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What Do the Numbers on Binoculars Mean for Users?

what do the numbers on binoculars mean
Written by Erik Himmel

Take your binoculars and examine it. You’ll see numbers coming up into your notice. Ask your best pal who might be an ardent binocular user anything about the devices. The most frequent words you would hear from him/her are likely to be a collection of numbers, both familiar and vague. What do the numbers on binoculars mean? That’s a BIG question. But worry not! Today, I’m going to make the numbers easy, intelligible, and practicable. You will be able to choose the best binoculars whether for the purpose of hunting or night vision after understanding the value of these numbers.

So, What Do the Numbers on Binoculars Mean?

As I told binocular numbers are the indications that make a particular unit more attractive, noteworthy, and useful than others. Here’s an example.

You will always find a series name with two numbers, such as 10×50, 10×42, 8×42, or 8×25. These numbers relay the magnification, and the objective lens size respectively.

The first number in the sequence usually indicates how much the binocular will magnify the target for you. If 10 is the first number, your unit magnifies an object 10 times its original size. On the other hand, the second number refers to the diameter of the lens in millimeters. The lens size is critical as it indicates how much light the lens may assemble in a given situation.

Available numbers on binoculars:

In this article, I’ll discuss the following numbers.

  • Magnification
  • Field of view
  • Aperture
  • Exit pupil size
  • Eye relief
  • Prism systems
  • Coatings

All these properties bear numeric significance for hunters and birdwatchers, surveillance professionals, foresters, archers, and everyone who uses binoculars.


It is needless to say that the process of enlarging the apparent size of an object is known to us as magnification. However, the process amplifies the apparent size, not the physical size. The ocular lenses usually carry out the magnification process of the binoculars. When you use a binocular, ocular lens are the closest to the eyes.

You already know that the first number in the designation refers to the intensity of the magnification. For example, if you got numbers like 10×40 or, 8×42; that means the object you want to see through binocular will appear 10 or 8 times closer as opposed to the naked eye.

When you see multiple numbers before the ‘x’ sign, such as 12-36×70, you have got a zoom binocular. The magnification of your binocular is variable from the first number to the second number.

For the 12-36×70, you can see the object 12x to 36x bigger by just moving the zoom lever. Some binoculars may also show the numbers like 25/40×100. The sign ‘/’ between the first two numbers mean this is not a zoom binocular.

However, it can use several fixed-power eyepieces to obtain the different levels of magnification. The 25/40×100 binocular has eyepieces on rotating turrets. It gives you the opportunity to switch back and forth between 25x and 40x magnification.

Understanding the right magnification settings will help you choose the compact, high-end binoculars.

  • With the objects appearing closer, the field of vision will diminish.
  • For bird watching and sport viewing binocular with the magnification of 7 or 8 will be great.
  • Smaller ocular lenses are also good for these activities as smaller binoculars are easier and steadier to hold.

Field of View

This specific aspect is expressed using either degrees or meters visible at a 1000m range. Deer hunters long for a wide field of view, so they get a better chance to spot the target. Binoculars with a narrow field of view requires the user to pan the device around more, an action that would cost the hunter the spotting of the animal. So, a proper balance between field of view and magnification is all you want.

Aperture (Lens Size)

The second number after the ‘x’ sign refers to the size of the objective lenses’ diameter in millimeters. For the lens, size does matter. The larger lenses will admit more lights for clear viewing. The terms aperture indicates ‘opening of the lens.’

To measure the lens aperture size, we use the term ‘f-stops.’ Moving to start with one f-stop and then onto the next will double or separate into equal parts the opening of the objective lens. At the point when the lens measure is multiplied, multiple times the measure of light will enter the binoculars.

The size of the objective lenses also influences the physical size of a couple of binoculars. The large objective lenses will make the casing of the binoculars larger. However, makers use different materials for the casing, but the weight of the binoculars will definitely increase with the aperture number. If you’re using a handheld binocular, consider the following points.

  • Large sized apertures are useful for bird watching.
  • Thermal binoculars are also available when there is no light.
  • Buy a sturdy tripod with larger binoculars.

Exit Pupil Size

Divide the aperture by the magnification, and you will get the exit pupil. The number will be approximately between 4 and 8. Exit pupil is usually the diameter of the shaft of light coming to your eyes from the binocular eyepieces. The exit pupil refers that how well the binocular will perform in a low light condition.

The ideal exit pupil size of your binocular has to be equal or slightly smaller than the pupil of your eyes. Because of this, the binocular will deliver the maximum amount of light and bring the brightest images for its aperture. The more extensive the shaft of light, the more splendid the picture will be on the grounds that the light is hitting a greater amount of our retina.

Exit Pupil Size

A binocular that has too high amplification for its objective size usually delivers a darker view because the smaller shaft of light is achieving a little measure of the retina. Keep the following information in mind.

  • In the daytime, the pupil of the eyes will contract to the size of 2-4 mm while it will be 7 mm at night.
  • The number will vary through the ages.
  • Exit pupil size will not be an issue for sunlight, but you need to use a larger 6-7 mm exit pupil for dusk or night.

Eye Relief

Eye relief or the ‘exit pupil distance’ is the distance that your eyes have to be from the ocular lenses. It will ensure the optimum performance of the binocular. It will also indicate the standard distance of holding the binoculars from your eyes to get a clear view of the entire apparent view.

eye relief

  • The eye relief range for the standard binoculars is from few millimeters to 25 millimeters or more.
  • The longer focal length of an eyepiece will ensure greater eye relief.
  • Long eye relief will be useful for eyeglass wearers.

Prism Systems

All the binoculars have built-in prism systems that serve as the mirror. Prism system of binocular consists of two prisms. They are placed either one in front of another or which is called the roof prism system or the two prism can be placed in 90 degree angle position. This is called Porro arrangement of prism. The first prism creates a reverse image of the target object. The second prism then reverts the image to its original position. You should see through two eyes as two eyes are better than one. 

Prisms in a binocular are of two kinds. There are roof prisms and Porro prisms. Without going in the technical detail, I can only say that Binocular with roof arrangement of prisms is good for bird watching. However, they are a bit poor performer for the astronomical uses.

Two types of glasses are used to make prisms. One is a combination of boron and silicate flint glass and the other is glass made with barium. Barium glass is also known as Bak 4. These glasses have high refractive power. They offer clear and crisp images. That’s why users prefer BaK-4 over the standard BK-7. Read the points below to check the BK-7 or BaK-4 prism on your own, you can do the following.

  • Take the binocular and Direct it to a light emitting source; look at the eyepiece and it will tell you about the prism glass.
  • The exit pupil of the BaK-4 glass will be curbed, and it will also be evenly illuminated.
  • Glass made with born and silicate which is known as BK-7  may have grey, square-shape circumference in the exit pupil.

Coatings explained

Likely the essential factor in the quality of binoculars is the coating procedure. This inconspicuous, inward component decides the measure of light that enters the objective lenses. Proper coating limits the amount of light that is misled by being reflected far from the binocular.

The procedure of compelling coating has been improved. However, it is moderate, repetitive, and exorbitant to the producer. Setting aside the opportunity to comprehend the quality of coating will enable you to get the best binocular for your cash. We should take a look at the varieties in the coating that decide the quality of various binoculars.

coatings explained

At least eight lenses are in a couple of binoculars. A portion of these is reinforced together. For those that are not, there is an air-to-lens obstruction that the light should cross to infiltrate the lens and enter the binoculars. A typical binocular currently has around 20 such hindrances. So, it is obvious why coatings are essential to satisfactory light infiltration.

Different types of coatings

  • Single-layer coating: There will be a single layer of coating on at least one lens surface.
  • Fully coated lens: They have a single layer coating on every air-to-glass overlay.
  • Multi-coating: Multi-coating gives the lens a higher level of light transmission and image contrast.
  • Fully multi-coating: In this feature, all internal air-to-glass surfaces are coated.
  • Broadband multi-coating: This is the highest-quality of multi-coating. It has less of an increase in reflectivity at the ends of the range.

One interesting thing to note! After talking to countless people (from our community of outdoorsmen), I observed that some hold this notion about binoculars:

“Binoculars numbers are all about numbers.”

Technically yes! But, practically, they’re more than mere digits, and the numbers are actually indicative of the overall potentiality of a unit.

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12x42 Powerful Binoculars for Adults with Clear Low Light Vision - Large View Eyepiece Binoculars for Birds Watching Hunting Travel
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  • 【18mm Large Eyepiece Binoculars, Larger Image, Clearer Details】 2.25 times the image size of 12mm eyepiece binoculars (e.g. 10x25 12x25 10x21 compact binoculars). Large eyepiece makes full use of 12x magnification, delivers larger image ensure clearer details.
  • 【HD Vision with Quality Optics】 Large 16.5mm BAK4 Prism and Fully Multi-coated lens guarantee the key elements of an HD vision brighter view and crisp images. Perfect for birds watching, hunting, Concerts and Sports.
  • 【Clear Low Light Vision】 Adorrgon Binoculars deliver clear vision in weak light condition but not in not completely darkness. Perfect for Hunting or Concerts at night. Comes with Carrying Case*1, Eyepiece *1, Lens Protection Covers*1, Cleaning Cloth*1 and Neck Strap*1.
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About the author

Erik Himmel

A Part-Time Firearm Instructor

Hi, this is Erik. We all know firearms are dangerous, but only when one doesn’t know how to use and care for them. I have 15+ years of experience with different types of guns and for the last 10 years, I have taught numerous people how to hold and shoot a gun while staying safe and keeping the surroundings unharmed. My neighbors are some of my biggest admirers who enjoy talking to me about their guns, firearms safety and maintenance.

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