What do the numbers mean on a pair of binoculars? An objective view

best binoculars
It's good to know your objective lens diameters from your magnification factors (Image credit: Getty)

Whether you’re enjoying the sight of a bird colony on a lone, guava-coated sea stack, watching climbers inch their way up the Eiger’s North Face or you’re on a stake-out, The Wire style, with your fellow officer, having the right pair of binoculars is important.

You see, there are different bins for different situations and much of that has to do with the numbers they’re labelled with, such as “8 x 24” or “10 x 42”. But, what do these numbers mean on a pair of binoculars? Well, unsurprisingly, it’s all to do with their optics.

The first number which, on mainstream bins, is often either 8 or 10, is the magnification factor. The higher the number, the greater the magnification. For example, if an osprey was soaring through the sky 100 meters away, viewing it through 10x magnification would make it appear only 10 meters away. You might think that the higher than magnification, the better. However, it’s not quite as simple as that, which we’ll come to in due course.

Next up, the second number, is the objective lens diameter in millimeters. The larger the objective lens, the more light your binoculars can hoover up. Not only this, the difference is exponential. So, if you were to double the diameter of your objective lens, it’ll collect four times as much light – that’s a lot of photons! This makes the diameter of the objective lens particularly important in low light settings.

The magnification consideration

Backpacker on mountain using binoculars

The more zoomed in you are on an object, the more constrained your field of view is (Image credit: Getty)

As mentioned, you may think that having loads of magnification is the ideal scenario. However, the more zoomed in you are on an object, the more constrained your field of view is. You don’t want to be that person who is so closely studying a ptarmigan’s lovely winter coat that you completely miss a golden eagle swoop down and snatch it away. Less magnification gives a wider view, so you can enjoy studying your subject but still see what’s going on around it.

It’s also harder to pick out distant subjects and move with them when you’ve got high magnification. To take the ptarmigan/eagle analogy further, even if you did spot the eagle’s approach, tracking the bird’s flight once it’s back in the air is much easier with less magnification and a wider field of view.

Finally, the last downside to higher magnification is that your own movements are also exaggerated. Unless you’ve got your binoculars attached to a tripod, you’ll know just how much you move, even if trying to hold your bins as steady as possible.

So, while some magnification is obviously desirable, there’s definitely a balance to be struck.

Being objective about the objective diameter

View of the Summer Triangle from forest floor

Studying the night sky is much easier with a larger objective lens (Image credit: Getty)

We’ve already established that the larger the objective lens, the more light it lets in and that the difference is exponential. This is because when you double the diameter of a lens, you actually increase the surface area by a factor of four. The objective lens takes all those lovely photons and transmits them to the ocular lens, which is where your eye takes over.

Increased light capture is why optics designed for viewing objects at night have large objective lenses. Think of the lens size on an astronomical telescope, it’s pretty big. That’s because that cosmos out there is pretty darn dark. With this in mind, it makes sense that binoculars with large objective lenses are well suited to nighttime use.

It’s worth bearing in mind that a large lens equals a bigger, heavier pair of binoculars so, once again, there’s always a balance to be struck.

What magnification and objective diameter are best?

Three people birdwatching in woodland

Regardless of whether you're hiking, birdwatching or both, there's the ideal combination for you (Image credit: Getty Images)

So, now your know what the numbers mean on a pair of binoculars. Hurrah! As you’ll have noticed, there’s no perfect pair for every scenario and finding the right magnification and objective lens size is a balancing act.

The most popular binocular magnification and objective lens size combo is 8x42, like that present in the excellent Nikon Prostaff 7S. This pretty much hits the sweet spot for birders, giving enough magnification to appreciate an animal’s grace, while still giving the user a wide field of view and the opportunity to effectively track it when it moves. A 42mm lens lets in just the right amount of light.

You should carry binoculars on every adventure. If you’re popping your bins in your daypack and heading out on a hike, you’ll probably want something a little lighter and more portable than what a birder would take to the nearby reserve or estuary. Therefore, most bins designed for general outdoor use, often referred to as compacts, have much smaller objective lenses, usually around 25mm or 28mm. 

If wildlife watching from the trail is your bag, a magnification factor of 8 gives you a decent field of view, like the Kowa YF 8x30 bins, which are a great beginner option. On the other hand, if you use your binoculars as a navigational aid and expect to be looking at mainly stationary or slow-moving subjects that might be a considerable distance away, a factor of 10 works well.

Jupiter and moons

Jupiter gives up its secrets more readily to optics with large objective lenses (Image credit: Getty Images)

Stargazing binoculars have both a high magnification factor and a large objective lens, often 42, 50 or even higher, like the 15x70 Celestron Skymaster. It’s worth getting a tripod once the objective lens starts to get overly large, as the bins will obviously be bulkier. This also helps to keep the image still. Jupiter and it’s Galilean moons may be careering around the sun at approximately 29,236 miles per hour (neat fact: it's the fastest planet in our solar system), but it’s so far away that it’ll seem like it’s not really going anywhere fast.

So, that's the magnification and objective lens sorted. Perhaps it's time to get some expert tips and learn how to use binoculars properly.

Alex Foxfield

Alex is a freelance adventure writer and mountain leader with an insatiable passion for the mountains. A Cumbrian born and bred, his native English Lake District has a special place in his heart, though he is at least equally happy in North Wales, the Scottish Highlands or the European Alps. Through his hiking, mountaineering, climbing and trail running adventures, Alex aims to inspire others to get outdoors. He is currently President of the London Mountaineering Club, training to become a winter mountain leader, looking to finally finish bagging all the Wainwright fells of the Lake District and hoping to scale more Alpine 4000ers when circumstances allow. Find out more at www.alexfoxfield.com