times and employs objective lenses, the ones out front, that are 42 millimeters in diameter. A 10×42 combines the same size objectives with 10X power. These numbers are important because the bigger the objectives, the more light that enters the barrels. The higher the power, the less light that reaches your eyes. If you want maximum brightness at any magnification, get bigger objectives. If lens and prism coatings are equal (more on this later) a 10×50 will be brighter than a 10×42, which is brighter than a 10×32, etc. But an 8×50 is brighter than a 10×50 and a 6×50 is brighter yet.
To determine potential brightness, divide objective diameter by power. A 10×42 = 4.2mm, a 10×32 = 3.2mm, 8×42 =5mm. These final numbers are the diameter of the exit pupils (EP), the circles of light that exit the eyepieces. You can see them by pointing a binocular toward a bright wall or sky and holding it two feet from your eyes while looking at the eyepiece lenses. The diameter of those little EP windows relates to your pupils, which dilate from about 2.5mm in full daylight to 7mm in near darkness. An EP smaller than your own pupil will result in reduced light transmission to your retina, i.e. a “darker” image.
Now that you know what all the numbers mean, how do you use this knowledge to buy the right binocular? Begin by deciding how much power you need. More is not always better because additional objective diameter to increase brightness also increases bulk and weight.
A big binocular might prove too uncomfortable to carry and use. Lots of magnification sounds good, but it results in a narrower angle of view. You see less field-of-view at 10X than 8X. Hand tremors, heat waves and dirty air are also magnified at higher powers.
In general, 6X or 7X are best for glassing tight cover where whitetails hide. An 8X is a great all-round choice, while 10X should be reserved for searching big, open country. Anything higher is a real specialty item best used with tripod or other solid support. Most of us can’t handhold a binocular powerful enough for judging details such as antler tines or horn rings at significant distances. That’s a job for a 15X-60X spotting scope.
After choosing power, select objective lens size to provide adequate exit pupil size. A 5mm EP is adequate 98 percent of legal hunting time (3/4-hour before and after the sunset.) A 4mm EP is more than bright enough from 1/2-hour before and after the sunset. A 3mm EP is fine for bright daylight and lightly cloudy days because your pupils shrink to that size or less in bright light. Below 2.5mm EP becomes too small for comfortable viewing. But, regardless of EP size, there are two more things you should look for to increase brightness.
Coatings Make the Difference
The difference between the best binoculars and the also-rans isn’t the glass, but how that glass is coated. Lenses coated with multiple layers of anti-reflective elements transmit more light than raw glass, and these coatings add no weight or bulk. The more layers the better. A fully multi-coated 8×32 will transmit a sharper, brighter image than an un-coated 8×50. Good coatings provide maximum brightness bang for your buck. A binocular advertised as “fully coated” must have all air-to-glass surfaces coated with one layer. “Multi-coated” means one surface has multiple coatings, but the rest could have just one layer. “Fully multi-coated” means all surfaces must have multiple coatings, and these give the highest light transmission.
Prisms Matter, Too
Prisms in binoculars bend light, reducing barrel length, and erect what would otherwise be an upside down image. There are two basic prism styles, Porro and roof. The Porro puts a dog-leg in the barrels. Roof prisms are stacked neatly atop one another so each barrel is straight and compact. An optically superb Porro model costs about a third less than an equal roof prism, but is a bit more delicate. You’ll have to protect it from vibrations and jarring. Porros are more easily knocked off axis than are roof prisms. Highest light transmission comes through BaK-4 prisms as opposed to BK-7, so ask for those.
To get the highest light transmission through roof prisms, buy models that have been Phase Coated. This coating improves sharpness. HD or ED lenses can sharpen by reducing color fringing, but this isn’t much of a problem until power goes above 15X. Money is better spent on anti-reflection coatings.
Other than those features, a binocular company’s guarantee is worth quite a bit. If one guarantees its instrument for a year or two and a competitor guarantees a similar product for a lifetime, that suggests the lifetime guarantee won’t be needed because the binocular is so well made. And if it does malfunction, you’ll save money on repairs or even a new binocular.
Shop The Sportsman’s Guide great selection of Binoculars!
Ron Spomer has been photographing and writing about the outdoors for nearly four decades. He’s written seven books, hunted on six continents and been published in more than 120 magazines. He’s currently rifles’ editor at “Sporting Classics,” Travel columnist at “Sports Afield,” Field Editor at “American Hunter” and “Guns & Ammo” — Optics Columnist at “North American Hunter,” Contributing Editor at “Successful Hunter,” Senior Writer at “Gun Hunter,” and TV host of “Winchester World of Whitetail.” He will write on Shooting Tips weekly for sportsmansguide.com. You can read his blogs and catch some of his YouTube videos at www.Ronspomeroutdoors.com.