Last Updated On January 30th, 2019

Binoculars are something that most people would have heard of at some point in their lives. If you’ve ever been to the theatre, explored hobbies like birdwatching, or went on a sightseeing vacation, you might have owned a pair of binoculars. Yet, despite their popularity, many people may find themselves wondering, “How do binoculars work?”

Most have no idea how it works, how to measure power and strength, or how to separate one option from another. So, what’s the answer? If you are like me and you’ve been thinking about it, then you’ve come to the right place. By the time you finish reading, you’ll know all of the science behind the mystery of binoculars.

Here’s the Answer…

man thinking deeply while holding a pair of binoculars

The Basics

Let’s start simple, they are like two telescopes joined together. Binoculars use a series of prisms (to direct light), mechanical elements, and lenses to produce an improved or magnified image of distant things. They are like two telescopes that serve as peepholes so you can use both eyes to look through. These two parallel optic tubes along with prisms and lenses help to deliver a greater field of depth. Most believe that binoculars offer a far more immersive experience than spotting scopes because they use both eyes to see.

If you’ve been shopping for binoculars for a while now, you might have noticed that some of them seem to have a more streamlined design while others are bulkier. This is because the physical size and appearance of binoculars depend on the kind of prisms used within the construction.

The prisms in your binoculars are designed to correct the orientation of what you see both vertically and horizontally so that the image appears natural. Without the prism, your binoculars would still magnify what you see, but the image would be flipped and upside down.

Types of Prisms

There are currently two types of prisms available: porro prism and roof prism.

1. Roof Prism

In a roof prism, the glass elements have a more fixed alignment with each other, making the binoculars look streamlined. This narrower and more compact design means the binoculars do not normally need to be re-aligned.

2. Porro Prism

In a Porro prism, the glass elements are offset and often has a Z-shaped configuration. This design offers greater scope and depth, but it also makes it chunkier overall. The design also means there is an occasional need for re-alignment.

Binocular Magnification Explained

If you’ve never had this explained to you before, then how binoculars make images appear bigger is bound to be something of a mystery to you. The first thing you need to know is that most binoculars come with two numbers written on their casing, such as (8×42).

The first number (8) refers to something called magnification. When it comes to binoculars, magnification is the number of times larger an image will appear compared to if you were looking at it with your naked eye. This means that a magnification of (8x) would make an image eight times bigger. The level of magnification you need will depend on what your activity is. For instance, if you’re watching a horse racing event, then your viewing requirements may be different from if you’re bird spotting.

The second number printed on the casing (42) refers to something called the “objective lens diameter” of the image. This is the lens that is used to direct light through the binoculars. The larger the number is, the clearer and brighter the picture in your binoculars will appear. If your eyes are exposed to a greater amount of light, then they can capture and focus on the image more clearly. In most cases, binoculars that have an objective lens diameter of over 30mm will be classed as binoculars of a “standard” size. On the other hand, those with a lens diameter of less than 30mm are seen as compact accessories.

Binocular Zoom Explained

a person looking through a pair of binocularsThough technically, all binoculars allow you to “zoom in” on the image that you want to view, you’ll need to have this explained to you if you want to understand why some models are classed as “zoom binoculars.”

When there’s an additional number printed before the “x” on your casing, this indicates that your binoculars have multiple levels of magnification to choose from. If the numbers on your casing are separated by a dash or a dot say for example (10-30×60), then it could mean that those binoculars are “zoom” models. This entails that you can adjust your view according to the level of magnification that you need. In our example (10-30), means that you can make the image ten times, up to thirty times bigger.

Although zoom binoculars often seem like a good idea, they’re not always the best option for serious hobbyists. This is because zoom models can harm the quality of the image that you view. When you zoom in on an image with these binoculars, you may be able to magnify the picture, but it’s unlikely that you’ll focus on a great deal of additional detail, as the amount of light available to you will stay the same.

Binocular Strength and Binocular Power Explained

The concept of strength is complicated because there are many different things that can contribute to the power of your binoculars. For instance, if you’re simply looking for the one with the greatest degree of power, then you may search for a pair of binoculars that allow you to magnify an image by 11 times or more. However, you’ll also need to think about how you can get as much focus and clarity as possible in your picture if you want to get the best experience.

Exit Pupil

woman looking through her binocularsWhen it comes to answering the question: “How do binocular’s work?” you may find yourself learning about something called the “exit pupil.” It is the number that you get when you divide the objective lens diameter on your binoculars by the magnification. Let’s use (8×42) again as an example. From this, we are able to calculate the exit pupil and derive at 5.25 as the answer. This number represents that diameter of the beam of light leaving the lens of the binoculars when you hold your device towards a specific light source. The exit pupil of your binoculars should be slightly smaller than the pupils of your eyes when it adapts to darkness. This way, you can direct the best possible amount of light to clearly see what you are looking at. Most average adults have pupils that open to 7mm in the dark. This means that any instrument with an exit pupil greater than 7mm will be just wasting light.

Field of View

Another thing you may need to consider when looking for the right binoculars is the field of view or more usually called FOV. This refers to the area of space that you can see through your binoculars. Usually, this number is determined by the design of your binoculars’ optics. (You’ll read about optics in a moment).

FOV is often expressed in one of two ways, either as the degrees of field, or the width in feet. When expressed in feet, we call it “linear FOV.” When it’s expressed in degrees, we call it “angular FOV.” Again, the field of view that you need will depend on how you use your binoculars. For activities such as astronomy, a wider FOV will be necessary because it allows you to see a larger portion of the sky.

Binocular Optics Explained

Finally, when you’re searching for the ideal binoculars, it’s also worthy to talk about binocular optics. Binocular optics refers to a number of things. Here, we will focus on eye relief.

Eye Relief

an old captain looking through a pair of binocularsThis term describes the optimal distance between the eyepiece lens of the binoculars and your eye. Manufacturers typically add eyecups to make sure that your eyes remain at the right distance from the eyepiece lenses. However, if you wear glasses, then you might need to adjust the distance to get better eye relief.

High-level binocular manufacturers may be able to offer dioptric adjustments to one of the eyepiece lenses which means that they can adjust the eyepiece lens according to your eye prescription requirements. This way, you’ll be able to use your binoculars freely without relying on your glasses at the same time. If you’re sharing your binoculars with other people, then you may want to find one with eyecups that can be adjusted. For instance, if the cups are made from rubber or latex, you can just fold them back to place your eyes closer to the lenses. Alternatively, you may be able to purchase binoculars with eyecups that twist in and outwards to ensure the proper view.

Usually, binoculars with larger degrees of eye relief will come with a smaller FOV as standard. However, if you have issues with your sight, you might be able to reduce eye strain when you use these models. Consider how much relief is necessary for you, then buy the binoculars that give you the greatest control over your view.

What Kind of Binoculars Are Right for You?

binoculars on a wooden surfaceNow that you can answer the question “How do binoculars work?” you should have an easier time choosing the make and model of the best binoculars that are suited for you. Just remember that the higher the magnification level you select, the steadier your arms should be. If you want to work with anything over 12 times magnification, then you may need to invest in an additional tripod or stand to hold your binoculars during long periods of bird or train watching.

Additionally, you’ll also need to think carefully about the primary purpose for using your binoculars, and whether you have any special requirements that need to be considered (such as eyeglasses prescriptions) before making your purchase.

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