The standard go-to binocular configuration for most hunters, birders, and wildlife observers since at least the Carter administration has been the 10x42, but advances in materials and optical technology are making the larger 10x50 binocular an appealing choice.
So, which is best for you? We’ll consider the advantages and disadvantages of each and offer some insights that are useful no matter what binocular size and magnification you’re looking for.
WHAT THE NUMBERS MEAN - MAGNIFICATION
Every binocular has two sets of numbers separated by an X. The first number is the magnification. So a 10x42 is a 10-power optic. A 10x52 is also a 10-power.
The second number is the size of the objective lens, expressed in millimeters. So the 10x42 has a 42mm objective lens (that’s the big lens that points toward whatever you’re looking at through the optic). A 10x50 has a 50mm objective lens.
Because the job of a binocular is to allow us to see distant objects clearly, you’d think that magnification — the more the better — is the most important consideration. But magnification has a cost. The more magnified an image, the less you can see of its surroundings. This width of the image is called field of view, and the more magnification an optic has, the narrower its field of view.
If you’re shopping for a binocular to scan wide landscapes, you want a wider field of view, so you’d probably opt for a 6- or 8-power binocular. But if you’re more interested in seeing close details of a bird, or a buck, then you might pick a 10- or even 12- or 15-power binocular.
Magnification also amplifies what’s called hand shake, or the apparent movement of the image when seen through the optic. Think about high-power telescopes that are used to observe stars and planets.
They have to be stabilized by heavy tripods or other mounts (sometimes entire buildings) to minimize movement, since any little shake will cause the stars to be blurry. The same thing happens with binoculars; the more magnification, the more you’ll notice any little shake or movement.
Ten-power binoculars are about the highest magnification that you can hold without this blurriness caused by your hands. I know a few uncaffeinated folks with low blood pressure and steady dispositions who can hand-hold a 12-power binocular, but they’re a rarity.
By the time you get up to 15- and 18-power binoculars, you’d better have them mounted on a tripod or other stabilizing support, or they’ll be so shaky they’re almost useless as precision observation instruments.
WHAT THE NUMBERS MEAN – OBJECTIVE LENS
Think of objective lens size as a useful shorthand for portability. A compact binocular, like the kind you can tuck in a jacket pocket, typically has a 24mm objective lens and a hand-sized chassis.
At the other end of the spectrum, a 50mm or even 56mm objective lens is built around a chassis about the size of a loaf of bread. With size, of course, comes weight. So these larger binoculars are not only heavy, but they’re not especially portable.
That’s one reason the 42mm binocular is popular. They’re built on a chassis small enough to tuck into a binocular harness or to wear on a strap around your neck, but big enough to do some serious glassing.
The objective lens size also allows more or less light to enter the binocular. So a binocular with a larger objective, all other things being the same, will deliver a brighter image than one with a smaller objective lens. That’s important if you’ll be using your binoculars in times where there’s not much light, like at sunrise and after sunset. But a brighter image, even in full daylight, will appear sharper and more detailed than a darker image.
DOES EXIT PUPIL MATTER?
Besides objective lens diameter, the reason for brightness is what’s called “exit pupil.” That’s the little bright disk of light you see if you look through the lens of a binocular (or a rifle scope, or a spotting scope) at arm’s length. Exit pupil is calculated by dividing the objective lens size by the magnification. So a 10x42 would have an exit pupil of 4.2mm, or 42 divided by 10. A 10x50 would have an exit pupil of 5.0mm, or 50 divided by 10.
That 5mm exit pupil delivers 16 percent more light than the 4.2, which is a meaningful amount in low-light conditions.
There’s a reason those big celestial telescopes have tremendously large objective lenses; they’re trying to maximize brightness of distant stars, and their magnification is so intense they need a big lens to create a usable exit pupil. The trade-off? These optics are hardly portable. On the other end of the spectrum, those little pocket-sized 8x24 binoculars are wonderful for travel but they’re lousy for counting the horn rings on a bighorn sheep a half-mile away or glassing in low-light conditions.
SO… 10x42 OR 10x50?
The reason that 10-power binoculars have become so popular, then, is because they’re a good balance of magnification and portability. But what about the question that brought you here? Is a 10x42 somehow better than a 10x50?
Even five years ago I would have said that if you’re looking for a walk-about binocular for hunting or birding or even travel, the 10x42 is the best combination of magnification, chassis size, and brightness.
I like a 10x42 for almost all my Western hunting pursuits, and it’s the configuration I have on my windowsills at home and in my pickup’s jockey box. They’re simply versatile, and they fit in nearly all my regular-sized chest harnesses.
Because they’re so popular, every brand has a 10x42, which means they’re widely available and there’s a model for every budget.
But for those times when I’m willing to sacrifice just a bit more weight in exchange for a slightly brighter image, I’ll opt for a 10x50.
Happily, over the past few years, manufacturers have developed lighter chassis materials, used thinner internal lenses, and produced a number of 10x50 models that rival the 10x42’s size while delivering more optical horsepower.
Models that pack 10x42 handling in a 10x50 build are Zeiss’s new SFL 50 binoculars, The Crossfire HD from Vortex, Bushnell’s Engage, and Maven’s B.6. I’d also put Swarovski’s new 10x52 NL Pure in that category. Of this collection, the Zeiss and the Swarovski are not only the most expensive, but they pack the most optical performance in the 10x50 class.
Is the 50mm objective lens really brighter?
In my head-to-head tests of a selection of 10x42 versus equivalent 10x50s, the answer is unequivocally yes, 50mm lenses are measurably brighter, and their 5mm exit pupils provide between five and 10 minutes more viewable light in evening glassing conditions.
That doesn’t sound like much, but if you’re a twilight hunter waiting for the buck of a lifetime to step out of the timber, or a birder trying to cipher subtle characteristics on a bird tucked back in the shade of a thicket, that’s a meaningful advantage.
The 10x50 class has the secondary advantage of providing a little more heft that settles nicely in the hand, and doesn’t get as jostled by the wind.
The downside is that there are relatively few 10x50 models on the market, and prices for premium models can be pretty stiff.a