What Is The Meaning Of The Numbers On Binoculars?

Getting a new set of binoculars is fun! You will find them super useful for a variety of uses, but you may be pondering just what those numbers are written on the side of your binoculars symbolize.

Buyers must check out and fully understand all the numbers written on a binocular when shopping for one. You did not purchase your optics for a math lesson. But don’t worry, there is no math involved. Though, to those who are going to buy their first binocular, it could seem so overwhelming and confusing.

Understanding these numbers can help you choose the best binoculars for your needs and avoid overspending on unnecessary features. Moreover, it will help you better operate your binoculars, so you don’t miss a single thing.

Table of Contents

What is Binocular?

Binoculars are a crucial piece of equipment that can enhance daily activities such as hunting, bird watching, urban exploration, hiking, and sporting events. Primarily binoculars take a distant image, enlarges it through a series of lenses for viewing

Binoculars, for the most part, are built relatively straight-forward and simple. There are the lenses at the end of the optical tube called the objective lens. The objective lens, which is measure in millimeters, gathers light and focuses the light on the ocular lens. The ocular lens is the minor lens in the eyepiece. Binoculars are two small scopes mounted side-by-side so the user can view the desired image with both eyes instead of just one eye.

What does 20x50 mean on binoculars?

Binocular numbers are the signs that make a particular unit more attractive, noteworthy, and useful than others. When inspecting a pair of binoculars, you will see two numbers expressed as 20×50mm, 8×42mm, 10x50mm, 10x42mm. These numbers describe the binocular’s capabilities. The first number in the sequence (20) in 20×50 is the magnification; this will magnify the object ten times its standard size. The second number (50) in 20x50mm is the aperture. Furthermore, the two numbers can also express the exit pupil.

This article will enlighten you through all the essential things and numbers you need to know to choose a suitable binocular.

Which is the best magnification for binoculars?

Magnification is one of the needed features one should take into an explanation. It is pointless to say that the course of enlarging the apparent size of an object is branded to us as magnification. Nevertheless, the process amplifies the apparent size, not the physical dimension. The ocular lenses typically carry out the magnification process of the binoculars. When you use a binocular, an ocular lens is the closest to the eyes of the user.

You already know that the first figure in the designation refers to the strength of the magnification. For example, if you got data like 20×50mm or, 8×42mm; that means the object you want to view through binocular will appear 20 or 8 times closer and bigger as opposed to the naked eye.


Aperture means the opening of the lens. It is the size of the binoculars’ objective lens. It is also the second number (50) in 10×50 measured in millimeters. The lens size is vital, as it indicates how much light the lens may assemble in a given situation. More substantial aperture results in more light, which means it can produce a brighter and clearer image, even in low-light conditions. A smaller aperture, collect less light.

The size of the objective lenses affects the physical size of a couple of binoculars. The broad objective lenses will make the casing of the binoculars bigger. However, makers use different materials for the housing, but the weight of the binoculars will increase with the aperture digit. If you are using a handheld binocular, consider the following points.

  • Large-sized apertures are suitable for birding.
  • Thermal binoculars are also accessible when there is no light.
  • Buy a sturdy tripod with more giant binoculars.


The exit pupil is the number of light rays that enter the objective lens and exits the ocular lens. The exit pupil is that tiny column of light that you can see through your binocular when you hold them far from your face. You can achieve the measurement of the exit pupil by dividing the aperture lens size by the magnification size, for example, 10x50mm, aperture (50) divided by the magnification (10), equaling 5 exit pupil.

Mathematically; 50 (aperture in mm) divided by 10 (magnification) = 5mm 

A higher exit pupil implies that the binoculars will work effectively in dim light. For well-lit environs, an exit pupil of 2.5 to 4 is sufficient. The wider the column is, the more light will be collected by your eyes when using the binoculars; this can vary from person to person. The ideal exit pupil magnitude of your binocular has to be the same or slightly lesser than the pupil of your eyes. For this reason, the binocular will deliver the supreme amount of light and bring the brightest images for its aperture. In usual conditions, the exit pupil should be somewhere around 5mm. The more extensive the shaft of view, the more splendid the picture will be because the light is hitting a more considerable amount of our retina.

  • Night or boarding: 7mm
  • Bright light: 1-2mm
  • Average daylight: 3-4mm
  • Dawn or dusk: 5-6mm

Eye Relief

Eye relief is the range that your eyes have to be from the ocular or optical lenses. It will safeguard the optimum performance of the binocular. It will also indicate the standard range of holding the binoculars from your eyes to get a clear view of the entire magnified field of vision. This Eye relief number is significant for corrective specs wearers. Since you will need to wear your corrective lenses to see through your binocular, finding a set that has ample eye relief to house your eyeglasses is crucial. Several binoculars only have an eye relief of 15mm (or less!), which is not nearly enough for most eyeglass wearers. Be sure to look for a longer eye relief as it fits your situation.

  • Long eye relief will be valuable for eyeglass wearers.
  • Eyepiece with longer focal length will ensure more excellent eye relief.
  • Standard binoculars eye relief range is from few millimeters to 25 millimeters or more.

Field of Vision

The field of vision is the area of the scene you can see using your optic. It is another significant binocular number you should know. Many factors are determining this number, such as the lens thickness, the eyepiece, the position, and the magnification.

You can express the field of view using either degrees or meters; it is vast when it is about 60 degrees or more. Thus, selecting a binocular with a wide field of view makes it easier to observe fast-moving objects such as birds or viewing sports.

Each degree is equal to 52.5 feet at 1000 yards. For instance, a binocular with a 6.5-degree field of view would provide you a picture that is about 341.25 feet at 100 yards.

Systems Prism

All the binoculars have in-built prism systems that serve as the mirror, between the aperture lens and the objective lens is the prism. There are two types of prisms in everyday use;

i. Porro-prism

ii. roof-prism

Roof prism system means making prism visible to the other. Porro prism involves placing the two prisms in a 90-degree angle position. The first prism generates a reverse image of the target object. The second prism then returns the image to its original position. Without going in the technical detail, Binocular with roof arrangement of prisms is suitable for horse racing and bird watching. However, they are a poor little performer for astronomical uses.

The main types of glasses used in making prisms are two. One is a combination of silicate and boron flint glass, and then the other is glass made with barium. Barium glass is the same as Bak 4; these glasses have high refractive authority. They offer clear and crisp pictures. That is why users prefer BaK-4 over the standard BK-7.

i) Porro Prism

Porro Prism has a much more iconic and traditional design compared to other types of prism. The Porro-prism can be identified by their offset tubes; the objective lenses are not lined up with the ocular lenses, the ocular lenses are usually closer together than the objective lenses, but the opposite can also be exact, particularly in compact models. The Porro-prism binoculars are often optically superior to the roof-prism binoculars, especially in medium-priced class binoculars. Porro-prism binoculars have a single pivot between the two halves of the binoculars, therefore easier to adjust for the inter-pupillary (distance between the eyes).

ii) Roof Prism

Roof-prisms are in-line inside the optical tube; this makes for a more compact set of binoculars. Roof-prism binoculars have straight tubes (the objective lens is in-line with the ocular lens), which makes them more compact. The roof-prism binoculars usually have two pivot points between the pipes and are a little more challenging to adjust for the inter-pupillary (distance between the eyes).

Focusing binoculars

Focusing is another essential thing you should spend time understanding when choosing binoculars. To make it simpler to follow, let’s first look into focus knob.

what is the meaning of the numbers on binoculars

The focus knob falls between the barrels to help produce more precise images. However, attaining the sharpest focus may take you a portion of a stint.

To verify if you have set the sharpest focus or not, make sure the focus knob is located decisively under your index finger when you hold the binoculars.

There are two ways of focusing binoculars:

i. Center Focusing

Center focusing use a focus knob to focus on the right and left, which will take a while. The benefit of this here is that it targets both very close and far away.

ii. Individual Focusing

This method adjusts the focus for each eye by rotating the diopter adjustment ring. Once this is done, objects from 40 yards to infinity all have the sharpest images.

This method of focusing would be suitable for intermediate and extended objects, rather than objects within proximity. For example, Individual Focusing is mostly used in astronomy and marine observation.

Types of glasses

Extra-low dispersion (ED)

This type of lens is known to avoid or at least lessen the chromatic deviation as it provides more possibilities to focus and direct the light. In short, the image created will be a lot brighter and spotless.

Most expensive optics will have an Extra-low dispersion glass lens as it permits users to take pictures that are flawless, sharp with no color fringe.

Some interesting thing about the Extra-low dispersion glass:

Extra-low dispersion glass is not so steady when there is a variation in temperature. The focal length may alter a bit.

Extra-low dispersion glass also has a low level of refraction, which means it needs curved elements for the same focal length.

High definition (HD)

The term High definition, surprisingly, is not a type of glass. Instead, it implies the result of the Extra-low dispersion lens, which is of high resolution.

Fluoride Lens (FL)

Fluoride Lens is another version of Extra-low dispersion glass, which is classified as a unique and high-quality sub-category.

High Transmission (HT)

Just as the title recommends, the lens is well known for its fantastic broadcasting properties.

Prism Glasses

There are names like BK7, BaK-4, and SK-15. But with little knowledge of what they are. BK7, BaK4, and SK-15 are three significant types of prisms that most people would pick. Let’s discuss briefly this binocular dimension

BK7 is made out of borosilicate glass, highly reflective, with a refractive rate of about 1.52. BK7 can be used for Porro prism and roof prism.

BaK-4 is made out of a Barium crown. It has a far advanced refractive index, which makes the pictures well-edged and brighter.

SK-15 is an intermediate of the two types mentioned.

Glass Coating

Aside from all the prism types, types of glass, and glass elements, glass coatings are also a critical influence that can make the difference. If the layers are unable to match the prisms, you would not be able to use the binoculars.

For beginners, coatings are a kind of formula that allows the higher broadcast of light-waves into your eyes. In most situations, the way light reaching your eyes depends a lot on how it goes through coatings.

Diopter Adjustment

There are two common types of diopter adjustment. These are:

i. Standard Diopter Adjustment: This is a general type of dioptre adjustment ring. You can find it on the Roof and Porro-prism binos.

ii. Lockable Diopter Adjustment: It looks reasonably like the standard version from the left. Yet, sometimes the lockable ones are used on binoculars with higher specs to make sure the setting will not be changed unintentionally.

Exceptional Features

Some binoculars come with some typical features, such as gas-filled and weather-proofing.

Gas Filled

The binoculars are occupied with gases such as nitrogen to prevent condensation. It is also convenient when there is a need to move binoculars to places that have a temperature difference. 


  • A weather-proof or waterproof optic: the optic should not allow in-flow of water.
  • A weather-resistant optic: it can survive the light shower but not in highly wet conditions.
what is the meaning of the numbers on binoculars


Travel: for traveling purposes, the size and the weight are going to be two of the most fundamental factors to consider. A roof prism would be more suitable to bring take along. It should also have an objective lens ranging from 20mm to 32mm.

Hunting: what matters more here is the objective lens. It is advisable to use binoculars with an objective lens of 35mm to 42mm

Marine: waterproof pair of binoculars is a must. If you want a clear view, then choose a magnification of 8x or 7x.

Astronomy:a magnification ranging from 8x and 12x, large a tripod, and objective lens of 42mm are all you needed in this case. A tripod would do a lot of help.


Binoculars come in multiple sizes for different resolutions. I have listed a few general types of binoculars that are chosen by most people.

Wide Angle Binoculars: it is used to capture a fast-moving action over an ample space such as wilderness terrain.

Standard Size Binoculars: A standard binocular for everything from observing wildlife to watching sports

Compact Binoculars: it is lighter and smaller, perfect for taking to the concerts, theater, or on trips.


Binocular sales are segmented into three stages of price: low, medium, and high-end. Here are some suggestion brands for each division:

Low price: Nikon

Medium price: Pentax and Vortex

High-end: Zeiss, Leica, Swarovski

Top 3 Best Recommended Binoculars

Leica 40319 Camera Trinova HD Binoculars
Leica 40319 Camera Trinova HD Binoculars
Close focus under six feet; Innovative carrying system for quick and comfortable use; Compact construction
Vortex Optics Diamondback 10x42 Roof Prism Binocular
Vortex Optics Diamondback 10×42 Roof Prism Binocular
Multi-coated optics and phase corrected prisms; Waterproof and fogproof protection; Close Focus: 5 feet
−$28.28 $200.72

3 Best Recommended Choice For The Binocular

1. Vortex Optics Diamondback 10×42 Roof Prism Binocular

Leica 40319 Camera Trinova HD Binoculars
66 Reviews
Leica 40319 Camera Trinova HD Binoculars
  • Close focus under six feet
  • Innovative carrying system for quick and comfortable use


  • Dimensions: 10.6 x 8.3 x 6.7 inches
  • Weight: 1.6 pounds

About This Binocular

  • Objective Lens Diameter 42 mm
  • Tethered actual lens covers, comfort neck strap, deluxe custom-molded carry case included in the box, and Rain-guard. Field of View: 6.6 degrees 345 feet/1000 yards
  • VIP Unrestricted Lifetime Warranty means Vortex Optics will fix or repair at no cost to the customer, no questions asked.
  • Roof prism full-size 10x42mm binoculars provide 10x magnification through a 42mm objective lens.
  • Multi-coated optics and phase-corrected prisms
  • Waterproof and fog-proof protection
  • Close Focus: 5 feet
what is the meaning of the numbers on binoculars

2. Vortex Optics Diamondback HD Binoculars

Vortex Optics Diamondback 10x42 Roof Prism Binocular
887 Reviews
Vortex Optics Diamondback 10×42 Roof Prism Binocular
  • Roof prism full-size 10×42 binoculars provide 10x magnification through a 42mm objective lens.
  • VIP Unconditional Lifetime Warranty means Vortex Optics will fix or repair at no cost to the customer, no questions asked.


  • Dimensions: 5.9 x 5.6 x 1.75 inches
  • Weight: 1.53 pounds

About This Binocular

  • Fully multi-coated lenses increase light diffusion with anti-reflective coatings on all air-to-glass surfaces. The dielectric layer provides clear, color-accurate, bright images. Armorer coating protects exterior lenses from oil, dirt, and scratches.
  • 10x magnification & 42mm objective lenses, these diamondback HD bingos are improved with Select glass elements to supply an exceptional resolution, cut chromatic aberration, and provide outstanding color fidelity, sharpness, and light transmission.
  • Adjustable eyecups rotate up and down for relaxed viewing with or without eyeglasses. The Center focus wheel adjusts the focus of both binocular barrels at the same time. Diopter (positioned on the right eyepiece) adjusts for differences in a user’s eyes.
  • Argon purging and O-ring seals offer water and Fog proof performance in all environs. Rugged construction endures recoil and impact, Includes glass Pak harness.
  • Rubber armor provides a non-slip grip, durable external, and secure protection. Binoculars are tripod adaptable, allowing use on a tripod or car window mount.
what is the meaning of the numbers on binoculars

3. Leica Trinovid 10 X 42 HD Binocular

Vortex DB-215 Optics Diamondback HD 10x42 Binoculars
6,146 Reviews
Vortex DB-215 Optics Diamondback HD 10×42 Binoculars
  • 10x magnification & 42mm objective lenses, these Diamondback HD binos are optimized with select glass elements to deliver exceptional resolution, cut chromatic aberration and provide outstanding color fidelity, sharpness and light transmission
  • Fully multi-coated lenses increase light transmission with anti-reflective coatings on all air-to-glass surfaces. Dielectric coating provides bright, clear, color-accurate images. Armortek coating protects exterior lenses from scratches, oil and dirt


  • Dimensions: 9.2 x 7.8 x 5.1 inches
  • Weight: 2.4 pounds

About This Binocular

  • The Trinovid 10X42 HD suggest excellent contrasts, optimum color reproduction, and excellent light broadcast
  • The compact design ideally balanced and less in weight
  • Large fields of view offer excellent panoramas, even for distant, unpredictable terrain
  • Outstanding close-up limit: under 6.2 ft / 1.9 m
what is the meaning of the numbers on binoculars


The components of binoculars have changed over the years, but the basic construction of binoculars and how they work has remained unchanged. All binoculars still have three major components. The objective lens collects light and focuses that light in the user’s eyes, the ocular lens that magnifies the image, and a set of prisms that overturn the image and turning the image right-side-up. Choosing binoculars is not an easy job. It has many records that you need to spend lots of time researching and understanding.

As you can see, there is a bunch of useful information you can get from just those two numbers printed on the side of your optics. Once you understand what you want and what the numbers are, then choosing a proper pair of binoculars should not be difficult for you. Above are all significant binocular numbers you should know before making any purchase decision.

Author Profile

Gabriel Tackett
Double major in Engineering and Geology at the University of Minnesota. Experienced shooter & hunter for over 15 years. Certified NRA officer for over 10 years working as a writer at Ballachy.com .

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