What the Numbers on Binoculars Mean

The Biggie Numbers

Let’s sift through the numbers, starting with the biggies: magnification and objective lenses.

Magnification

The magnification power of binoculars indicates how much closer the image will appear compared to what you see with your naked eye.

You might think, “Darn tootin’, blow it up as big as it can get!”

But the thing is, with more magnification, the more EVERYTHING is magnified including every little movement and that can get real nauseating, like literally, real quick.

Plus, while you get more detail with higher magnification, the trade-off is a narrower field of view.

When choosing what’s best for you, consider how much magnification you want balanced with how broad a field of view you want.

Of course we want to see everything in fine detail, but we also need to be able to scan the scene comfortably.

As birding folk, we prefer to stay in the 8-10 range, with 8 being our go-to. At that magnification, we’re able to maintain a fairly wide field of view, keep our views bright, and avoid visual shakiness all while getting good close-ups.

Objective lens diameter

The larger the objective lens size, the greater a binoculars light gathering ability, as it’s the objective lenses that determine how much light enters the binoculars.

With larger lenses, comes more light, and, thus, brighter images.

BUT, larger objective lenses result in larger binoculars. And, larger binoculars are — you guessed it — heavier.

That being said, getting a brighter image doesn’t necessarily mean you need to be lugging around an anvil-like pair of binoculars.

Much more than just the objective lenses determine the weight.

Full-size binoculars (over 40 mm) or at the very least mid-size (30- 40 mm) are the best options for more light and more vibrant views (though plenty of compact binoculars (under 30 mm) do well light-wise).

Our go-to is typically 42mm. With that size, it’s not too heavy and we’re able to see details in most lighting conditions.

If low light performance doesn’t matter to you because you are absolutely, positively ONLY going to be using your binoculars in bright light, then by all means, go ahead and avoid the extra weight that comes with a larger objective lens.

But, as much as you may wish every day was a bright and sunny one, that’s just not the case.

Other Important Binocular Specs

Again, magnification and objective lens size are the most important considerations, but they aren’t the only considerations when it comes to getting more into your eager eyes. (To the tune of “Private Eyes,” cue a Hall & Oates earworm…)

Field of view

Field of view is measured in feet per 1,000 yards.

Yeah, you heard that right.

We’re mixing feet and yards because who needs consistency when you’re trying to wrap your head around numbers?

This number is the width of the area you can view from 1,000 yards away. With a wider field of view, the more you get to see without having to swivel your skull.

At a minimum, try to stay above 300 feet when choosing binoculars, at least for birding.

Angle of view

This is simply field of view given in degrees – so the angle of the view you’re getting. A bigger angle of view means you’re getting a wider view.

Minimum focus distance

Minimum focus distance or close focus refers to the shortest distance at which a pair of binoculars can clearly focus on something.

For birders, this is actually pretty important! While it’s true that the main goal of binoculars is to see what’s far off, there are also situations where we want to see more detail of something not too far from us.

Ideally, choose binoculars with a close focus of 10 feet or less.

Eye relief

Eye relief is critical, especially for those of us who wear glasses <raises hand 🙋🏻‍♀️>.

It’s measured in millimeters and indicates how far you can hold the binoculars away from your eyes while still being able to see the entire view.

If you’re wearing glasses, you’ll want at least 15 mm and if your eyes are going commando, 10 mm to 15 mm is perfect.

Exit pupil

The exit pupil – the beam of light that hits your eye through the ocular lens – appears as a bright circle at the center of each eyepiece when you hold your binoculars about a foot from your face.

It’s calculated by dividing the objective lens diameter by the magnification strength.

For example, on a 10×50 pair of binoculars, you divide 50 (the objective lens diameter) by 10 (the magnification) to get a 5 mm exit pupil size.

The larger the resulting number, the brighter the image.

Sticking with the 10×50 example, say you wanted to keep that 10x magnification, but dropped down to a smaller objective diameter, say, 42 mm. In that case, your exit pupil would drop to 4.2 mm and the view, in theory, wouldn’t be as bright.

So, even with two binoculars of the same magnification, the one with the larger objective lens is going to have a larger exit pupil, thus, the brighter image.

Weight

Jeez Louise, now that we’ve got all the hard math behind us, let’s take it easy with something simple: weight.

The higher the number, the heavier the binoculars are.

Easy peasy lemon squeezy.

FAQ

What specs are best for birding?

With bird watching, a magnification of anywhere from 6x to 8x is the sweet spot, though going up to 10x is warranted if you’re doing some serious long-distance viewing. For the objective diameter, 32 mm to 42 mm is a good range to stick to, though the higher end of that is my preference.

If you’re looking for a place to start, go with 8×42. From there, look for a close focus of at least 10′, a field of view of at least, 300′, and an eye relief of at least 15 mm if you wear glasses.

So, there are objective lenses and ocular lenses?

Yes! The objective lenses are the larger glass lenses at the front of your binoculars (not the ones near your face) and then the glass opposite those lenses, there’s an ocular lens for each eye to look through.

What about zoom binoculars?

Zoom binoculars may seem tempting, but, I promise, the trade-off is not worth it, at least with birding.

What other factors are there to consider when picking out binoculars?

Aside from the numbers on binoculars, there are a few other factors to consider when looking at binoculars.

• Optical performance. This includes things like image clarity, color accuracy, and contrast. Look for binoculars with high-quality optics to ensure a better viewing experience.
• Ergonomics. How comfortable are the binoculars to hold and use. This includes features like a non-slip grip and a well-placed focus wheel.
• Durability. Choose ones that are built to last and can withstand outdoor conditions. Look for binoculars with a strong, waterproof design and high-quality materials.

How does the prism type impact binoculars?

There are two main types of prisms in binoculars: roof and Porro. Roof prisms offer a more compact design, but they may not perform as well in low light conditions compared those with a Porro prism. Porro prisms, on the other hand, tend to offer brighter images, but tend to be bulkier.

Do most binoculars have adjustable eyepieces?

Yes, most binoculars come with adjustable eyepieces to accommodate the distance between your eyes. This is known as the inter-pupillary distance, and it can vary from person to person.

Any final tips for picking out the best binoculars for me?

Keep these thoughts in mind when you head out to buy binoculars:

• Do your research and read reviews from other birders to get an idea of which binoculars are highly recommended.
• Test out different pairs in person, if possible, to get a feel for the weight, ergonomics, and overall image quality.
• Consider your budget and don’t automatically go for the most expensive option. There are plenty of high-quality options at various price points.
• Finally, remember that the best binoculars for you may not be the best for someone else. It’s all about finding the right fit and what works best for your specific needs and preferences.