Good binoculars will open your eyes to a world you didn’t know existed. With sharp and clear optics that let in a lot of light, suddenly the smudge a 1,000 yards away turns into a deer. That odd white patch of snow high up on a rocky cliff? It’s a mountain goat. That great blue heron on the other side of the river? You can watch it hunt for frogs with any decent set of binoculars.
First off, is it really a set of binoculars for just one binocular? The short answer is that you can refer to binoculars however you want and everyone will understand what you mean. (If you want to get technical about the plurality of binocular language, skip to the bottom.)
Meanwhile, let’s get to the important questions: How do binoculars work? What do the numbers mean? How do you choose the best binocular for you?
Binocular Magnification Power
Binoculars are commonly referred to with two numbers in their core specifications — something like 8×42 or 10×32. The first number is always the amount of magnification. So if you look at an image through an 8×42 binocular, you will “see” the image magnified so that it will appear 8 times closer to you than it really is. Or 8 times larger than it appears with your naked eye.
The goal of binoculars is to let you see things more closely, so more magnification is better, right? Not necessarily.
Most people can hold a 10x set of binoculars with a reasonably steady hand that physically lets their eyes process what they’re seeing. Once you jump up to 12x, however, quite a few people have a hard time keeping the binocular steady enough to allow their brain to process what they’re seeing. So their natural shakiness creates more of a blurred experience (for many people).
Some hunters and bird watchers, for example, will purposely choose 12x or 15x binoculars because getting closer is most important to them — and they sometimes carry shooting sticks or monopods to help hold large and heavy binoculars steady.
What Is Objective Lens Diameter?
The second number in an 8X42 binocular is the measurement of the objective lens, which is the large lens at the end of the binoculars. The measurement is in millimeters, so the 42 in 8×42 refers to a 42mm objective lens.
The objective lens diameter is important because it is the most important factor in how much light your binoculars can gather. A larger lens lets in more light, which makes it easier to see what you’re looking at. Larger lenses also result in seemingly brighter images, especially in low-light conditions around dawn and dusk.
What Is Binocular Exit Pupil?
Exit pupil is a measurement that indicates how much light can leave a binocular and enter your eye during typical usage. The bigger the exit pupil number, the brighter the image can possibly appear to you. How is this connected to human pupils?
The human pupil can contract and expand from about 2 mm to 8 mm (aging eyes lose elasticity over time and contract and expand less). To calculate the “exit pupil,” you divide the diameter of the front objective lens by the magnification power. So, in an 8×25 binocular, 25 divided by 8 equals 3.125. So, 3.125 is the exit pupil for an 8×25 binocular.
This essentially means that during the day, when your pupil is contracted at say, 2.5 mm in diameter, plenty of light will be able to enter your eye through the binocular and you’ll see a bright image.
At dusk, when your pupil has opened to 5 mm, your eye is essentially ready to accept more light than the binocular is going to deliver: Your eye is at 5mm while the binocular’s exit pupil is 3.125. The end result is that smaller exit pupil numbers mean that the binocular will deliver less light, which starts to make a difference at dawn and dusk when your pupils want more light, leading to images that appear darker to you when viewed through the binoculars.
Consequently, what all this really means is that most every binocular works very well in bright light on bright sunny days. It’s when you start to use your binoculars at dawn and dusk that exit pupil becomes a more important metric.
How might exit pupil factor into your binocular purchase decision?
An 8×42 binocular has an exit pupil calculation of 5.25, which, in low-light conditions would typically result in a much brighter image than an 8×25 binocular with an exit pupil of 3.125.
However, what if you want a bit more magnification? What if you wanted to consider a 10×42 binocular? A 10×42 binocular gives you 2x greater magnification but the exit pupil drops to 4.2. Is the extra magnification worth the lower exit pupil?
That’s a tough question. Let’s answer it in terms of hunters and hunting. For most western hunters who are using binoculars to cover longer distances, the best all-purpose binocular is 10X42. A 10×42 binocular is powerful enough to scan far off ridges but it’s light enough to pack around. The exit pupil of 4.2 is pretty workable overall.
If you hunt areas with shorter distances, 8×42 is better because it’s lighter and has an even greater exit pupil. If you’re looking for a great binocular for an aging person who might have shaky hands and aging eyes, 8×42 usually results in a better experience than a 10×42 binocular.
If you hunt generally dense woodlands where it’s difficult to see far distances, an ultralight 8X25 binocular is usually the best choice simply because it’s lighter and more compact. However, if you want a lighter image because you’re constantly in the shadows, you’ll want to try binoculars with a higher exit pupil.
Glass Quality and Binocular Coatings
While glass lets light pass through it, it also reflects some light. In a standard binocular, there could be 10 or more glass surfaces, all of which will reflect some light as it tries to pass through. Any light that is reflected and doesn’t pass through a glass surface will reduce the amount of light that can reach your eye.
In addition, reflected light and imperfect glass can cause glare and aberrations.
To combat the physics of light and glass, manufacturers use really good glass. For binoculars, for example, “BAK4” represents excellent glass. BAK7 is of lesser quality than BAK4, but it’s usually fine for larger optics like spotting scopes — for most people, most of the time, that is. (Optical snobs and various kinds of professionals are usually willing to pay much more for the very best possible experience, which most recreational users won’t notice or truly need.)
To reduce the light reflection, the glass is also coated with thin chemical films. If a binocular has “coated” optics, it means that at least one lens has an anti-reflective coating. A “fully coated” binocular has glass surfaces that have all been coated at least once. A “multicoated” binocular means that one or more glass surfaces have been coated multiple times. A “fully multicoated” binocular means that all glass surfaces have been coated multiple times. You want binoculars that specifically say they have fully multicoated lenses because it is usually associated with reasonable binocular quality compared to the cost.
“Phase shift” coatings are for straight-barreled “roof prism” binoculars, and they help correct shifts in light waves, which, if everything is manufactured perfectly, also helps increase image quality.
More expensive binoculars usually start with higher quality glass. Then they also have multiple coatings of a given manufacturer’s secret formula, fully multicoating everythting. Plus, they tend to have more exacting manufacturing tolerances, all of which results in clearer, brighter images that more fully reflect accurate colors and clarity in the wild.
Are binoculars that cost more than a $1,000 worth it? Only if you a) can afford them, and b) you will use binoculars for hours on end, for many days out of the year . . . for a purpose that is very important to you. (Most hunters haven’t invested in a good set of binoculars, so midrange and better binoculars make very good gifts for hunters.)
What Is Field of View (FOV)?
One last term you’ll see as you look for binoculars is Field of View (FOV). FOV is how many feet wide you can see at 1,000 yards. A small FOV means you’re looking at a small section of the forest. Big means you’re looking at more information. Generally, FOV shrinks as magnification increases when the objective lens size remains the same. So, an 8×25 binocular will show more area at 1,000 yards than a 10×25 binocular — the 8×25 binocular will have a larger field of view despite having less magnification.
You can get compact, lightweight binoculars in sizes that usually have an objective lens of around 25mm. So 8×25 or 10×25 are generally small and easy to pack and carry. The downside is the exit pupil, which means they won’t work as well at dawn and dusk.
Midsize binoculars will have objective lenses around 32 or 35, so 10×32 binoculars will be larger than 10×25 binoculars (and they’ll have a better exit pupil and work a bit better at dawn and dusk).
Full-size binoculars usually have 42mm or greater objective lenses. So an 8×42 or 10×42 binocular is considered a full-size binocular. Of course, larger binoculars for hunting, birding or marine use can get very large and heavy. You could get 12×50 or 15×56 binoculars, but these are usually only for people who know exactly why they want a very large binocular with high magnification.
Roof Prism vs Porro Binoculars
Most modern binoculars are now “roof prism” because they have straight barrels that let them be more compact. It takes a bit more precision to manufacture great roof prism binoculars, though, so they’re typically more expensive than porro prism binoculars.
Porro binoculars have that old-school, classic look where the binoculars are much wider at the objective lens than the lenses at your eyes. Because they’re easier to manufacture, you can get excellent quality at a lower price point. They’re just bigger and often (but not always) less waterproof and fogproof.
Waterproof and Fogproof Binoculars
If you will be using your binoculars in bad weather, you want waterproof binoculars. If you’ll be using binoculars in cold weather, you want fogproof binoculars, too. Fogproof binoculars are usually filled with an inert gas like nitrogen so that moisture-laden regular air doesn’t condense inside of your binoculars when the temperature shifts.
The Quirk of Binocular Language: Binocular, Binoculars or ‘Pair’ of Binoculars?
Just in case you’re wondering, technically we can refer to a single binocular product as a binocular — but that just seems weird to use for most people, so we turn binocular to plural when we’re asking a buddy to give us a turn with their binocular.
We say, “Can I see your binoculars?” even though they obviously have just one product in their hands.
So a pair of binoculars or a set of binoculars would technically refer to at least two binocular products together. But pretty much nobody thinks or talks this way.
All this means is that you can use the term binoculars like you use the term pants — you don’t put on multiple pants every morning, and you certainly don’t wear pants in pairs. But everyone understands what you mean when you put on a pair of pants. If you run into someone who is on a high-horse about the vocabulary around binoculars, just roll your eyes and remember not to invite them to any parties.