How Do Binoculars Work? The Perfect Guide

by Raymond Archambault
women looking through binoculars

Binoculars are a handy tool with a broad range of applications as far-flung as surveillance, hunting, and sporting. Their versatile use is almost limitless, and they’ve been incorporated into nearly all outdoor activities and hobbies.

These tools come in handy in allowing us to see far distances in great detail. How do binoculars work? How does this small device magnify a distant image and enhance its visibility as though it’s a stone’s throw away?

In this guide, we’ll explore how these lenses have the uncanny ability to magnify an image and present us with a crisp, clear viewing experience. While we’re at it, we’ll also discuss the various types of binoculars, the aspects that differentiate them, and the essential things to take note of before purchasing the best binoculars for yourself.

Brief History of Binoculars

The invention of the first telescope in 1969 paved the way for a world of stargazing possibilities while just using a piece of glass. We could see clearer and farther than ever before, and this was just the beginning.

Granted, you can only see through a telescope with one eye, and you have two. Resultantly, a French inventor known as J.P. Lemiere crafted the first pair of binoculars.

While the application for the initial patent was in 1825, it took an extra 29 years before the patenting of the Porro prism system we use today was circulated in the market. That was the beginning of modern binoculars.

How Do Binoculars Work?

In a nutshell, binoculars can be described as two small telescopes, side by side, with adjustable eyepieces mounted on each side. The eyecups are hinged at the center, allowing them to be closed to fit various-sized faces and separated. To put it simply, the objective lens absorbs a light path and captures an image.

The second lens then magnifies the image, allowing it to be crisp, clear for your eye. The focus wheel also allows you to have a narrower field of view, depending on what you’re focusing on. With that being said, let’s discuss each step of this process to understand how binoculars work.

1. Light Refraction

It defines how light bends when it goes through various materials, for instance, the glass in the objective lens in a set of binoculars. The light refracts (bends) as it passes through the objective lens. By altering the lens, binoculars manufacturers can regulate the light.

2. Magnification and Lenses

In an objective lens such as a convex lens, the middle is thicker than the sides. Resultantly, the light refracts towards the center, paving the way for the lens to focus on distant light rays of light into a small image that’s projected a short distance away.

Think of the eyepiece lens as a magnifying glass that enlarges the small image, making it clearer to view. The lens will take the image onto which the objective lens projects it and magnify it for you to view.

3. Prisms

There’s one setback when it comes to using convex lenses for light refraction. When light refracts through a convex lens, the light rays may pass through, causing an upside-down image. You might assume that the eyepiece lens sorts out this problem, but that couldn’t be further from the truth.

Instead, prisms are incorporated to rotate the image for your viewing. To put it simply, these prisms are large glass wedges that rotate and mirror the image. It takes two prisms to rotate the image to a 180-degree angle, with each effectively altering the image by 90 degrees.

Given that it takes two prisms in each tube to rectify the image, every set of binoculars will have four prisms.

Roof vs Porro Prisms

The Porro counterparts are more popular due to their affordability and are considerably older than roof prisms. They are the original design used in the first modern binoculars that eventually evolved into the handy devices we know today. Nonetheless, they make for bulkier, larger binoculars.

On the other hand, roof prisms are designed with portability in mind as they are compact binoculars. However, don’t let their size fool you as they are costlier than Porro binoculars.

Porro prisms are arranged adjacently at a 90-degree angle, allowing image refraction from one prism into the next. The technology is old and has been implemented for centuries.

Given that they are aligned horizontally, Porro prisms are bulkier and bigger but easier to build, which explains why they are cheaper than roof prism binoculars. However, those prisms are easier to misalign. All it takes is one drop to destroy a set of roof prisms. Similarly, most Porro prism binoculars aren’t weatherproof.

4. Focus

Most types of binoculars have a large center ring that can rotate to focus the barrels concurrently. That’s perfect for quickly zooming in on an image and seeing it in clear view without a hassle.

Additionally, you’ll typically find an adjustment ring in most binoculars. It lets you focus on one barrel independently from the other. As a result, you can compensate for the variations between your eyes and focus on the crispest image possible.

5. Field of View (FOV)

It refers to the area in full view. It’s a measure per 1,000 yards of the number of feet you can see through your device. Usually, greater magnification indicates a smaller field of view. As you’re dialed in, you’ll notice a smaller overall area but significantly clearer.

When it comes to reduced magnification, you might not view as much detail, but you’ll be taking in a larger surface area at a go.

A wider field of view is perfect for pinpointing targets on the move or those that are difficult to see. That’s ideal for bird watching or hunters trying to discover their prey. With a smaller FOV, you’ll get a detailed look at a small area that is best suited for surveillance activities.

6. Eye Relief

If you’re unfamiliar with how binoculars work, you might try to put the eyepieces against each eye. However, you’re meant to hold them a short distance from your face to maintain clear visibility of the FOV.

The specified distance you’re supposed to hold them from your eyes is known as eye relief. The spec is the most crucial if you wear contact lenses or prescription glasses.

If the eye relief is overly small, you’ll lack adequate room to get close enough to view the image without your eyewear colliding with eyepieces. Therefore, you should opt for binoculars with at least 11mm eye relief.

7. Exit Pupil

The width of the light beam piercing through the eyepiece is known as the exit pupil. If you hold the binoculars at a short distance from your face, you’ll observe the exit pupil as a small round dot of light in the eyepieces.

Usually, a larger exit pupil is better. The larger it is, the brighter the image is. That’s particularly essential in low-light conditions where a small exit pupil can deter you from seeing clearly.

Locating the exit pupil is a breeze in any binoculars. Once you divide the objective lens by the magnification, you’re good to go. Therefore, for a set of 8 by 48 binoculars, you’d divide 48 by 8 for an exit pupil to get 6mm.

man holding binoculars

Source: Pexels

What Do the Numbers on the Binoculars Imply?

You’ve probably seen binoculars marked with various numbers and are a tad curious as to what these numbers signify, for instance, 8 by 48, 11 by 44, or 12 by 26. Fret not, as they don’t have anything to do with math.

Magnification

The first number, followed by the X, represents the level of magnification of the binoculars. Therefore, for a set of binoculars listed as 8 by 48, they have a magnification of 8X. Likewise, 11 by 44 binoculars have a higher magnification of 11X, and so forth.

That means, when you look through the 8X binoculars, the image you see will be seemingly eight times larger through the lenses compared to your naked eye. Furthermore, consider it as the distant object appears 8 times nearer than when you view it via your binoculars.

Objective Lens

The second number represents the size of the objective lens measured in millimetres (mm). As mentioned, this lens is located at the end of your binoculars and takes in the light and image in view. For instance, the 8 by 48 binoculars mean the diameter of the objective lens is 48mm. Therefore, the binoculars have an 8X magnification coupled with a 42mm objective lens.

Generally, a bigger objective lens takes in more light and shows a crisper, clearer image and enhanced low-light performance. Contrarily, it also implies that the binoculars will be bigger and less compact, making it a trade-off.

Various Sizes of Binoculars

Like everything else, size matters in binoculars. The size you opt for will be based on your intended use.

Compact

These binoculars are best suited for adventurers on the go. Compact binoculars are therefore not ideal for situations that call for magnification.

Midsize

These binoculars are the happy medium. Midsize binoculars are perfect for excellent magnification and the broadest range of options.

Other types of binoculars are:

  • Mini or foldable
  • Wide-angle
  • High-powered
  • Zoom
  • Image Stabilizing
  • Waterproof
  • Autofocus
  • Tripod-mounted

Conclusion

Binoculars are two simple telescopes connected on each side for each human eye to view through one. While the objective lens allows light in, capturing the image and sending it to the eyepiece lens for magnification and your viewing pleasure, prisms have to spin the image.

That’s because the rays of light get crossed, and the image gets flipped when you view it through your objective lens. These prisms can be aligned in a straight line (roof prisms) or horizontally (Porro prisms).

Furthermore, there’s a wealth of specialty binoculars best suited for various needs, ranging from wide-angle to zoom counterparts. Regardless of the option that you select, the fundamentals of how they function are standard. Their endgame is to allow you to view farther than you ever could with the naked eye.

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