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How Do Binoculars Work?

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Updated on: 

October 6, 2021

How Do Binoculars Work?

The Science Behind Binoculars

The Science

The refraction of light is key to how binoculars work because it explains how lenses work. Refraction is The way light bends when it goes from the air to a different material. Placing a stick in water is a refraction of light because the stick appears bent, however it is not. A lens is a curved piece of glass and when light rays hit a piece of glass such as a lens, they slow down and bend. If the lens curves, so its outside is thinner than its middle, it's called a convex lens. As light rays enter a convex lens, they Sunagor BCF Mega Zoom Reviewbend in toward the middle—as though the lens is sucking them in. That means a convex lens brings distant light rays into a focus. It's also called a converging lens because it makes light rays come together (converge). Looking at things through a convex lenses makes them appear bigger—so convex lenses are used in things like magnifying glasses.

A different kind of lens curves the opposite way, with the middle being thinner than the outside is. This is called a concave lens. (You can remember this easily if you think that a concave lens caves in in the middle.) A concave lens makes light rays spread out like the lines of a firework. Imagine light rays coming into a concave lens and then shooting out in all directions. That's why a concave lens is sometimes called a diverging lens. It makes light rays shoot out (diverge). Concave lenses are used in movie projectors to make light from the film spread out and cover a bigger area when it hits the wall.


Binoculars are basically just two small telescopes side by side on the same device, but the difference is that binoculars show you three-dimensional images which you can see from the same perceived viewpoint. As there are two eyepieces, you are able to look at the magnified object with both eyes at once and you won’t need to close eye to avoid ruining the viewing experience.

Binoculars use lenses to bend light and magnify far away objects. The word lens actually comes from the Latin word for lentil, and this is because the shape of a glass lens looks a little bit like a lentil.

Binoculars use two lenses. The first lens is the objective lens and it basically catches light rays from far away objects and created a focused image. The second lens picks up the image from the first lens and magnified it so you can see it.

You can use this information to make your very own basic telescope. If you take two magnifying glasses and use the one closest to your eye as the eyepiece and the one further away from your objective lens you should be able to magnify objects which are difficult to see with the naked eye.

The problem with this basic concept is that the light rays cross over and it results in an upside down image.  This isn’t a problem if you’re looking at celestial objects (in fact most telescopes provide you with upside down images), but it is a problem if you want to look at things a little closer to home.


Prisms are pieces of glass which basically act like mirrors. Prisms are very important because without them your objects would not only be upside down but they would also be backward! That’s not really what you want when you’re watching sports or going bird watching, but luckily prisms are there to ensure you see the image the right way up.  They reflect the light to correct the orientation of the image so you see the image as it is, it’ll just closer!

The two most popular types of prisms that you are likely to come across are Porro prisms and roof prisms.

Porro Prism

Porro prisms are wide, their prism is shaped like a Z, and they are able to give you good depth perception. These prisms give you a wide view so you can see more of you surroundings and so they are a good choice for spectator sports or watching animals. They also tend to give you a brighter image than a comparable roof prism because the way roof prisms reflect light reduces light transmission by up to 15%. Porro prisms tend to be cheaper than roof prisms as they are easier to make, but this has been changing in recent years.

Roof Prism Binoculars

Roof prisms have their eyepieces roughly in line with their objective lens and they tend to be smaller than Porro Prisms. Roof Prisms are also lighter, more compact, they are easier to waterproof and they are a little more durable. Roof prisms reflect light six times whilst Porro prisms only reflect their light four times, and this is why roof prisms tend to give you a darker image than a Porro prism of the same calibre.


A binoculars magnification is shown by a pair of numbers, say 10 x 50. The first number tells you how powerful the magnification is, i.e. how many times the closer the image in question will appear whilst the second number shows you have large the diameter of the objective lens is. The objective lens is the light gathering lens, so together these numbers show you how close the image will appear and how much light the binoculars can utilize.


You will have to focus your binoculars in order to get a clear view of objects which are not at a fixed distance. Some binoculars have independent focus which requires you to focus each eyepiece separately, whilst central focused binoculars normally have a central focusing wheel.

Field of View

The field of view refers to how much you can see through the binoculars, i.e. how much of the surrounding area can you see depending on how far away you are? A larger field of view is important if you need to see the whole landscape i.e. for sports or watching groups of animals, but more magnification and less field of view may be important if you need to hone in one one specific object.

Eye Relief

Eye relief refers to how far away your eye can be in order for you to properly see the magnified image through your binoculars. The larger the binoculars focal point is the greater the eye relief will be. Eye relief can go beyond 2.5 centimeters, but others are as small as a few millimeters. Eye relief is an important considering for people who wear glasses because they will typically need to position their eye further away from the binocular.


Binoculars have multiple air-to-glass surfaces and all these surfaces can lose light. Binoculars lose light through reflection because, instead of contributing to the image, reflected light reduces the contrast between the image you want to look at and its background. You can reduce this and improve your image quality through the use of optical coatings.

These coatings are designed to give you a sharper, brighter image. Your coating options range from coated optics, fully coated, multi-coated and fully multi-coated. Coated optics will have just a few of the surfaces coated, whereas fully multi-coated will have all their air-to-glass surfaces coated.


We hope you have found this article on how binoculars work useful. Binoculars have many important applications for work as well as leisure and it’s important that you understand how they work so you know what to look for if you decide to buy one. As you can see binoculars are not the most complicated instruments in the world, but they are made up of a few essential components which enable them to work efficently.

But, Binoculars?

Okay, so hopefully you can see where we're heading. If you want to see something in the distance, you can use two convex lenses, placed one in front of the other. The first lens catches light rays from the distant object and makes a focused image a short distance behind the lens. This lens is called the objective, because it's nearest to the object you're looking at. The second lens picks up that image and magnifies it, just like a magnifying glass magnifies an image on paper. If you put the two lenses in a closed tube, hey presto, you have a telescope. You can make your own telescope easily enough with a couple of magnifying glasses and a cardboard tube wrapped around them.Nikon Aculon A211

Binoculars are basically just two telescopes in one device, but when light rays pass through a convex lens from a distance they cross over. Hence why far away things often look upside down when looking at them through a magnifying glass.

So binoculars have a pair of prisms (big wedges of glass) inside them to rotate the image through 180 degrees. One prism rotates the image through 90 degrees (flips it onto its side), then the next prism rotates it through another 90 degrees (flips it onto its side again), so the two prisms effectively turn it upside down. The prisms can either be arranged in a back-to-back arrangement (known as roof prisms) or at 90 degrees (known as Porro prisms)

The prisms explain why binoculars are heavy and why they are sometimes quite chunky in the middle. Field glasses, which are compact binoculars like the ones shown in the photo here, flip the incoming images using only lenses. There are no prisms, so field glasses are smaller, lighter and more compact—but the image quality is poorer.

Is this becoming any clearer?

Less Complicated Break Down

No? Okay so basically the 2 lenses you look through in a pair of Binoculars refract the light in certain ways to capture images from far away to be clearer than they really are. Like glasses lenses help you see things differently, so do Binoculars.


Born and raised in the south coast of UK, a small town close to Cornwall, I have adventure in my blood. Ever since a young age, I have always been into the great outdoors and particularly love watching the natural wildlife. Birdwatching is what I enjoy the most, and thus my natural interest in binoculars began. I founded and run Best-Binoculars.UK a site where I talk about various aspects of and review binoculars. Hope you enjoy my ramblings :)

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Born and raised in the south coast of UK, a small town close to Cornwall, I have adventure in my blood. Ever since a young age, I have always been into the great outdoors and particularly love watching the natural wildlife. Birdwatching is what I enjoy the most, and thus my natural interest in binoculars began. I founded and run Best-Binoculars.UK a site where I talk about various aspects of and review binoculars. Hope you enjoy my ramblings :)

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