How Do Binoculars Work – Binoculars Parts And Their Function

Binoculars work using a set of lenses that are cleverly arranged to magnify distant objects and make them appear closer and larger. Prisms reverse the inverted images of the lenses and reduce the length of the binoculars by redirecting the light path.

How Do Binoculars work

Binoculars work on the principles of optical laws, precisely by taking advantage of the properties and the behavior of light as it travels through different media. Let’s take one apart to see how binoculars work, what the essential parts of binoculars are, and how they work together?

Binoculars consist of three optical parts to work:

  • Eyepiece or Ocular: to focus on and enlarge the projected virtual image
  • Prisms: to correct the image orientation – flip the image vertically and horizontally
  • Objective Lens: collects incident light and concentrates it in the focal point, usually a lens system of 2 or more lenses to compensate for aberration

binoculars parts explained

Easy explanation of how binoculars work

To understand the parts of binoculars and their functions, it is best to take binoculars apart and have a look at the insides. That way you can see the individual assemblies and get a better idea of how binoculars work. There is an optical system and a mechanical system to look at.

Optical Parts Of Binoculars

Binoculars consist of three optical assemblies that refract and focus light to enlarge a distant object and make it appear closer. These three essential components are the objective lens, prism system, and eyepiece. Let’s take a closer look at these parts to understand their functions and how binoculars magnify.

Objective lens

Binoculars parts- lens of binocularsThe objective with its large collection lens is located at the front end or the bottom end of the binoculars, depending on how you look at it. The collection lens is aimed at the object of interest, it captures the light.

The larger the aperture – the diameter of the lens, the more light-gathering power the binoculars have, thus the brighter the image will appear. For this reason, hunters or stargazing enthusiasts choose binoculars with large lens diameters. Lightweight compact binoculars that are great for hiking or occasional use may have a much smaller aperture of around 25 mm.  The size of the aperture is expressed in the second number of the binoculars rating ie. 8x42

The virtual intermediate image created by the objective lens is upside down and mirrored. To correct this, the prisms are used inside the binoculars.

The prism system

Without prisms in the binoculars, the observer would see an inverted image. The prisms correct this, and they also help to reduce the binoculars’ length somewhat.

The prisms are made of crown glass and serve as correcting mirrors. As the light beam passes through the prisms, multiple reflections flip the upside-down and inverted image projected by the objective lens, so the observer can see a normal-looking image.

There are two principal types of prisms used in binoculars. These are the Porro prisms and different roof prism designs.

Porro prisms

porro prism binocular parts
Porro Prism

There are two prisms used in Porro prism binoculars, they are set up at right angles to each other.

The light rays are reflected by the internal surfaces and inverted from top to bottom in one prism and from left to right in the other prism.

The advantage of binoculars with a Porro prism system is that these prisms are much easier and cheaper to manufacture and require little space as they are set up next to each other.

Porro prism binoculars are often of shorter build than roof prism binoculars.

Roof prism

roof prism assembly
Roof Prism Assembly

This prism system usually consists of two prisms, of which at least one has the shape of a roof edge.

One side of a prims glass body must be coated to compensate for the phase shift of the different wavelengths and to reduce color fringing, this makes roof prism a bit more expensive than Porro prisms.

The image correction in roof prisms binoculars may follow a more complex beam path than in Porro’s, but the result of flipping the upside-down and mirrored image from the objective lens is the same.

Roof prisms allow much slimmer, more compact built binoculars. Binoculars with those prisms are a bit narrower and more elegant compared to models with Porro prisms but are usually a bit more expensive since a more elaborate production is required.


binoculars eye pieceThe eyepieces, also called ocular, are located at the front of the binoculars and are the two lenses that we look directly into during observation.

Eyecups (rubber extensions on the ocular barrel) are often used to keep the correct eye distance to the eyepiece and provide some protection from disturbing oblique stray light.

The eyepiece usually consists of two or more lenses.

When the binoculars are properly focused, it allows the human eye to see the image that’s projected by the objective lens and the prism system.

Mechanical Parts of Binoculars

Diopter Adjustment

binocular parts diopter adjustment

Many people do not have the same strong eyesight in both eyes. To allow for fatigue-free observation through binoculars the different eyesight must be compensated so both eyes can see a focused image.

Usually, the right eyepiece can be fine-tuned to compensate for the dioptric difference by adjusting the focus of the ocular lenses.

To adjust, close your right eye and focus on an object using the left eye (the eyepiece without diopter compensation) by turning the Center Focus Wheel until the image is sharp and clear. Then fine adjust the sharpness on the second eyepiece with the aid of the Diopter Adjustment.

Focus Wheel

To get a sharp and clear image when viewing objects at different distances it is always necessary to adjust the focus of the binoculars. Turning the focus knob (or focus wheel) will push out or pull in the eyepieces (or just one of the eyepiece lenses)  so the focal point of the eyepieces converges with the focal point of the objective lens.

Barrel-bridge with Hinge

Binoculars are simply two telescopes that are mounted next to each other. They need to point exactly in the same direction to allow an observer to see simultaneously through them. The barrel-bridge holds the binoculars’ barrels in parallel alignment to each other so the optical axis is parallel (the light beam is collimated).

Hinges that join the bridge allow us to adjust the distance of the eyepieces to the viewer’s individual eye distance.

Barrel or Tube

The barrel is the housing that holds all the optical parts together. The housing protects the optical components and holds them in a stable position so they don’t shift from mechanical shock or when dropped.

Many of the better binoculars have O-ring seals to keep water and moisture out. In many high-priced models, the interior is also filled with inert gas to ensure water resistance even when submerged under water.

Path of Light in Porro Prism Binoculars

beam path in porro prism binoculars

How Prisms Invert the Image

  • Image from viewed Object appears horizontally and vertically inverted
  • The first prism inverts the image vertically
  • The second prism inverts the image horizontally
  • A small image is projected at the end of the focal length of the objective
  • The ocular magnifies the image and presents it to the eye

How Do Binoculars Work - Binoculars Parts And Their Function

How the image gets into Focus

To present a focused and sharp image to the viewer, the focal point of the ocular lens must converge with the focal point of the objective lens.

How to Calculate the Binoculars Magnification

The magnification of binoculars also referred to as “power”, is an important rating factor, which helps to decide on the intended use of the instrument. It indicates how many times an image appears magnified when viewed through binoculars compared to being viewed with the bare eye.

The magnification number is the quotient of the lens focal length and eyepiece focal length. In the example above it would be 240 / 24  = 10

Binoculars magnification is expressed in the first number of the binocular rating ie. 8x42

The most popular magnification factors are 8x and 10x. At 10x magnification, objects 100 meters away appear as if they were 10 meters away.

binoculars parts inside roof prism binoculars
Collecting lens, prism assembly, and eyepiece of roof prism binoculars

Summary Binoculars Parts And Their Function

Quiet interesting how do binoculars work, and how they are basically put together from a very simple arrangement of three optical parts: Eyepiece, Prisms, and Objective Lens.

The Objective lens is the large lens at the front end of the binoculars,  it faces the viewed object. The diameter of the objective lens is called the aperture. The size of the lens determines the resolution (sharpness) and how much light can be gathered. The captured image from the objective is mirrored and upside down.

Prisms are used to correct the image. When the light rays of the inverted image pass through the prisms, they reflect off the inner surfaces of the prism and exit as a normal, real-looking image. The prism types are Porro prisms and roof prisms.

The Eyepiece or ocular is the part where the user looks into.  The image that the objective lens has collected and projected at the end of its focal length, is magnified by the eyepiece and presented to the viewer’s eye.