Understanding Binoculars In 2024 (Expert Advice)

If you want to have the best experience with binoculars, it is of utmost importance that you understand how their mechanism works. Otherwise, you might wind up with a pair of fine binoculars inside a box in your basement. You should make an educated decision when it comes to getting a pair of these tools.

The best decision comes from knowing what you are getting into. But don’t worry, we got you covered into all the details of understanding binoculars.

Understanding Binoculars

Ever wondered what is inside binoculars?

It turns out that knowing this is less of a pragmatic thing of satisfying one’s curiosity. This dissection will help you better understand what works for you when it comes to these gadgets.

Binoculars are practical and technical at the same time. Let’s start with the basic anatomy. The three main parts are:

  • The objective lens – the large lens at the far end of the binoculars
  • The ocular lens – the eyepiece
  • A prism – a triangular-shaped triangular section of glass block that reflects and refracts light to make the binocular body be smaller than a telescope

The mechanisms through which these three parts work together are pretty technical and the difference between their size and quality is what sets different types of binoculars apart.

You may also like: Telescope vs. Binoculars

Understanding binoculars magnification

When you hit the store looking for a pair of binoculars, the most important part you should look at is a pair of numbers separated with an “x”, i.e. 7×50 or 8×30. The first number and the “x” indicate the magnification or power. So, if it reads 7x, this means that these binoculars magnify an object seven times.

To make it easier, we can also say that a pair of binoculars with a power of 7x, makes objects appear seven times closer than if you saw them with the naked eye.

Most of the binoculars you will see in the market usually come within a range of 5x and 8x. This is usually all you will need. Lower than this will offer a poor magnification but a larger magnification is not the best experience either. A magnification of 10x is still pretty good, but anything beyond that becomes an issue.

If you magnify at 12x, you will not be stable enough to hold the binoculars still and objects will look shaky and blurry. This size of magnification is perhaps only useful when watching celestial bodies but only if you use a tripod to stabilize the binoculars.

Understanding binoculars specifications

Ok, so we got the magnification cleared out for you.

But there are some other specs that you cannot overlook. Check them out before going to the store.


Magnification is represented by the first number and the “x” in the set that is found in all binoculars. The last number refers to the size of the objective lens or aperture. This number actually indicates the lens’s diameter in mm.

Here’s what you need to know about aperture: the wider the brighter the image will be. This is simply because a wider lens will capture more light.

Have you ever heard of the inverse square law of light brightness? It says that as you double the distance between a light source and an observer, brightness is reduced four times (or the square of the distance). If this distance is tripled, then brightness dims nine times, and so on.

This law applies to binoculars too, of course.

To compare their brightness, all you have to do is square their aperture. For example, a 7×50 mm lens (50²=2500) will gather four times the amount of light of a 7×25 mm (25²=625) one, but the bigger aperture is only twice as wide.

Another thing to know about aperture is that the larger it is the poorer the resolution. Or, in simpler words, the wider the objective lens, the less detail you will get.

Normally, you would choose binoculars with a larger objective lens to get clearer images. But these binoculars have a drawback and that is that they have to put a lot of weight. This is not practical for activities such as hiking or camping, situations in which you want to reduce weight.

Field of view:

The field of view refers to how wide is the frame of what you can see through the binoculars. This is usually expressed in degrees or in the linear diameter (in feet) of the circular image you can get to see in the binocular at a distance of 1000 yards.

Every time you divide that 1000-yard distance by ten, you also reduce the field of view by ten. So, if your binoculars offer a field of view of 5°, this means that a distance of 1000 yards, the diameter of your circular field of view will be 260 ft (5°x52 ft).  At a distance of 100 yards the field of view will be 26 ft, and at 10 yards it will only be 2.6 ft.

We know what you’re thinking.

“I’m gonna go get me a pair with a pretty big field of view.” Stop right there.

A larger field of view is usually achieved at the expense of clarity of edge of a field. So, don’t go crazy about the field of view unless you are guaranteed that the binocular is sharp from edge to edge.

Exit pupil:

When you look at the eyepiece from a distance, you notice a circle of light in each one. This is the exit pupil.  This is the diameter of the beam of light that actually enters your eye’s pupil (the “looking” part of your eye).

To find the diameter of a binocular’s exit pupil, simply divide the diameter of the objective lens by the magnification. This is pretty easy since these two numbers are labeled into the binoculars. So, binoculars with a set 7×50 will have an exit pupil with a 7.1 mm (50 mm/7).

A very efficient exit pupil is the one that matches the size of your pupil; this is when the image will look the brightest since it is when all the light is entering your pupil.

During the day, your pupil contracts to less than 4 mm. Binoculars with an exit pupil of 6 mm will waste light in the daytime as not all of the light will enter your contracted pupil. This means that a 7x50mm binocular, with its 7 mm exit pupil will not be as brighter as a 7×28, with a 4 mm exit pupil, since your eye will only perceive light in less than 4 mm.

If you do little use of binoculars in low light conditions, it is pointless to get a larger exit pupil if it makes no difference during daylight.

Now, the story changes during the night and if you are young. The pupil of the average youngster dilates to up to 7 mm in conditions of very low light. People on their 50s will usually have a dilation of no more than 5 mm.  Accordingly, binoculars with large exit pupil are recommended to young people who use them in low-light conditions.

For birding during dusk or dawn, a 4mm to 6 mm exit pupil is the most useful. In fact, any activity during daylight can be done perfectly with these binoculars as that is the approximate size of your pupil contracts too.

Still can’t decide?

You are probably better off with binoculars that have a too large exit pupil rather than a too small one. It makes more sense to waste some light with a large exit pupil than to beg for more light during dusk.

If you are past the middle age, there is also no point in getting a too big exit pupil since your pupils do not dilate as large anymore.

You may also be interested in our detailed post on binocular sizes.

Relative Brightness:

Most manufacturers also specify a detail known as relative brightness. The higher the relative brightness, the brighter the image in the binoculars. To get this number, all you need to do is to square the diameter of the exit pupil. For example, binoculars with an exit pupil of 4 mm will have a relative brightness of 16 (4²=16).

But wait!

Relative brightness does not consider aperture differences between binoculars. Take for example a 5x20mm and a 20x80mm. Both of these binoculars deliver an exit pupil of 4 mm, but you would not say that they perform the same in low light conditions. The larger aperture delivers a lot more light into that exit pupil than the smallest binoculars.

Relative brightness, then, is only useful to compare the low-light performance of binoculars of similar aperture. Of course, it is useful when the size of your pupil is the same as the exit pupil.

Don’t get ripped off.

Some manufacturers boast a “50% relative light efficiency”. Now, note that this specification is not the same thing as relative brightness. This specification really has very little justification since it takes into account the higher light transmission of coated optic versus uncoated lenses. However, all modern binoculars have a coated lens. This statement then is just to make the unwary shopper think that a “50% relative light efficiency” refers to brighter binoculars.

Twilight Factor:

The twilight factor is more important to consider low-light performance than exit pupil and relative brightness. For example, a 5x20mm binoculars and a 20x80mm binoculars both have an exit pupil of 4 mm but the 80mm is much better in low light because they gather 1600% more light than the 20mm.

This light-gathering capacity and the “twilight factor” is what you should look at if you use binoculars mostly in low-light conditions. The twilight factor takes into consideration light gathering and magnification.

How do you obtain the twilight factor, you ask?

To obtain the twilight factor, multiply the magnification by the objective lens diameter. Then, obtain the square root of that product. A pair of binoculars 5x20mm will have a twilight factor of 10 and a 20x80mm has a twilight factor of 40. You can easily see how this number better explains the better performance of the latter despite having the same exit pupil and relative brightness as the former.

Light-gathering and magnification (which determine twilight factor) are two factors that determine how much detail you can see through the binoculars.

In simple terms, the larger an image the easier it is for you to see details. But if the image is smaller, the brighter it gets, the easier it is to see the same details in that image. So, it stands to reason that if brightness goes up, magnification can go down without affection image resolution, and vice versa.

The twilight factor helps you compare different combinations of aperture and magnification and determine the ones that best balance an increase in power with a decrease in magnification (or the other way around). The larger the twilight factor, the better the binoculars will perform in low light.

For twilight or early morning use, binoculars with a twilight factor of 17 and above are the best choice.

But one more time we must say. Although twilight factor holds more performance weight than exit pupil and relative brightness, it is still not your only guide when purchasing binoculars. There are other factors that can be equally important to get details in conditions of low light.  A pair of binoculars may have a better twilight factor than another might still not be as good in dim wooded areas, for instance, where a wider field of view and greater depth of field are more important.

A depth of field:

The depth of field refers to the distance in front and behind the point of sharpest focus that an image remains usefully sharp. A good depth of field reduces the need for refocusing all the time when animals, such as birds, move from place to place. The higher the magnification, the closer it will be the focus and the shallower the depth of field will become.

There is not really a standard for a good depth of field. When testing for depth of field, focus on a certain distance Place a small detail, like a small letter or something, 1 foot behind that point and 1 foot in front of it. If both of these details look equally sharp, then you can consider that the binoculars have a decent depth of field.

Eye Relief:

Eye relief is the distance between the eyepiece and your eye that allows you to see the entire field of view. A long eye relief is important if you use glasses since these stand in the way of you getting close enough to the eyepiece to see the entire field.

You probably noticed already that eye relief is important for people having any type of sight condition. If you are farsighted or nearsighted, you usually can use the binoculars with your eyeglasses off as these usually have enough focus to accommodate.

If you suffer from severe nearsightedness or astigmatism you will need binoculars with good eye relief-15mm or 17 mm longer. Regular eye relief-10 to 15 mm can be used with sunglasses but you will lose some of the fields of view. If the eye relief is too short, your eyes will have difficulty blending the two circles into one.  Therefore, an eye relief below 10 mm is not recommended if you must wear glasses.

Interpupillary Distance:

The distance between both pupils, from center to center is called interpupillary distance. If a pair of binoculars cannot be adjusted to make the exit pupils with your pupils, then you will see a shadow that cuts part of the image.

Understanding different types of binoculars

To this point, understanding the different types of binoculars is probably easy now. You have seen that the best binocular is determined mostly by the type of activity you will be using them in.

Let’s take a look at the two basic types of binoculars and what are their pros and cons.

1.Porro Prism Binoculars

The easiest ways to identify Porro prism binoculars is by noting that the eyepieces are not in line with the objective lenses. This is because of the design of the Porro prism used for its elaboration.

  • Prisms in these binoculars are spaced pretty wide, producing a better stereoscopic image.
  • The design cannot be too compact.
  • They usually come with a low price tag because they are easier to elaborate.
  • More moving parts, which becomes an even worse problem if they get wet or dusty.

2.Roof Prism Binoculars

In this design, the prisms are aligned with each other in a straight line. Because of this, these binoculars can be more compact and easier to carry. The easiest way to identify these binoculars is by noticing that the eyepieces and the objective lenses line up with each other.

  • They are not as complex, so water and dust are not as much as a problem if they get in.
  • Back in the days, the image quality of roof prisms was compromised by the fact that they are aligned. However, the really good roof prism binoculars are in the same quality range of the Porro prism binoculars.
  • The design is simple and compact.
  • These type of roof prism binoculars are usually high in the price, so you do not want to go cheap if you want a good quality image


The one thing you must understand about magnifying power is that more power is not the same as better.

We believe we have made that point pretty clear by now. Many people make the mistake of thinking that in all cases and for all uses, more powerful and longer range binoculars will always be the best option. As you have read already throughout this article, there are other equally important factors that you must consider before making the right choice for binoculars.

Understanding HD binoculars

You have probably read on the description of binoculars as being HD or ED. These terms are used interchangeably in marketing and they virtually refer to the same thing. Truth be told, claiming binoculars to be HD is more like a marketing stunt since there is really no standard that helps determine what a “high definition” lens is.

In fact, not everything created “HD”, “ED”, “XD”, etc., is created equal. Some of the factors that make a piece be considered “HD” may have nothing to do with the glass they are made of.  Other elements such as coatings, baffling, or lens element geometry might factor in, actually. Our advice is not to get flashed by claims of that sort and make your decision based on what you have learned.

Understanding distance in binoculars

Quite simply put, if you want a pair of binoculars that allow you to see from a very long distance, there are a few things you need to remember.

One, in order for you to have binoculars that are good at long distance, you need a greater magnification. But we have already seen that magnification is not all about it. You also need clarity in order to get details. In order to achieve this, you need to capture more light, hence a bigger aperture, hence larger binoculars. Some of the best compact binoculars have that as well.

Second, larger binoculars mean more weight on you, which means you will get tired from carrying them more easily, however there is an exception we did a review on the best binoculars under 200 that will help you decide on a good pair of lightweight binos.

Third, when binoculars have a magnification of 12x, even with very distant objects, you will need to stabilize it since your pulse is not steady enough to not lose details in the object you are watching. Unless you are astronomy-watching, a “long-distance” pair of binoculars is not really necessary.

We have already gone through the aspects involved in good binoculars and not all of them are dependent on magnification.

 Understanding zoom in binoculars 

Maybe you are familiar with binoculars that can adjust their zoom, that is to bring the object closer or further away. This is usually used when you want to see objects with varying distances from you. However, we will not recommend these binoculars because adjusting magnification only sacrifices all the other aspects that we have mentioned above.

If you buy yourself a good pair of permanent magnification and aperture, you will have the guarantee of a great pair of binoculars that have adjusted to the conditions you will use them more. If these binoculars have a great depth of field, you will rarely need to zoom in or zoom out.


Understanding magnification numbers for monoculars and binoculars

Monoculars basically have the same mechanism of binoculars. The numbers indicating power and aperture in binoculars indicate the same for monoculars. To understand the numbers in a pair of binoculars is the same as understanding those of a monocular.

Using a monocular poses an advantage of lighter weight and a more compact design. They usually come in a nice bag and are, of course, relatively cheaper than binoculars.

If you are going to go for a monocular, apply the same principle for choosing we have given you for binoculars throughout this article.

Understanding digital binoculars

With digital cameras technology on the rise, it is not surprising that this technology is put into binoculars. Actually, digital binoculars have been on the rise during the past years.

Perhaps the first thing you need to know about digital binoculars is that they are more expensive.

This is because they offer features that the regular binoculars cannot, such as more stability, video and photo taking, electronic distortion control, night vision, and image sharpness among others.

Digital binoculars do not use prisms since the light entering them is converted into an image electronically. This poses an advantage in not having to include the weight of those plus the instability contributed by prisms and other movable parts in regular binoculars.

Some other interesting features that you will find in your digital binocular are:

  • autofocus
  • digital zoom
  • no issue with eye relief
  • digital zoom
  • electronic distortion control

Despite these gadgets being state of the art, they still rely on the amount of natural light they receive.  This means that even digital binoculars will need to pass on the light through them and will deliver power and clarity depending on the amount they get. If you are interested in buying the best binoculars under $100, click here.

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