Optics Technology

How Binoculars Actually Work (Without the Jargon)

Have you ever looked through a great pair of binoculars and thought, how is this even possible? Binoculars and other optics provide us with an entirely new perspective on what we’re looking at. Like that an American Goldfinch looks simply “yellow” from far away but is actually a combination of multihued feathers including lemon, mustard, olive, and different blacks stitched together to create its own unique color. Or that most Tree Swallows have metallic blue feathers that shimmer like oil slicks while Blue-gray Gnatcatcher’s have “eyeliner,” delicate white eye rings around its eyes.

Binoculars allow us to truly see the unseen and seek out the wonders of the natural world around us. So, how do they work!?

Inside every pair of birding binoculars, compact binoculars, or lightweight binoculars is a surprisingly simple system designed to do a few things really well: gather light, flip, focus and magnify it, and deliver it to your eyes in a way that feels natural.

No engineering degree required. Here’s how binoculars actually work — plain and simple.

Step 1: The Objective Lenses — Catching the Light

The big lenses at the front of your binoculars are called objective lenses.

The objective lens' job is to collect as much light as possible from the environment around you—whether you’re bird watching in your backyard at sunrise, coastal whale watching on a sunny day, or wildlife viewing on an overcast hike.

Bigger objective lenses = more light
More light = brighter, clearer images

That’s why many binoculars for hiking, binoculars for travel, and binoculars for outdoors balance size and brightness. You want enough light without carrying something bulky.

Pro tip: think that you’ll be using your binoculars mostly in low light environments? If so, opt for a larger objective lens.

The size of your binoculars objective lens is denoted by the millimeter rating on them. 25mm, 32mm and 42mm are all common objective lens sizes.

Tradeoffs: Bigger is better? The bigger your objective lens, the more light you’re able to capture, ergo brighter images. BUT, that will also result in larger heavier optics. Want something more compact and lightweight? A 25mm or 32mm binocular will be smaller and lighter than a 42mm pair.

So, at this stage, the light entering the binocular is: upside down, backwards, and probably out of focus.

Step 2: Prisms — Flipping the World Right-Side Up

When light enters binoculars, it doesn’t travel in a straight line. It gets flipped upside down and backward—just like in a camera. So, we use a prism to correct for this.

Prisms act like mirrors inside the binoculars, bouncing light around until the image is upright and facing the right direction again. Without prisms, everything you’d see would be upside down.

Nocs uses exclusively modern roof prisms which feature a sleek and compact design compared to the traditional wider Porro prisms. Both do the same job: straighten the image so your brain doesn’t have to.

It’s important to note that the reflective surfaces and prism coatings in the internal optical system of binoculars all play a role in reflecting light and getting it through the prism while keeping things bright and clear. Nocs uses premium Swiss designed BaK4 prisms which offer better light transmission than common BK-7 prisms.

Step 3: Focusing — Bringing the Image Into Sharpness

Before the image reaches your eyes, it must be focused.

When light passes through the objective lens, it converges toward a focal point. The focus wheel adjusts internal lens elements forward and backward to fine-tune where that focal point lands.

If the focal point lands exactly on the plane of the eyepiece, then the image is sharp or “in focus”. If it’s slightly in front or behind the focal point, then the image is blurry. Turning the focus wheel moves the internal focus lens millimeters at a time to dial that focal point precisely to your eyes.

Did you know that people who wear glasses or contacts can use binoculars without their personal corrective lenses? That’s because they can use the focus wheel and diopter (which adjusts for left and right eye variances) to adjust to their own personal focal point.

Step 4: Magnification — Bringing Things Closer

Your binoculars magnification rating tells you how much closer something will appear when looking at it through a pair of binos. An “8x” binocular means objects look eight times closer than they do with your naked eye. But magnification isn’t everything.

Higher magnification can reduce: image stability (how “shaky” you perceive the image), brightness and field of view.

Simply, the objective lens (front lens) makes a small, real picture of the object that you’re looking at. The ocular lens/eyepiece (back lens) acts like a magnifying glass that enlarges that picture for your eye.

An “8x” binocular makes an object appear eight times closer because the eyepiece enlarges the image formed by the objective lens.

8x and 10x are popular binocular magnifications since they tend to create bright, stable and wide images at common objective lens sizes.

Step 5: The Eyepieces — Delivering the View

At the back of the binoculars are the ocular lenses or eyepieces—the lenses you look through. They take the focused, flipped, corrected image and present it comfortably to your eyes.

Good eyepieces make viewing feel: natural, relaxed and easy on the eyes (and eye sockets). Twist up eye cups allow you to adjust how far your eye will rest from the eyepiece to create the best image. Ever get that annoying black silhouette encroaching on your view? Try adjusting your eyecup placement, that’s usually the culprit.