Binoculars
by Chris Woodford. Last updated: February 14, 2024.
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: Reflections of a passing ship in a large pair of binoculars onboard the USS Sioux. Photo by Juel Foster courtesy of US Navy and DVIDS.
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Contents
How binoculars use lenses
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: New technology is constantly making old inventions obsolete, but there's still no substitute for a really good pair of binoculars. Photo by Brooke Moeder courtesy of US Air Force.
Types of 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.
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.
The optics of binoculars
Photo: 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.
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. It's called the eyepiece. 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. Binoculars are simply two telescopes side by side, one for each eye.
Artwork: How to make a telescope from two lenses. The objective lens makes a focused image of the object. The eyepiece lens makes the image bigger.
But there's a catch. When light rays from a distant object pass through a convex lens, they can 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).
Artwork: How prisms correct the inverted image and turn it the right way up. The eyepiece lens takes the corrected image from the prisms and magnifies it, as before.
In practice, in a pair of binoculars, there are four prisms (two for each "tube"), and they're tightly packed inside the two "tubes" you look down. If you've wondered why those tubes are the shape they are, the reason is simply because each one has to house two prisms inside it.
Artwork: The path that light rays take through the lenses and Porro prisms in a typical pair of binoculars. It's not that clear from our artwork, but one of the prisms is arranged at 90 degrees to the other (in other words, one is mounted horizontally and the other vertically).
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.
Artwork: Key features of a typical pair of Bushnell Porro prism binoculars. Right: You can see the objective lens (blue), the two prisms (orange), and the central focusing screw (red). Left: The eyepiece focusing mechanism (yellow) is pulled out and shown in more detail. Now you can see the two eyepiece lenses, the compound ocular lens (top) and the field lens (bottom), separated by an air gap that increases or decreases as you turn the focus ring (mounted on the outside of the eyepiece). Artwork from US Patent 3,744,872: Binocular with improved prism mount by Alfred Akin and David Bushnell, July 10, 1973, courtesy of US Patent and Trademark Office.
