How Binoculars Work ?

Why do I like binoculars so much?

Why do I feel like being able to see as far as a hawk can see is some sort of superpower? I love binoculars, and I’m unapologetic about that. They feel like a natural extension of my eyes from which I can watch almost everything because I trust the optics would be great. But how are all of these possible? How do binoculars, these small optical instruments, perform such wonders that leave us in awe? If you’ve been asking similar questions, I’ll take a few minutes to break this down for you in this guide. Leggo!

A tale of two telescopes

If you’re trying to understand how a binocular works, I think it’s best to start by discovering what actually makes a binocular. According to Britannica;

A binocular is an optical instrument, usually handheld, for providing a magnified stereoscopic view of distant objects, consisting of two similar telescopes, one for each eye, mounted on a single frame.

How Binoculars Work

Both of the telescopes are aligned and hinged at the center.

They can be separated or closed to fit different face sizes and come with a thumbwheel to adjust the focus of both telescopes either jointly or separately.

Now that we’ve mentioned how the binocular derives its power from two telescopes aligned together, it’s only right that we discuss how each of these telescopes also works.

How a binocular works

Binoculars leverage the science of optics and a few clever tricks with light and lenses.

Your binocular will come with two lenses. There is the objective lens, whose main objective to gather light and capture the image. Then there is the ocular or eyepiece lens whose purpose is to magnify the image, so it becomes much clearer to the eyes looking through the binocular.

Below, we discuss three key processes going on in your binocular when you try to watch birds, gaze at stars, or do any of those things you enjoy;

1. Light refraction

The objective lens that captures the image is a convex lens.

A convex or converging lens has the middle thicker than the outside. Through this, it is able to capture light (image) from a distant object. Because of its refractive properties, the lens causes the light to bend and converge together at a point behind the lens. This converged image from the objective lens is what the eyepiece lens then captures, magnifies, and passes on to the viewer. But the process is not as simple as it seems. The objective lens causes the image to be inverted, which brings us to the next process below.

2. Image reinversion process

The objective lens captures the image, and the ocular lens magnifies it.

But there’s a light inversion process that occurs before the eyepiece lens can pick up the image from the objective lens. The issue with convex lenses is that while their refractive property helps them capture light, the light crosses over, so what you’ll have on the lens is an up-side-down mirrored image. There has to be a way to correct this image, and there is!

This is where prisms come into play.

These prisms are large wedges of glass, and they help to rotate and reflect the image, so they don’t appear inverted to the eyes looking through the eyepiece lens. Two prisms are usually needed to do this, as each prism can only rotate an image by 90 degrees. So, your binocular will have four prisms in total, two in each telescope.

 

There are two main types of prisms employed for the image correction process.

It’s either porro or roof prisms are used. Porro prisms are the most common types and are set side by side, horizontally at 90 degrees. Although they are less expensive to manufacture, they are larger and lead to a bulkier binocular.

Roof prisms are set on a straight line, so the light follows a straight path. They are more complex to manufacture, more durable, and more expensive. And because they are set in a straight line, binoculars with this type of prisms are more compact and streamlined.

Image reinversion process
Image credit: Britannica

3. Magnification

The eyepiece lens is a magnifying glass.

It picks up the image projected from the objective lens that passes through the prism. This image is then magnified, so it becomes a lot clearer when you look through the eyepiece or ocular lens.

Different binoculars have different magnification powers.

This is usually denoted as a number such as 6x, 7x, 8, 10x, 40x, etc. A binocular with a magnification power of 10x will produce a virtual image that is ten times larger than the subject.

Talking of magnification, the magnification power of your binocular will depend on the ratio of the focal length of your objective lens divided by your eyepiece lens’s focal length. Your ideal magnification will depend on your intended use of the binocular.

A lot of people, however, make the mistake of thinking larger is always better. It is not! You might check out our post on what numbers on a binocular means so you can get a better grasp of what this means.

Adjusting the focus on a binocular

Being able to control focus helps you focus on a subject so you can easily get a clearer image. Usually, there is a large center ring for this. The ring spins and is often used to control the focus for both telescopes simultaneously.

But there are also binoculars with diopter adjustment rings that allow viewers to control the focus of one of the barrels of the binocular separately from the other.

how a binoculars work

Conclusion

The logic behind the workings of a binocular is a fascinating one.

First, the light travels from the far-distant subject and gets picked up by the objective lens. The objective lens picks up this light and forms an inverted image behind its converging lens.

This light passes through the prisms where they are reinverted and reflected to the eyepiece lens. The eyepiece lens picks up the light, magnifies it, and makes it available for your viewing pleasure. It’s simple yet amazing!

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