How do Binoculars and Telescopes Work?

It took hundreds more years before we worked out the mechanics and physics of how light bends.

We figured out that when the medium changed from say, air, into another, such as water or glass, it would change direction in a very specific way.

This refractive index (how much something bends light) of disparate substances creates the illusion seen at the right. This illusion is given by comedian Steven Wright as the reason he refuses to take a bath!

It was the invention of prisms that finally gave us insight into how light worked, and that made everything that followed possible.

Gradually we worked out that smooth curves and flat surfaces created in high quality glass could alter light paths, produce spectrums, provide magnification, and even correct vision defects, but that took centuries and an understanding of how the lens inside our eyes worked, too.

Modern Times

Most people have used binoculars at some point in their lives, so they understand the principle of making something appear to be closer and allowing them to see more detail. When it comes right down to it, binoculars seem to be little more than paired, low-powered, telescopes…except they’re not…as we’ll see below.

The most obvious difference between telescopes and binoculars, aside from two barrels vs. one, is that many modern binoculars are often “bent” (left example in picture below), whereas telescopes are straight.

The left one has a magnification of ten powers (10x) and an objective lens (the one furthest from the eyes) of 50 millimeters (mm), meaning it can gather a lot of light. The eyepieces are not aligned with their objective lenses allowing the objective lenses to be even further apart, so it is also designated WA, or Wide Angle, meaning its field of view is larger than typical. This one can see a width of 122 meters at a distance of 1,000 meters (367 ft. @ 1,000 yards).

Straight binoculars (middle example) called Galilean are very old, uncommon, and far less efficient. They employ what is called a negative lens to produce a usable upright image; however, this results in a darker, smaller image, with fuzzy outer edges.

You’ll also find “straight” in the compact and more expensive DCF models (right example) that are sold today. What makes them different from each other?

DCF can stand for two things. Lately people have begun to use it to indicate that one eyepiece lens of a pair of binoculars can be adjusted independently in case your eyes have a significantly different focus. In that case it is taken to mean “Diopter/Center Focusing” and suits ANY adjustable eyepiece binocular.

More traditionally, and with modern “straight” binoculars, the original meaning of “Dach Center Focus” refers to the fact that the prisms inside are shaped like the roof of a house, where “dach” is the German word for “roof”. This allows them to be straight and smaller.

However, when light is reflected inside a prism, if it bounces off a surface at less than 90º, light escapes the prism. In addition, the light must take a more complicated path in a roof prism, so the resulting image is often less bright and has a lower contrast than “bent” binoculars.

The leftmost bent model contains a porro prism. There are two main reasons for its existence:


  1. They are much less expensive to produce, and they act to increase the focal length (and thus magnification) without making the binoculars physically longer; and
  2. When you look through a conventional biconvex lens the image is inverted, and it is flipped from left to right as well.

Each section of the porro prism rotates the image 90º, or 180º in total. This reorients the image into not only being right-side-up, but being correct in the left-to-right direction as well. The total effect of this is to make it very natural to use binoculars, where if you track left or right, the images moves left or right appropriately. The same is true for up and down.

A single biconvex or converging lens will invert the image once it is past the focal point. It is therefore quite natural to ask why ordinary vision glasses or contact lenses work without inverting the image. They are, after all, just a single lens.

The function of eyeglasses is to move the focal point slightly forward or backward until it settles directly on the retina so we can see a clear image. There is very little magnification involved.

The difference is that when two lenses are closer together than the focal length, they act as a single lens, and the image remains in the same orientation. This is what happens with the lens of your eye and a vision correction lens slightly in front of it, acting as one lens since it is within the focal length of the lens. Once the light passes beyond the focal point the image inverts, as in the diagram above.

Binoculars can be used to observe the Moon, track satellites, and watch the International Space Station as it passes by, but they are less useful for observing planets. They are ideally suited to birdwatching, sporting events, concerts, and other activities where you are not very “close to the action”. There is a reason that we don’t use telescopes for such activities. See this site for more info


Astronomical telescopes are generally uncorrected for orientation. Telescope users don’t care if the image of Mars, Jupiter, or Saturn is inverted. What does it matter if you see Jupiter “right-side up” or “upside down”?

Once you take a photo, you just turn it around and it’s just fine, right? If astronomy magazines didn’t turn the image around, how would you even know if you hadn’t observed the object for yourself?

If you were to use a telescope for bird watching, however, you would soon see the problem. The image would be swapped in the up/down direction, and the left/right direction.

To make matters worse, each movement you make with a normal telescope is the opposite of what you would expect; and, due to considerably higher magnification, the movements are quite exaggerated, too. As long as the telescope is not a Newtonian Reflector (explained below) we can correct the image to be upright, if desired, though depending on the type of telescope, not necessarily correct from left to right.

For example, if you were standing on the Moon, looking back at the Earth with your eyes, your view would be like the first image. A refracting telescope, which is entirely lens-based, would see something more like the middle image. If we were to employ a type of erecting prism (called a star diagonal) with that telescope, the image would be correct in the up/down direction, but reversed in the left right direction, as shown in the third image.

Types of Telescopes


These are the simplest telescopes. Galileo Galilei was among the first to use one to look at the stars. He’d heard of the concept, and built his own telescope. It was a magnificent 7-power (7x), which was two or three times more powerful than a mariner’s navigation telescope. Nothing compared to what we can achieve nowadays, but with that, he changed our view of the Universe forever.

The problem is that glass lenses can introduce chromatic aberrations (rainbow halos) around each light source. This make viewing details difficult, so this must be corrected by using several different lenses that are made of different materials. For example, silica glass (like your everyday windows), bend light in one manner, whereas flint glass bends it in another. Combining different types makes the light converge on the same spot and the halos disappear, and allows viewing Andromeda and Orion.

A fully-corrected refractor telescope could cost $5,000 but even modest hobbyist telescopes have some correction for chromatic aberration. A decent one can be had for $200.

Newtonian Reflector

A typical Newtonian Reflector is larger and thus gathers much more light than a Refractor telescope, because instead of lenses it uses mirrors to collect light. Large high quality mirrors are much easier to make than a lens of the same size. This means bigger light openings so brighter, more detailed images.

Consider: One 8 centimeter (cm) lens on a refractor telescope, vs. one 16 cm mirror in a reflector telescope. It is twice as big, so twice the light perhaps? No, this is where the square law comes into play. An 8 cm circle has an area of ~50 cm2, but a 16 cm has an area of ~200 cm2, or four times the light gathering power. Area increases as the square of the radius, quadrupling when you double the radius.

How does it work?


In this image, light enters from the right, hits a curved mirror to be sent to a flat mirror, where it is redirected into the eyepiece. Mirrors don’t have the same chromatic aberration problem, which is why many observers prefer them. There is little between your eye and the light, so the image is sharp, clear, and bright.

Newtonian Reflectors gather so much light that they are affectionately called “light buckets”. In fact, to observe the Moon, my own telescope has an aperture restrictor that is just 8% of the size of the whole scope. By removing the cap and placing it on the holder, the Moon is still very (very) bright, but doesn’t hurt your eyes. For stars and planets, of course, the dust cover is removed completely. More info here

Catadioptric Telescope

This combination style blends the best of both worlds by utilizing both lenses and mirrors. Catadioptric telescopes come in several varieties but they all possess several advantages.

First, they have a low-powered lens on the front called a corrector plate (unlike a Newtonian) that keeps dust out, which makes maintenance easier. This corrector plate reduces aberrations in the light and helps to create a sharp image all the way to the edge of the view—we call this a “flat image”. Second, they send the light back and forth down the tube two times, which means the tube can be much shorter. This makes it lighter, easier to manufacture, and much more portable.

Tech Makes it Easier

This is also the type of scope where you will most often see an Auto-finder. Other scopes on equatorial or altazimuth mounts can have motor-drives that can keep them centered on a particular object, but you have to locate it first yourself.

Sophisticated catadioptric scopes can have onboard computers that have a rudimentary memory of the sky and can locate guide stars on its own. This means you can point it at Polaris (the North Star) for instance, and then it can self-locate. Now all you do is type in 40 Eridani or Mars and it will automatically point at it and keep it in the view for you. This makes time exposures easy, and makes collecting 1,000 3-second images over the course of an hour relatively simple.

Stellar Photography

This image (#2) of Neptune is composed in that fashion. After many images are collected over an hour, they are dumped into software which sorts through them, keeping all of the consistent bits and throwing away the garbage.

The blue image is typical—barely discernible as anything at all—and the image on the right is post-processing. That picture of Neptune, the farthest planet, would be unobtainable from Earth any other way.

If you want a better picture of Neptune, it’ll cost you US$895 million in 1980 dollars and it’ll take years to get within 1/10th of an Astronomical Unit (AU), the average distance between the Sun and Earth. It was done in August of 1989 by the Voyager 2 probe, and looked like this. Personally, I’ll take the lower resolution and save my millions for a rainy day…

The Romans had Lenses

In ancient Rome, the very wealthy would use single lenses held between two fingers to get a sharper look at plays and events in the Coliseum. Lens-making was still a very primitive science; getting one that worked was a matter of luck for the jeweler when splitting a crystal. They used emerald because it was a fairly soft, workable crystal, and was naturally fairly clear with few inclusions (unwanted air bubbles, or opaque foreign minerals).

If it was made thin enough, the green colour was not a significant impediment to its use. They were certainly not even remotely as good as modern vision lenses, offering only small areas of focus, but preceded Salvino D’Armate’s invention of eyeglasses in 1285 C.E. by over 1,200 years, so make of that what you will.

The Takeaway

Binoculars are great for terrestrial observation and relatively close objects like the Moon. Opera Glasses are actually based on the Galilean design with no prisms because they require such low magnification to be useful. They also provide a wide field of view so you can see the whole stage at once without having to move your head around.

Telescopes can let you look billions of years back in time to see the light of ancient stars that is only just arriving, or light from Mars that is very fresh. Astronomy is one of the sciences where amateurs are responsible for more discoveries than professionals!

Hobbyists regularly report comets, and near-Earth Asteroids before a professional ever sees it. Best of all, if you discover something new, you get to name it after yourself.

Binoculars and telescopes both have their uses, and there is some crossover, but picking the right instrument will give you the best view for the purpose. Clear skies to you!