How Binoculars Work and Binocular Parts

J.P. Lumiere created the first real binocular in 1825, introducing prism-erecting systems to correct image orientation. Classic binoculars, which were initially made using glass lenses and Galilean optics, were named after Galilei Galileo. After all, he’s the one who introduced the telescope to astronomy, so he deserves recognition for his significant achievement.

The binocular is an optical instrument composed of two identical telescopes mounted together so that you can see through them using your two eyes rather than one. In this article, we’ll explore how binoculars work and binocular parts in-depth.

How Binoculars Work: The Short Answer

There are two objective lenses placed at each end of the binoculars to collect the light ray from the distant object and create a focused image a short distance behind the lens. Following that, the eyepieces are used to pick up the image, magnify it, and present it to the eyes. Prisms are also required to correct the image orientation. 

To fully understand the binoculars’ mechanism, you need to be aware of their parts.

Binocular Parts

Several components make up a binocular. Before we delve deep into each of them, let’s just give you an overview of a binocular’s parts and essential functions.

  1. Objective lens: collects the emitted light rays from the distant object.
  2. Ocular or eyepiece: displays the magnified image to the eyes.
  3. Prism: fixes the inverted image.
  4. Focus wheel: focus adjustment.
  5. Binoculars barrel: holds all binocular components together.
  6. Barrel-bridge with hinge: keeps the binoculars in parallel alignment to each other.
  7. Diopter knob: provides viewing customization to compensate for the different eye strength of the viewers. 

Objective Lens  

The objective lens is the convex lens located at the end of the binocular, nearest to the object. It catches the light ray from the targeted object and creates a focused image, then directs it to the eyepiece.

There are different sizes of the objective lens. The larger the lens diameter is, the more light is collected, leading to a brighter image. 


The ocular or eyepiece is the part that’s closer to the viewer’s eyes. This lens is noticeably smaller than the objective lens. When the objective lens collects the light and brings it to focus, creating the image, the eyepiece picks up that image and magnifies it.

This part is surrounded by rubber extension, called eyecups. Eyecups are used to maintain distance between the eyepiece and your eyes, providing some protection from disturbing infalling stray light. 

Binocular lenses are coated to reduce the light reflection to ensure that the produced image is sharp and clear; a binocular lens without a coating may lose up to 5% of its clarity.

There are four types of coating: coated, fully coated, multi-coated, fully multi-coated.


The prisms are located between the objective lens and the eyepiece. Prisms are very important as the objective lens produces up-side-down images, serving as a correcting mirror. Without prisms, you’ll see an inverted image.

How Does It Work? 

One prism flips the image by 90 degrees (vertically), and the other one flips it by another 90 degrees (horizontally), making a full 180 degrees rotation correcting the inverted image created by the objective lenses.

Two types of prisms are commonly used in binoculars: Porro prism and roof prism.

Porro Prism 

This type is used in the classic binoculars. In fact, the reason behind the classic binoculars being larger is that the prisms are arranged at a right angle to each other, which increases its sight too.

Porro prism binoculars have several advantages. For one, they have a broader objective lens than roof prisms, so they produce a better image. Moreover, they’re less expensive compared to roof prism binoculars. However, their drawback is their size, making it difficult to hold them for a long time.

Roof Prism

The roof prism system consists of two prisms arranged back to back. 

Roof prism binoculars appear compact and slimmer, but they actually feature a more complex light passing process and require greater manufacturing accuracy. That’s why they’re expensive to manufacture.

Despite their high price, some customers prefer roof prism binoculars because of their slim and elegant design.

Focus Wheel

The focus wheel is used to adjust the binoculars’ focus if an object appears blurred to get a more precise and sharper image.

This mechanical part is essential. It keeps your eyes healthy, as our brain will automatically try to compensate for the blurred vision, which causes eye fatigue, headache, and dizziness.

Binoculars Barrel or Binoculars Tube 

A binoculars barrel or binoculars tube is used to cover and protect the optical parts. They hold them all together so that they don’t shift when dropped. You’ll also find that some advanced binoculars come with an O-ring to keep water and moisture out.

Barrel-Bridge with Hinge

Since binoculars consist of two individual telescopes mounted together, they need to point precisely in the same direction to allow an observer to have one simultaneous view through them. The barrel-bridge keeps the binoculars in parallel alignment with each other for a parallel axis. As for the hinges, they help the viewer adjust the eyepieces’ distance to their eye distance.

Diopter Knob or Diopter Adjustment

The diopter knob is a ring that’s directly placed behind the eyepiece lens. It allows you to focus the eyepiece lenses individually. Diopter knobs are used to compensate for the differences in the eye strength of the viewer. Mostly, it helps those who wear glasses to focus without stressing their eyes.

Final Thoughts

As we’ve seen, binoculars and their operation method are truly impressive and worth speculation. It’s wonderful how such a visual tool can provide enlarged images of distant objects.

The fact that they’re made of two telescopes mounted on a single frame is equally astonishing! We hope that now you have an in-depth overview of how binoculars work and binocular parts that make them operate the way they do. 

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