What Do Binocular Numbers Mean?
Luke Cuenco 10.15.24
We’ve all probably been there at some point when it comes to optics in general – what do all those binocular numbers mean? Your confusion shouldn’t give you any embarrassment, however, optics is a whole industry in addition to being a field of science that is still heavily researched and developed today. Binoculars are perhaps one of the most familiar forms of optics to most people outside of things like camera lenses. While most people intuitively understand zoom levels (3x, 5x, 10x, and so on), most people start to struggle with these numbers once they start getting combined with other numbers as part of the same product – so today we’ll explain those numbers and see how these numbers affect magnification, light-gathering ability, field of view, and more. It’s super simple!
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What Do Binocular Numbers Mean?
Magnification: The First Number
The first number is probably the one you’re most familiar with—it’s the magnification, or how much closer things will look compared to the naked eye. So, with 8x magnification, objects appear eight times closer. Sounds simple, but there are a few things you should keep in mind. Higher magnification gives you more zoom, but it also makes the view shakier—especially beyond 10x, where a tripod can help keep things steady. Additionally, higher magnification usually narrows the field of view, making it harder to track fast-moving subjects like birds or if you’re trying to spot individual shots on a target. That’s why you’ll find that most pairs of binoculars tend to stick right around the 8x or 10x magnification range, and binoculars with higher magnifications typically feature tripod compatibility.
Objective Lens Diameter: The Second Number
The second number represents the diameter of the objective lens (in millimeters). In an 8×42 binocular, this means that the diameter of the lens itself from edge to edge is 42 mm. The size of this lens affects how much light your binoculars can gather and more light is typically associated with better image clarity and quality.
A bigger lens gives you better low-light performance, so a 50 mm lens can pull in way more light than a 30 mm one, making your view brighter and sharper—perfect for those early morning or late evening range or hunting sessions. There is, however, a catch to having this extra performance: bigger lenses also mean heavier, bulkier binoculars and with that an associated increase in overall price – another number most people are intimately and probably angrily familiar with. If you’re going on a long hunt or stalk, you probably won’t want to lug around that extra weight unless you really need the added brightness or magnification. It’s all about finding the right balance between how much light you want and how much weight you’re willing to carry.
Light and Exit Pupil
These are numbers that go beyond what’s typically advertised on the outside of the binoculars. The exit pupil is the measurement of the diameter of the small beam of light that exits the eyepiece. A larger exit pupil will typically deliver a brighter image – especially in low-light situations. There is an easy way to calculate the exit pupil on any optic in case you don’t have its specifications handy and you’re looking to see if it might be a good fit for a low-light situation. A larger exit pupil is great for low-light situations because it delivers more light to your eyes. Your pupils dilate to about 5-7 mm in the dark, so an exit pupil in this range gives you better nighttime performance.
Exit Pupil = Objective Lens Diameter ÷ Magnification
So, with 8×42 binoculars, the exit pupil is 5.25 mm. This means the light beam exiting the eyepiece will be 5.25 mm in diameter, which is suitable for low-light conditions, as it aligns well with the eye’s pupil size when dilated in dim settings. A larger exit pupil also tends to give the user better viewing comfort by making it easier to align your eyes with the binoculars.
Field of View (FOV)
Field of view (FOV) is all about how much of the scene you can see through the binoculars, usually measured in feet at a distance of 1,000 yards. A wide FOV is great for following action, like birds darting through trees or animals on the move when hunting or stalking because you can see more of what’s going on without constantly adjusting your view. It gives you that extra bit of context, which can keep you from making a dangerous shot, or just giving you the best view of the scenery imaginable.
Similar to FOV there is Apparent FOV which gives you a feel for how immersive your view is. You can think of it as how “panoramic” everything looks. It’s easy to estimate: just multiply the angular FOV by the magnification. For example, if you have 8x binoculars with a 7-degree angular FOV, you get an apparent FOV of 56 degrees (7° × 8 = 56°). The wider that apparent FOV, the more it feels like you’re right in the middle of the action, almost like looking through a window rather than a pair of binoculars. On the other hand, narrower apparent FOVs allow you to see greater detail on specific objects rather than the broader scene. In addition, you’ll typically also see less edge distortion, and experience less eye fatigue with a narrower apparent FOV.
Practical Example: Choosing Your Ideal Binoculars
Let’s say you want a pair for birdwatching. You’d probably benefit from something around 8×42, in fact, I’ve been using a pair of Vortex Razor HD 8x42mm for this exact purpose over the last couple of months:
- 8x magnification: Provides a good balance of zoom and stability.
- 42 mm objective lens: Sufficient for low light, like dawn or dusk, but not too heavy for handheld use.
- 5.25 mm exit pupil: Ideal for early morning or evening viewing.
For more casual use, something like 10x25mm might work if you don’t need as much light but want a smaller, lighter option.
- 10x magnification: Provides you with better detail or a “closer look” at the bird you’re looking at
- 25 mm objective lens: While not good in low light, it will work during most times of the day when birds are active and will reduce the overall cost of the binoculars
- 2.5 mm exit pupil: Quite small in the grand scheme of things and pretty much will reduce the useability of the binoculars to daylight hours only.
Long-range shooters and hunters on the other hand may benefit from some seriously powerful binoculars like a pair of 16x70mm binoculars.
- 16x magnification: Delivers high detail and brings distant objects significantly closer, ideal for things like stargazing or long-distance viewing. However, it may be challenging to hold steady without a tripod due to the high zoom level.
- 70 mm objective lens: Excellent for gathering light, especially in low-light conditions like dusk or nighttime. This large lens size is great for seeing more detail in dim environments, but it also makes the binoculars heavier, bulkier, and more expensive.
- 4.38 mm exit pupil: While not as large as some low-light optimized binoculars, it’s still decent for low-light use. It should perform well at dusk and dawn, but it might not be as bright as binoculars with an exit pupil closer to 5 mm or larger, especially if your eyes are fully dark-adapted.
In the end, it’s about matching binocular specs with your needs. If you prioritize portability, go for smaller lenses and lower magnification. If you’re all about long-range shooting and hunting you’ll probably want to get the biggest set of binoculars that you can get your hands on but you’ll also have to be prepared to pay the price as these large models typically fetch a premium price tag.
I’ve found a lot of use out of models Like the Bushnell R5 series both in their 10×42, and 12×50 configurations for spotting targets at long range. Meanwhile, my Vortex Razor HD 8×42 binoculars are the perfect pair to bring along on a hike or just use at home for casual birdwatching. I’ve also been dabbling with Vortex’s 18×56 Kaibab HD binoculars which are great for checking groups on the range without the need to setup a tripod. There’s something out there for everyone but with an understanding of the numbers, you should now be able to narrow down your options to get the exact pair of binoculars you need.
