How Do Binoculars Work?

Understing the working of a binoculars

how binoculars work

How do binoculars work? This review will help you answer this question. visual acuity in birds is said to be the most outstanding. Especially for the birds of prey. The bald eagle has forward-facing eyes which allows the bird to have a wide binocular vision. One with the lens that can change and adapt to the environment.

Unlike birds, mammals have less ocular abilities. For that reason, gadgets like telescopes, spotting scopes, monoculars, and binoculars are constantly used. To bring images forward. But how exactly do they magnify an image? Let’s find out!

The Technicalities of Binoculars

In the 17th century, Hans Lippershey invented the first telescope. As the technology evolved over the years, an inventor named Lemiere created the patent for the 1st binoculars. This was in 1825.

30 decades later, another inventor by the name Ignacio Porro invented a binocular with a Porro prism system. Thanks to great minds and technology, the binocular has visual abilities like never before.

Below, you will learn about every detail of the binoculars. From its structural integrity to the technical bits of how it refracts light to give of its images.

The video below answers the question: how do binoculars work?


How do binoculars work?

Structure of a Binoculars

Unlike the telescope and the monocular, a binocular can be viewed with both eyes, both have circular glasses where you place your eyes to allow for light to get into the binocular. Due to the interpupillary distance, the focus system may produce different imaging for different eyes. But, the system tries to keep it focused on one area


That is the first set of lens on the binoculars. It is closest to the eye, where you will look through to view an image. Being the first lens, most binoculars will have caps for the lens.

This is to protect it from UV light that could damage the lens. According to the brand or the make, the design of the eyepiece can be manipulated. For some, it can twist or tilt to allow for a sharper focus on the image.

Objective lens

Following close to the eyepiece is the objective lens. This is where the light is trapped or captured so that it can be refracted. Light refraction refers to the bouncing of light from one surface to another to allow for image reflection.

The objective lens is measured in millimeters to get its diameter. It appears as the number after the magnification power of the binoculars. I.e, 12×40, forty is the objective lens diameter.

The type of lens used in the binoculars can be either of two:

  • Convex
  • Concave

Convex lens

Also known as converging lenses, they capture light and gathers it up in the middle to allow for a central focus system in a binocular. Once the light enters the binoculars through the lens, it is bent through its path of light.

The reason for the converging of light is due to the outer shape of the lens, slim outer ridges, and a fat midsection.

Concave lens

Unlike the convex, this lens diverges any light received through the lens. The shape is the complete opposite to the convex with wider edges and a thin midsection.

This is not very common with binoculars as it disperses light which could cause glares in the images.

Both lenses can be used to refract light in the binoculars but, over long distances, the image could get averted. Meaning the image mag looks upside down. This is where the prisms come in.


This is a mirror-like glass that regulates all the transmitted light to give off an even image at the other end of the binoculars. The interior structure of binoculars had been constructed in a way that it will allow the prism to rotate an image 180 degrees.

The image will no longer appear averted. The 180 degrees is not from one mirror. The light is bounced off from one 90 degrees angle to another 90-degree angle. How the mirrors are arranged brings about the difference between prisms.

Two types of prisms can be used to make binoculars.

  • Porro prism
  • Roof prism

Porro prism

This prism has a wide recollection area to allow for more and better trapping of light. With the wide space for correction, a better image is achieved. Most brands don’t use this type of prism due to the weight and also they don’t transmit light well over long distances.

Besides those features, the arrangement is steady and allows for better imaging.

Roof prism

Here, the mirrors are arranged in a back to back-formation. This is to allow for phase shifting. When the image is initially split into two as in enters the binoculars. This prism will collect the light back together and refract it evenly for a clearer image.

It is best used for compact binoculars as it is lighter in weight. It is also easy to work with and to adjust.

The prisms use a mirror-like glass with reflective capabilities, but they can be different types of glasses.

Types of prism glass

2 types of glasses can be used to make the prism.

  • BAK4
  • BK- 7


Superior in nature, this Barium Crown glass can transmit light better than any other glass.


Made from Borosilicate Glass, it offers standard light transmission and it’s less expensive compared to the Barium Crown glass.

Mechanical Design of Binoculars 

Focusing and adjustments are constant techniques used with binoculars. This is essential to modify and improve the clarity of images seen under the gadget. Under this section, there are two types of focus techniques used in binoculars. The design is the basis of how the binoculars work.

Independent focus

Used mainly with military-grade equipment, this technique allows for manual adjustments of the focus center on the binoculars. The eyepiece is made in a way that the diopter can be focused and adjusted to fit the needs of the surrounding.

They are used for heavy field use as the materials used are also of high quality in the market. The price of binoculars with this will also be expensive.

Central Focusing

The very common arrangement used in binoculars. This one will use a rotating technique to be able to adjust the focus system of the tubes to the middle. It also allows for focusing on individual eyepieces to adjust any errors or mismatching from using both sides of the binoculars.

It is also applicable to the focusing wheel and the diopter, the adjusted eyepiece. The lens will be adjustable with the focusing wheel only without the use of the interchangeable eyepieces.

The difference between the two is adjusting the diopter and the focusing wheel. They work towards a focused image but with different techniques.

Binoculars parameters

Now let’s take a look at some of the tiny details that make each binocular different from the other.

Lens coating

Depending on the type of glass you choose for your binocular, you would need some corrective coating to prevent colour distortions, chromatic aberrations, and glares from your images.

Coated optics

With only a single layer, this type of coating offers the least protection from reflective light.

Fully coated optics

Better than the coated version. This one has more layers of coating. Unless there are plastic lenses, the layering could provide an adequate coating.

Multi-coated optics

The multi-layered coatings are placed over more surfaces.

Fully multi-coated optics

This means all air to glass surfaces have been fully coated. It provides the best cover for reflective lighting.

These apply to all binoculars in the market. Since the Porro prisms do not require any corrective coating, the roof has specially designed coats to enhance the image. It could provide clearer, sharper and brighter images.

Phase coated

Also known as p-coating. This is very common with roof prism binoculars. They rectify any geometric differences that may be observed with the prism during the process of phase shifting.

Metallic coating

The prism can be coated by aluminum or silver. They offer less reflectivity of between 85% to 98%. But because they do not tarnish, they provide a relatively good coating for the prism.

Dielectric mirror coatings

These use a combination of both low and high refractive indices. The Porro prism doesn’t need any coating as they have the highest refractive index of 100%. Mixed with a low refractive index can provide up to 99% reflectivity.

That is better than the metallic coating.

Other terms used to refer to glass and coatings are ED and UD. These means ultra-low or extra-low Dispersion of light. They also offer good imaging quality for the binoculars.

Exit pupil

This refers to the field of view from the lens. If you hold up the binoculars, you can see a bright reflection on the midsection if the eyepiece. That is the exit pupil.

The measurement is also expressed in millimeters where the diameter could be long or short. The longer the diameter, the more light can enter the binoculars. Consequently, this light can be used to view images under low light areas or in extremely dark areas

Field of View

This refers to the area under which you can view a magnified image with the binoculars. Under specifications, you can see it is measured in feet over the yards or meters.

The field of view goes hand in hand with the diameter of the objective lens. The larger it is, the more you can be able to view it. It can also be affected by the magnification power.

When the power is low, the field of view is tighter but more focused. If the power is high, the image may need to be refocused.

There are two terms used to refer to the field of view:

  • Real
  • Apparent


This is the actual imaging view where you see once you look through the binoculars. With no justifications whatsoever, it can easily be used to view birds as they fly under high speeds.


Expressed in degrees, this is the angle of view. The larger the angle the wider the field of view. It is also affected by the magnification power on the binoculars.

Close Focus Distance

This refers to the nearest spot of focus for binoculars. It could range from as little as 0.5 meters to 30 meters. It varies according to the brand and the magnification power of the binoculars.

With a close focus distance, you can easily spot fine details that wouldn’t have been seen with the naked eye.

Magnification power

This is the number of times binoculars can expand an image. Depending on the brand of the binoculars, the power could range from 2× to 60×. Spotting scopes have the highest magnification power while the Monoculars have the least.

Eye relief

This refers to the spacing between where you place your eyes and the eyepiece of the binoculars. While the lens needs the surface area to trap light, you also need to be able to look through the glass to see.

The longer the eye relief, the more comfortable it will be. It will also allow for glass wearers to use the binoculars with ease.

Types of Binoculars

There are different types of binoculars structured differently to offer specific features over others. Not that any binoculars will not magnify an image, but either is more suited for specific jobs than others.

Low vision binoculars

This is a very common type of binoculars. Used by military plus individuals for night surveillance, working in the dark plus other outdoor activities done in low light environments.

This type of binoculars has the highest diameter of the exit pupil to allow for light to be easily trapped and increased brightness in the dark. This infrared light will be illuminated by the binoculars to allow for clearer images even in extreme darkness.

Image stabilized binoculars

Another unique type of binoculars is these. They have focus adjustment knobs to allow for images to be adjusted either manually or automatically depending on the brand. These types of binoculars are best used for astronomy and bird watching. They can stabilize an image seen to be fast-moving.

Binoculars for hunting or bird watching

The best binoculars for birding have wider fields of view and higher magnification power to allow for target shooting and focused watching. While watching birds could need an image stabilizer. If watching something over long distances, binoculars with high power are suitable.


This physics lesson could have been the most detailed you have ever read. Just to do a quick recap, the whole idea of magnification by binoculars depends solely on the light. The light refracted from the objective lens travels through the prisms and out to show you a magnified image.

The focal path of light is what gives the ability to bring images closer. With this information, the question of how do binoculars work is answered

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