This website is reader-supported. When you buy through links on our site, we may earn a small affiliate commission.

What Lens Do Binoculars Use?

Last Updated: 29.11.21


Whether you’re looking at reviews of binoculars for children or simply want to know more information, it’s quite important to know that binoculars work through using two convex lenses to bring the image closer and turn it upside down, allowing us to see a clear, focused picture.



Understanding binoculars

Throughout the course of history, humankind has always been fascinated with things that are many miles away and are hard to reach. Questions have been asked, research has been done and, eventually, everybody came up with a simple answer to be able to watch something that is happening far away: binoculars.

Together with telescopes, binoculars have become the wonder-kid of the science of optics, using lenses which are able to engage in some pretty clever tricks together with the light and get us a much-zoomed-in image of whatever we want to be able to observe.

When Hans Lippershey managed to invent the first telescope in 1609, it was clear that it was only a matter of time before his technology would be perfected and shrunk down enough to be able to fit into a device for both eyes.

While the first application for a binoculars patent happened in 1825, it wasn’t until 1854 that a scientist called Ignacio Porro patented the Porro prism system we use today. Furthermore, there’s been a slew of other technological advancements since then, including roof prism technology, digital capacity, and even night vision. 

However, the truth is that most people already know the basic stuff about binoculars so, to make things better, let’s tackle on some info about the actual lenses used.


What are lenses?

To cut the physics lesson as short as possible, we’ll start like this: the process of magnification needs light to happen, so scientists have worked for centuries looking for perfect ways to capture it. One of the most important fruits of their prolonged effort is the lens, which allows us to control the direction and the quality of the light, depending on its shape.

As most people know, lenses come in all shapes and sizes. For example, the giant Fresnel lens used to surround a lighthouse lamp is designed to concentrate the light into a parallel beam, allowing those who watch it see it from great distances.

Binoculars also use lenses but, instead, theirs focus on doing the exact opposite job, gathering light rays which are far off so whoever is watching can clearly see things located at a great distance. In scientific terms, the process of light bending when going from air to a different environment such as glass or water is called ‘refraction’. 

After the light is first gathered in the lens, it then makes an instantaneous trip to the point of your focus, where an eyepiece lens captures it and gives you a magnified image. This trip is scientifically known as ‘the focal path’ and it can be longer or shorter depending on the size of the device, therefore explaining the size of some telescopes.

This is what gets the job done when it comes to binoculars, telescopes, and even glasses. Since we pointed out glass or water as alternate environments, one might ask how exactly can we get from light bending in the water to a pair of cool binoculars allowing us to study birds or do some whale-watching?

The answer, as one great mind said, is elementary. Since water sitting in a glass has a straight upper edge, though slightly curved, placing a glass on top of a newspaper and looking straight down will make the paper look completely normal, without anything out of the ordinary.

However, if the water suddenly gets a curved upper surface, a very simple experiment will show you that the print would now look magnified, basically demonstrating why binoculars and other such tools work the way they do.



Types of lenses

To put it mildly, a lens is a curved piece of glass shaped roughly in the form of a lentil. In fact, the word ‘lens’ comes from the Latin word for ‘lentil’, so the resemblance is easy to see.


When light rays go through a glass lens they will slow down and immediately bend with two types of possible outcomes: the first one happens when the lens curves in a dome-like trajectory, in such a way that its outside is thinner than its middle. When this happens, you will know you are dealing with a convex lens. 

As the light enters this particular type of lens, it will always bend toward the middle so that obviously means the convex lens is bringing distant rays into close focus. As such, another name for this is a ‘converging’ lens, quite aptly named because of its ability to make the light rays come together or, in other words, converge.

Simply put, looking at things through a convex lens makes them appear bigger and allows the observer to have a closer look so this is why they are used in things like magnifying glasses.


However, the second outcome is also quite interesting for our debate. There is another kind of lens which will curve the opposite way, with its middle being thinner than the outside, and is therefore aptly named a concave lens. What this does is make the light rays spread out, just like a series of big fireworks.

If this is hard for you to imagine, picture a lot of light rays coming upon the small shape of a concave lens, entering, and then shooting out in all directions on the other side. Therefore, these are sometimes called diverging lenses, because they make the light rays go crazy or, in settled-down English, diverge.


What about binoculars?

Since by now you have probably guessed it, we’re just gonna come out and say it: if you want to be able to look at something located far away or, in other words, if you want to build a pair of binoculars, you can use two convex lenses which you have to place one in front of the other.

This will create a spectacular process where the first lens will catch light rays from the far-away object and create a focused image a short distance behind the lens. Because this will be the nearest to the objective you want to look at, we will name it the ‘objective’.

The second lens will act just like any good apprentice and improve on the work of the first one, picking up the image and magnifying it, just like in the newspaper example above. Therefore, two such lenses in a closed tube will be the mathematical equivalent of your very own telescope. For the DIY people out there, a couple of magnifying glasses together with some cardboard tube should allow you to build one in no time.

Binoculars are, therefore, two telescopes put side by side, one for each eye. However, physicists had to work out one more kink here, because when light rays from a distant object pass through a convex lens they tend to cross over and show you the world upside down.

Therefore, binoculars usually come equipped with a pair of prisms, which are large wedges of glass located inside them and which manage to rotate the image through 180 degrees. These prisms are also the reason why quality binoculars are so heavy and something downright cumbersome. 

If you don’t like this, an alternative would be using field glasses which only flip the incoming images through lense use, being smaller and lighter. However, the downside is the image quality which is also poorer when compared to a pair of binoculars.



Some fun information

While Porro prisms are still the norm for today’s binoculars, the early 1900s saw some sturdy competition come along in the form of roof prisms. Named so due to their irregular roof-like structure, they managed to play a sort of ping-pong with the light inside them several more times than Porro prisms, therefore creating a longer focal path and greater magnification.

To this day, many people still prefer going with the good-old Porro due to their added depth and brightness while others prefer the higher magnification power of roof prisms. It’s really something which is up to every one of us.



Ioana Moldovan

Ioana’s professional experience in the optics field has helped her understand the value of passing her knowledge forward. Her curious personality helps her gather useful information for her readers and her goal is to make technical information fun and accessible to everyone.

Notify of
Inline Feedbacks
View all comments Protection Status