What Do Binocular Numbers Mean? How to Decode Them

Although you might think that you need to invest in a telescope when you want to turn astronomy into your new hobby, this isn’t true.

Your humble pair of binoculars can enable you to see a lot of what’s in the night sky. 

What can you see with your binoculars?

You’ll be able to see many details of the moon, planets, and their details, and even the brightness of a comet. 

But what do the numbers on your pair of binoculars mean, and why are they important?

Here’s your ultimate guide to making sense of your binoculars as well as how to find the best ones for your lifestyle. 


What Do Binocular Numbers Mean? 

You’ve purchased a brand new pair of binoculars. Congratulations!

While inspecting it, you might have noticed some numbers on them. If you hold your pair of binoculars facing away from you, you’ll see it has a model number.

These are usually made up of two numbers, separated by an “x,” and they refer to the magnification and size of the binoculars. The number before the “x” is the magnification, and the number after the “x” is the lens size.

So, if the binoculars are labeled “8×42”, this means that the binoculars have a magnifying power of 8 and a diameter of 42 – this diameter refers to the objective lenses of the binoculars.

The size of the objective lenses is important because it informs you of how large the binoculars are and, therefore how much light they will be able to collect in order to improve how well you can see. 

How Those Numbers Affect the Field Of View

How Those Numbers Affect Field Of View

It’s easy to assume that you should have lots of magnifying power in your binoculars, but this comes with a disadvantage. When you have more magnification, this reduces the area that you can view at one time.

This area, known as field of view, is measured in either degrees or meters. If you have a very narrow field of view, then this means you’ll have to move the binoculars around a lot in order to find what you’re looking for.

When it comes to field of view, the best field of view to choose for your binoculars depends on the type of activity you want to use the binoculars for.


For hunting, you’ll need to be able to watch moving targets, so a wide field of view is important. A magnification of around 8x would be sufficient.

As for lens diameter, it’s pretty standard for hunters to choose a diameter that’s 42mm. This works well when visibility is poor and provides a wider field of view for tracking animals. 


If you want to see views through your binoculars when you go hiking, you’ll need more magnification – aim for 10x or 12x – so that you can focus on objects that are far away from you.

The field of view will be narrower, but this is okay. As for the lens diameter, you will have clarity and brightness with a diameter of 25mm or 28mm. 

Bird watching

To track moving objects, a magnification of 8x is good. If you want to focus on stationary birds, you’ll want higher magnification.

Aim for a lens diameter of about 40mm or 42mm as this will perform well in low-light conditions.

Sporting events

If you’re using your binoculars at sporting events, you should aim for a magnification of between 8x and 10x.

This will ensure you can feel closer to the action on the field or pitch while still having a wide field of view. The lens can be around 42mm to help you see more details. 

How Objective Lens Size And Magnification Affect Focus 

How Objective Lens Size And Magnification Affect Focus

In order to focus clearly with your binoculars, you also need to take features such as its objective lens size (diameter) and magnification into account. 

Since the lens of the binoculars collects light, the bigger it is, the more light it lets in. This means that a larger lens will ensure that the image you’re looking at will be clearer and have more detail.

As for magnification, this is really the power binoculars have. While magnifying an object can help you to see it in more detail, too much magnification will make it difficult to see the sharpness of the image. It will start to become blurry. 

At high magnification, the view from your binoculars can also be blurry as a result of the Earth’s atmosphere, where heat waves and wind can cause various temperatures to blend together.

This makes the air interfere with the light that you’re trying to capture from an object in the distance, and it usually starts being a concern when you go over 60x magnification. 

So, it’s clear that higher power is not always the most important feature in binoculars! 

How Those Numbers Affect Exit Pupil Size 

How Those Numbers Affect Exit Pupil Size

The exit pupil size of your binoculars is another important thing to consider when using binoculars. It refers to the width of the beams of light that are leaving the binoculars’ eyepieces.

You calculate exit pupil size by dividing the objective lens diameter by the magnification of the binoculars. So, if your binoculars are 8×24, it means that the binoculars have an exit pupil size of 3mm

What’s a good exit pupil size to have?

If you’re trying to view birds or wildlife, or doing other activities with your binoculars during daylight hours, then exit pupil size won’t matter much.

At night, however, you want an exit pupil size that’s at least 6mm as this will help you to see more detail, such as when looking for stars in the night sky. 

What To Know About Binoculars And Eye Relief

If you wear glasses, you’ll want to be sure that your binoculars offer excellent eye relief.

This term can be defined as the ideal distance that your eyes should be from the binoculars’ eyepiece. If your eyes are further away than this distance, you’ll miss out on the edges of your view.

You’ll also see a smaller part of the image. You want long eye relief in your binoculars so that you’ll be able to see the whole picture while also being able to accommodate your glasses. You should always aim for eye relief that’s at least 15mm, but the longer the better. 

What About The Exit Pupil?

Earlier in this article, we defined what exit pupil means, but it’s worth mentioning that it affects how well your pair of binoculars will hold up in bad light conditions. So, the larger the binoculars’ exit pupil, the more light will be able to reach your eye. 

You need to know the size of your pupils so that you can choose the best exit pupil size for your binoculars. This is because everyone’s eyes are different and different people’s eyes will expand to different sizes according to the light outside.

If the exit pupil of your binoculars is too large for your eyes, this can cause you to lose out on some of the light that enters the binoculars. 

How To Measure Your Pupils

It’s easier than you think to measure your pupils to see how they change when the light changes. Here’s how to do a quick test.

  • To get your eyes to dilate, step from a bright room into a darker one.
  • Give them a few minutes to reach proper dilation. 
  • Then, take an opaque sheet of paper and make two small slits in it.
  • Look through the slits while you hold the paper about 12 mm or so in front of your face (it should rest against your cheek and eyebrow). 
  • You should be able to see two disks of light. If their edges almost touch, your pupil diameter will be approximately the same distance as the distance between the two slits (via Sky and Telescope). 

How To Calibrate Your Binoculars

In order to be able to focus clearly with your binoculars, you need to calibrate them. Here’s how. 

  • Start by turning the eyecups counter-clockwise so that they’re away from the body of the binoculars. If you wear glasses, keep them retracted. This allows you to have a wider field of view. If your binoculars have a rubber cup that goes around the eyepiece, use it for comfortable viewing.
  • Hold the binoculars by the barrels and look through them. Bend your binoculars up and down at the center to ensure that both your eyes can fit over the lenses. When you look through the binoculars, you should see a circular image.
  • Adjust the barrels if you feel that the binoculars are not fitting on your face properly. 
  • Next, to focus your binoculars you will have to look at an object in the distance that’s about 30 feet away from you. 
  • Cover one of your eyes with your hand. Is the image you see through the other eye blurry? If so, you need to adjust the focusing ring that’s located in the center of your binoculars. 
  • Then, close your other eye and focus on the image with the eye that’s uncovered. If you can’t see the object clearly, you have to use the diopter (wheel on the eyepiece) to adjust it. 

How To Use Binoculars If You Wear Glasses

Earlier, we talked about the importance of choosing binoculars with enough eye relief so that you can feel more comfortable when wearing glasses and using your binoculars at the same time.

There are other important things to know about how to wear binoculars comfortably if you’re an eyeglass wearer.

The eyecups of the binoculars are important to adjust if you wear glasses. You should retract them to their minimum setting, but you might only need to do so partially.

This depends on various factors, such as how thick your glasses are, how much eye relief you have in your binoculars, and the shape of your face. 

You will also need to set the binoculars’ barrels so that they match the distance you have between your eyes. Look through the binoculars and then adjust them so that you can see an image clearly through both lenses.

If your width isn’t correct for your eyes, this will cause the image to become black. You can easily adjust the width by using a lever or knob on the binoculars.

Since our eyes are never completely the same as each other, you want to accommodate the differences between them in order to comfortably look through the binoculars.

For this, you’ll have to use the diopter adjustment (it’s located close to one of the lenses). Basically, you will follow the same instructions we mentioned earlier when it comes to calibrating your binoculars.

You might have to do the process a few times to be 100 percent sure that both of your eyes can see clearly through the binoculars.

You might wonder if you can get away with not wearing your prescription glasses and if this is better to do. It really depends on the type of vision problem you have.

If you’re far- or near-sighted, you can give your glasses a miss when using binoculars because your vision will be corrected by how you adjust the focus.

How To Choose Binoculars For Astronomy

If you want to use your binoculars to be able to stargaze, it’s obviously important that they meet certain criteria because you’ll be trying to use them to see stars that are really far away!

Now, while that might make you think that you have to use high-power binoculars, the truth is that you should opt for low-power ones instead.

This is because you want to be able to hold the binoculars comfortably so that you can steady the images otherwise, they’ll be blurry. Aim for magnification of 10x or 7x. But remember, anything greater than 10x requires the use of a tripod to keep it steady.

The lens diameter you choose will be important because the lens is responsible for how much light can enter the binoculars. Try to choose binoculars that have at least 50mm lens diameter. 

General Tips For Buying The Best Binoculars

General Tips For Buying The Best Binoculars

Choose Between Roof And Porro Prism 

Roof and Porro prism are both used in many popular types of binoculars. But which one is better?

Roof prism means that the glass elements in the binoculars are in line, which makes the binoculars more comfortable to hold as well as more streamlined. In the Porro prism design, the glass elements are not in line, and this is beneficial for a greater field of view.

Consider Bak-4, BK7, or SK15 Prisms 

These refer to the materials that are used to make the prisms. Ultimately, BaK-4 prisms are the gold standard.

This is because it is able to transmit light better than other types while losing less light due to internal reflection.

Think About Multi-Coated Vs. Fully Multi-Coated Lenses

When lenses are multi-coated, this means that multiple surfaces have been coated. On the other hand, fully multi-coated lenses mean that every single surface, both inside and outside of the lenses, contains multiple coatings.

The latter is much more efficient when it comes to enabling you to see clearer images with color-correct features and better contrast.

Choose Binoculars That Are Well-Made

It’s important to spare some thought for the binocular construction. Some popular and durable materials that are used to make binoculars include aluminum alloy, which is strong yet light, and magnesium alloy, which has a good strength-to-weight ratio

To further ensure that your binoculars last a long time, it’s important to look for specs such as that they’re fog-proof and waterproof since you’re going to be using them outside. 

Related Questions 

What does the focusing ring on binoculars do?

The focusing ring puts the object you’re viewing into focus. By comparison, the diopter (found on the right eyepiece) tries to make up for the differences between your eyes. 

Are zoom binoculars worth it?

While they can give you a magnification of up to 30x, it’s common for images to lack brightness and sharpness


You can use your binoculars to see a variety of sights – you can even use them for astronomy! In this article, we’ve provided you with information to help you use your binoculars more effectively, whether you’re using them to birdwatch, spot wildlife, or locate a planet.

We’ve also looked at some general tips for buying the best binoculars for your needs, focusing on some important specs and features that everyone should know if they’re interested in investing in a pair of binos.  

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