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What Do the Numbers on Binoculars Mean: Complete Review

Binoculars are a piece of essential equipment for many outdoor activities such as bird watching, hunting, and watching sporting events. But what do all those numbers on binoculars mean? Here is a quick guide to help you understand what the numbers on binoculars represent.

Binoculars Numbers Explained

The first number is the magnification, or how many times larger the image will appear through the binoculars than it would to the naked eye. For example, 8x binoculars will make an object appear 8 times larger than if you were just looking at it with your eyes.

The second number is the diameter of the binocular lens in millimeters. This is the measurement of the front lenses of the binoculars. 

The final number is the exit pupil, also in millimeters. This is the measurement of the beam of light that exits the eyepiece. A larger exit pupil will make the image appear brighter, while a smaller exit pupil will make it dimmer.

In this article, we will also answer the questions: what do the two numbers mean on binoculars – and tell about binoculars magnification explained.

What Do Numbers on Binoculars Mean?

Most people don’t understand what numbers on binoculars mean. They see a pair of binoculars and think that the bigger the number, the better the binoculars. That’s not necessarily the case. The number on binoculars is just a way to tell you the magnification power and the size of the objective lens.

What do the two numbers mean on binoculars?

The first number is the magnification power, also called the control. This number tells you how many times an object will appear to be magnified when you look at it through the binoculars. So, if the number is 10, then the object will appear to be ten times bigger than it would if you were just looking at it with your naked eye.

The second number is the lens size, measured in millimeters. This is the lens at the front of the binoculars. A larger objective lens will let in more light, making the image appear brighter, while a smaller lens will make the image appear dimmer. You’ll see objects more clearly, even in low light conditions.

The table below gives an example of what the numbers on the binoculars mean.

Numbers on binoculars What do they mean
What do the numbers 10x42 mean on binoculars?Binoculars have a 10x magnification and a 42 mm objective lens size.
What do 10x25 binoculars mean?Binoculars have a 10x magnification and a 25 mm objective lens size.
What do 30x60 binoculars mean?Binoculars have a 30x magnification and a 60 mm objective lens size.

Other numbers

Binoculars can have other numbers and symbols on them, and here are some of them and their parameters meanings:

  • Exit pupil. This is the diameter of the light beam that comes out of the eyepiece. It is calculated by dividing the diameter of the binocular lens by the magnification. So, the exit pupil of our 7×50 binoculars is 7.1 mm. A larger exit pupil means that the image will be brighter, which is why this number is sometimes called the “brightness index.”
  • Field of view. This is the width of the area you can see through your binoculars. It is usually given in feet or meters at a certain distance, like 1000 yards. For our 7×50 binoculars, the field of view would be 38.5 feet at 1000 yards. A larger field of view is better for tracking moving objects.
  • Relative brightness. This number gives you an idea of how bright the image will be through the binoculars. It is calculated by multiplying the exit pupil by the square of the magnification. So, the relative brightness of our 7×50 binoculars is 250 (7.1 x 7.1). An enormous relative brightness means that the image will be brighter.
  • Eye relief. This is the so-called distance from the eyepiece lens to your eye. It is important for people who wear glasses because you need to be able to see the entire field of view. For our 7×50 binoculars, the eye relief is 18 mm.
  • Close focus. This is the shortest distance the binoculars can be used and still produce a clear image. For our 7×50 binoculars, the close focus is 16.4 feet.

So, when you’re looking at the numbers on a pair of binoculars, the first number is the magnification power, and the second number is the size of the objective lens. Remember that the bigger the magnification power, the narrower your field of view will be. And the bigger the objective lens, the heavier the binoculars will be. So, it’s a good idea to find a balance between the two that works for you.

Objective Lens Size (Aperture)

An essential factor in determining the quality of a binocular is the size of the objective lens. The larger the lens, the lighter it can gather, and the brighter the image you’ll see. This is especially important for night sky viewing or other low light conditions.

Wider lenses also have the advantage of an increased field of view. This means that you’ll be able to see more of the sky at one time through a wider lens.

There are trade-offs to consider when choosing a binocular with a large objective lens. The larger the lens, the larger and heavier the binocular will be. This can make it more difficult to hold steady, especially for extended periods.

Prism Systems

Prisms are one of the essential elements of a binocular. They are responsible for bending the light that enters the binoculars to produce a magnified image. There are two main types of prisms used in binoculars, Porro prisms and roof prisms.

Porro prisms are the older of the two designs and are characterized by offset configuration. This means that the binocular barrels are not in line with each other, and the prisms are positioned at an angle. This design is more efficient at bending the light, resulting in a brighter image. Porro prism binoculars are typically less expensive than their roof prism counterparts.

Roof prisms get their name from their shape – they are flat and aligned with the binocular barrels. This design is more compact, making roof prism binoculars more portable. However, this comes at the cost of image quality, as roof prisms are not as efficient at bending light as Porro prisms.

Understanding Angle of View

To understand the AoV of binoculars, one must first understand what AoV is. AoV stands for Angle of View and is the angle at which an object can be seen through a lens. The larger the AoV, the more of the object that can be seen.

There are two types of AoV: linear and angular. Linear AoV is measured in length units, such as feet or meters. Angular AoV is measured in units of degrees, arcminutes, or arcseconds.

To calculate the AoV of binoculars, one must first determine the focal length of the lenses. The focal length is the distance from the lens to the point where the object being viewed is focused. It is typically written in units of millimeters (mm).

Once the focal length is known, the AoV can be calculated using the following formulas:

Linear AoV = 2 * tan (f / (2 * d))

Angular AoV = 2 * arctan (f / (2 * d))

where f is the focal length and d is the distance to the object being viewed.

binoculars

For example, let’s say you have a pair of binoculars with lenses with a focal length of 10 mm. If you are viewing an object that is 10 meters away, the linear AoV would be 2 * tan (10 / (2 * 10)) = 2 * 0.01745 = 0.035. The angular AoV would be 2 * arctan (10 / (2 * 10)) = 2 * 0.01745 = 0.035 degrees.

The AoV of binoculars is important to consider when selecting binoculars. If you know the size of the object you will be viewing and the distance at which you will be viewing it, you can calculate the AoV that you need to see the entire object.

Eye Relief Number

The Eye Relief Number of Binoculars is the distance between your eye and the eyepiece lens of the binocular. It’s important to have the proper eye relief, especially if you wear glasses, to see the full field of view. The average binocular has an eye relief of around 15mm, but some models have less, and some have more. If you wear glasses, you’ll want to ensure you get binoculars with at least 18mm eye relief.

Understanding Field of View

Field of view (FOV) measures the angular width of the observable world that can be seen at any given moment through a binocular. It is usually represented as an angle (°), but sometimes as width in feet or meters at a specific distance. The term is not specific to binoculars and can be used in other optical instruments such as telescopes, cameras, and even the human eye.

The FOV of a binocular is determined by the design of the optics and the eyepiece. Larger objectives (the front lenses) and shorter focal lengths will result in a wider FOV. The eyepiece also plays a role, with shorter eyepieces providing a wider FOV than longer eyepieces.

The human eye has a relatively large FOV, with an average width of about 120°. This allows us to see a large area at any given moment, but it also means that our view is less focused. Binoculars, on the other hand, have a much smaller FOV, typically between 5° and 10°. This may seem like a tiny area, but it is enough to see a fair amount of detail.

One of the benefits of a smaller FOV is that it allows the binoculars to magnify the image. This is because the image is focused on a smaller area of the retina, which makes it appear larger. The trade-off is that a smaller FOV can make it more difficult to find and follow objects.

There are a few things to remember when choosing a binocular based on FOV.

First, consider what you will be using the binoculars for. If you need to follow fast-moving objects, you will need a wider FOV.

Second, remember that a wider FOV will result in a less magnified image. If you need a high level of magnification, you will need to sacrifice some FOV.

Finally, keep in mind that the FOV can be affected by the eyepiece. Using a binocular with a zoom eyepiece, the FOV will change as you zoom in and out.

The best way to choose the correct FOV for your needs is to try out different binoculars and see what works best. There is no single “perfect” FOV for all situations, so it is essential to find the one that works best for you.

Exit Pupil Number

The exit pupil number is the size of the light beam that comes out of the binoculars’ eyepieces and enters your eyes. The value is easily calculated by dividing the objective lens diameter by the magnification. The 10×50 binoculars have an exit pupil of 5 mm, while the 8×25 binoculars have only 3.1 mm.

The exit pupil number is crucial because it determines how much light will reach your eyes. A larger exit pupil means more light and, therefore, a brighter image.

The exit pupil is also a good indicator of how well binoculars perform in low light conditions. A larger exit pupil will allow you to see better in low light, while a smaller exit pupil will make it more challenging to see in low light.

When choosing binoculars, it is essential to consider the exit pupil number. A larger exit pupil will generally provide a better image and will be better in low light conditions. However, binoculars with a large exit pupil can be more expensive, so it is essential to find a balance between price and performance.

Close Focus (FoV)

The close focus number of binoculars is the minimum distance the binoculars need to focus on. This is important to know when choosing binoculars because it will determine how close you can get to an object and still see it.

If the minimum focus is 5 feet, then you can focus clearly on something at 5 feet away. Some binoculars have a close focus of just a few inches, which is excellent for looking at small objects up close.

FAQ

numbers on binoculars

The bigger the number, the better for binoculars?

The reason is that the bigger the number, the more light can enter the binoculars. This means that you can see more clearly, and the image will be less likely to be blurry.

What magnification binoculars should I buy?

If you are looking for binoculars for general use, a magnification of 7x or 8x is a good choice. If you are looking for binoculars for specific purposes such as birdwatching or astronomy, you may want to choose a higher magnification

What are the benefits of expensive binoculars?

Some benefits of expensive binoculars are that they usually have a higher level of magnification than cheaper binoculars, which can be helpful for bird watching or other activities where you need to see things at a distance. Expensive binoculars also often have better optics, providing a clearer image. Finally, expensive binoculars tend to be built to last longer and withstand more wear and tear than cheaper binoculars.

Bottom Line

Now that you know what the numbers on binoculars mean, you can make a more informed decision when purchasing a pair of binoculars. Remember the magnification you need, the size of the objective lens, and the exit pupil when making your decision.

If you’re interested in birdwatching or other forms of nature observation, 8x or 10x binoculars are a good choice. For stargazing, you’ll want to use binoculars with higher magnification, such as 20x or 30x.

We hope our article told you everything about the numbers on binoculars, but if you still have questions, leave them below in the comments.

About Valery Johnson

Hi, I am Valery and I love nature, the universe and the starry sky. Together with my friend Michael we share our practical knowledge in the field of astronomy and nature observation. We also test different optical instruments to see the strengths and weaknesses of different models. Very often we travel around our country, so we have the opportunity to test optics in different conditions and different seasons. Welcome to Michael's and my blog and we hope you find useful and practical information for yourself.

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