Binoculars
by Chris Woodford. Last updated: October 10, 2021.
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.
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.
How to choose a pair of binoculars: a quick buying guide
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:
- My number one tip is that binoculars you use constantly are
better than ones that sit in the car or the closet because they're
too heavy or cumbersome to carry or because they're so expensive you're
afraid to take them anywhere.
- It's vitally important to decide why you want the binoculars and
how you will use them before you start.
- If you want something for carrying in your pocket on walks, on
the off chance you might see an elephant walking down Fifth Avenue,
I'd go for a small and inexpensive pair of field glasses
that magnify perhaps eight times.
- If you want to do some serious birdwatching or astronomy, with
your binoculars used in a hide or a static location, invest in
something heavier, better quality, and more expensive.
- If you plan to look through your binoculars for any length of
time (such as when you're birdwatching or stargazing), good quality
lenses are worth the investment. Don't wreck your eyes. If you're just
going to be looking at the odd bird here and there, cheap field glasses
are fine.
- Don't assume that "more magnification" means "better binoculars".
The more the lenses magnify, the more the effect of your hand movements
will be magnified too. In other words, buy something that magnifies
more than about 8 or 10 times and you may find it impossible to keep
the lenses steady enough to see anything at all. Higher magnification
binoculars also show you less of the scene at a time (because it's
bigger). In other words, they have what's called a smaller field of
view. Steer clear of buying binoculars in newspapers with amazing
sounding magnifications (20 times, 30 times, or whatever it might be);
they're probably impossible to keep steady and focused.
- Binoculars are described with two numbers separated by an x. For
example, the field glasses in the photo above are 8 x 21. The first
number is the magnification. So they magnify up to eight times. The
second number is the size of the objective lens in millimeters. So my
field glasses have an objective lens 21mm across. The bigger the objective
lens, the more light enters, so the brighter the object will appear.
Whether you want objects to look bright depends on whether you want to
study the full moon (which can look almost blindingly bright through
binoculars) or birds at dusk. Again, considering what you will use your
binoculars for is all important. Binoculars marked 8 x 42, 7 x 42,
or 7 x 50 are great for all-round general-purpose use.
However, the bigger the second number, the bigger the lenses—and
the heavier (and harder to carry) your binoculars will be.
- Will you always use your binoculars on dry days... or sometimes in the rain? If you're
a hardcore birder, you'll need weather-proof and fogproof binoculars, filled with anti-fogging argon or
nitrogen gas (to stop them steaming up), and fitted with rubber grips and good rubber eyecups to
make a tight seal around your eyes. If you're out and about in very wet environments, you
might want to make sure your binoculars are fully waterproof when fully immersed for
short or long periods.
- Don't buy online until you're tried them in a shop. Go into a
shop and try a few different models. See how you like them (especially
their weight and general feel). Make a note of ones you like and then
buy online if you want to.
- Good binoculars are great, but small, light field glasses are—in
my view—unbeatable. They're small enough to put in your pocket and
light enough to carry. There's no risk of missing that golden eagle,
red kite, or baby dolphin!
- The best solution should be obvious to you now: if you can afford
it, buy a pair of fantastic binoculars for "best" and a pair of cheap
field glasses to carry in your pocket.
Photo: Try binoculars before you buy to make sure you like the feel of them.
Find out more
On this website
Books
For older readers
For younger readers
Articles
- Binoculars That Use Digital Trickery to Give You Super-Sight by Christina Bonnington. Wired. January 21, 2015. New binoculars use digital image processing to reduce fuzziness caused by rain, wind, and other atmospheric distractions.
- Pentagon to Merge Next-Gen Binoculars With Soldiers' Brains by Sharon Weinberger. Wired. May 1, 2007. The latest military binoculars pick up signals from the wearer's brain for hugely improved image detection.
- David Bushnell, 92, Importer of Affordable Binoculars, Dies by Jennifer Bayot. The New York Times. March 31, 2005. The story of how David Bushnell put binoculars in the hands of the masses.
- The Neurophysiology of Binocular Vision by
John D. Pettigrew, Scientific American, Vol. 227, No. 2, August 1972, pp. 84–96. A great, very clear introduction to how our eyes and brains enable our 3D visual perception.
Patents
There's much more technical detail here:
- US Patent 395,872: Binocular glass by James Briggs, January 8, 1889. An early design for field glasses that sit on your nose like a pair of spectacles.
- US Patent 3,531,177: A binocular construction using plastic foam and magnets by Alfred Akin (Bushnell), September 29, 1970. A design for lightweight, inexpensive, shock-absorbing, floating binoculars based on using plastic foam as the body material.
- US Patent 3,744,872: Binocular with improved prism mount
by Alfred Akin and David Bushnell, July 10, 1973. A typical Porro prism design from one of the pioneers of affordable binoculars, David Bushnell. This gives a good description of all the bits you'll find in modern binoculars and what they all do.
- US 20020109785: Digital record and replay binoculars
by Jack and Steven Hammack, August 15, 2002. Binoculars with built-in digital recording and a viewing screen.
