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Large US navy binoculars looking out to sea

Binoculars

by Chris Woodford. Last updated: June 13, 2013.

What if you could walk on the Moon or stare an elephant right in the eye? Binoculars and telescopes are the next best thing. They take you up to the action without having to move a muscle. Binoculars are based on the science of optics and some pretty clever tricks that lenses pull on light. But how exactly do binoculars zoom you from your armchair to the middle of the solar system? Let's find out...

Photo: A sailor in the US Navy keeps a lookout using binoculars from the bridge of an aircraft carrier. Photo by Lilliana Lavende courtesy of US Navy.

How lenses work

Lighthouse Fresnel lens

The way light bends when it goes from air to a different material (such as water or glass) is called refraction. (For a full explanation of how it works, please see our detailed article on light.) Refraction is the key to how lenses work—and lenses are the key to binoculars, telescopes, and glasses. But how do we get from light bending in water to a cool pair of binoculars that let us study the moon?

Water sitting in a glass appears to have a straight upper edge, even though it is very slightly curved (the curved edge has a special name: it's called a meniscus). If you place a glass on top of a newspaper and look straight down, the news print looks just the same as normal. That's because the top of the water is effectively straight. But if the water had a curved upper surface, the news print would look magnified. You can see this for yourself by trying the simple activity "Make a water lens" in our main article on lenses.

Photo: Lenses come in all shapes and sizes. The giant Fresnel lens surrounding a lighthouse lamp are designed to concentrate the light into a parallel beam so you can see it at a great distance. The lenses in binoculars do the opposite job, focusing light rays from far off so you can see distant things more clearly. Read more about how Fresnel lenses work.

Types of lens

A lens is a curved piece of glass shaped a bit like a lentil. (If you ever wondered where a lens gets it name from, that's where: lens comes from the Latin word for lentil.) When light rays hit a glass lens, they slow down and bend. If the lens curves like a lentil (like a dome), so its outside is thinner than its middle, it's called a convex lens. As light rays enter a convex lens, they bend 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.

Another kind of lens curves the opposite way, with the middle thinner than the outside. 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.

How binoculars work

A pair of field glasses with key features labelled

You can probably 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. (There's quite a good demonstration on this page at Birdwatching.com.) You can make your own telescope easily enough with a couple of magnifying glasses and a cardboard tube wrapped around them.

Simple artwork showing the key parts of binoculars and the path that light rays follow through them.

Binoculars are simply two telescopes side by side, one for each eye. But there's a catch. When light rays from a distant object pass through a convex lens, they cross over. That's why distant things sometimes look upside down if you look at them through a magnifying glass. The second lens doesn't sort out that problem. So binoculars have a pair of prisms (large 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.

Photo: Left: Key features of field glasses. You focus by turning the focusing screw in the middle. This pushes the focusing mechanism back and forward, increasing the distance between the objective lens and the eyepiece lens. Right: The path that light rays take through the lenses and prisms in a typical pair of binoculars.

How to choose a pair of binoculars: a quick buying guide for beginners

An inexpensive pair of field glasses

Photo: An inexpensive pair of field glasses.

The choice is big and baffling! So which binoculars do you buy?

I've bought two pairs of binoculars in the last few years—a really good compact, waterproof pair with roof prisms and a small, cheap pair of field glasses. I spent a long time researching the subject and getting confused by all kinds of unhelpful sales literature. I'm still no expert, but these are the handy tips I managed to figure out for myself:

Photo: Try binoculars before you buy to make sure you like the feel of them. Photo by Juan Antoine King courtesy of US Navy.

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Text copyright © Chris Woodford 2007, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007) Binoculars. Retrieved from http://www.explainthatstuff.com/binoculars.html. [Accessed (Insert date here)]

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