Why are Prisms Used In Binoculars Instead of Mirrors?

Binocular is the modern version of the telescope. This optical instrument is used to enhance the view of distant objects. It has the objective lens at the front edge to collect information. At the opposite edge, an ocular lens creates a magnified image of the collected information.

So it is clear that the purpose a binocular serves depends more on its optics than anything in its mechanism. However, manufacturers prefer prisms in modern binoculars as opposed to mirrors. But the question is, why are prisms used in binoculars instead of mirrors?

Let’s find the answer.

What Is A Prism?

To answer the question, we need to understand what a prism is.

A prism is an optical component commonly known as a three-dimensional shape. But the conventional definition of the function of prisms often appears to be cofusive. We better try to understand it in our own way.

There are three things we need to keep in mind to understand a prism.

  1. The first thing is it must be a three-dimensional figure.
  2. The figure must have identical ends. Identical ends mean there are two parallel triangles at the ends.
  3. The figure must have three rectangles facing each other from the front, back, and bottom. The prism with four rectangles faces is known as rectangular prism.
What Is A Prism

Now we can simplify the definition by saying that a prism is a three-dimensional figure with  rectangular flat faces and two identical ends.

Why Are Prism is Used In Binoculars Instead Of Mirrors

There are several reasons for using prisms in binoculars instead of mirrors at present. Binoculars with mirrors or glass are known as Galilean binoculars. In these kinds of binoculars, there is a  long distance between the eyepiece lenses and objective lenses. In short, the use of mirrors is common in some old-fashioned binoculars. Now let’s know the advantage of using prisms in modern pairs of binoculars over mirrors.

Why are Prisms Used In Binoculars Instead of Mirrors


Uses of prisms in binoculars have shortened the size of the binoculars. Carrying those old-fashioned half a meter-long binoculars seems odd these days.

Modern binoculars have a compact lens mechanism. Besides the objective and eyepiece lenses, there are additional lenses like convex lenses, concave lenses, and different types of prisms used in the optical construction.

Hence the incident light path hits the objective lens and travels inside the prism but in a shorter range. Thus the light travels the same distance as in the mirror but in a restricted range. Thus it makes the binocular more handy and portable.

Reflection and Image Quality 

Prism has more TIR ( total internal reflection ) than a mirror. Hence it provides more optical advantage. Mirrors often result in reflection distortion that reduces the image quality.

Also, the light absorption rate for mirrors is 95% whereas prism provides the appropriate TIR angels. In short, when it comes to prisms the ratio of reflected light and the incidence light is equal. Thus it produces high magnification and creates high-quality brighter images of the object you sight at. 


Mirror often comes with multiple coatings to perform at its best. Over time these coatings start to fade away and eventually disappear. Once it gets removed, whatever the reasons are poor sealing or tough environmental issues, the optics become useless.

You will not like those reversed and inverted images, will you? Also, changing the optics at frequent intervals will cost you a lot. On the other hand, prisms are more reliable and do not need any special maintenance to perform.

In fact, most hunting binocular manufacturers always prefer prisms to mirror when it comes to providing the best optics quality. Prisms, for instance, BAK4, can still produce better image quality even in rough weather and low light conditions. 

Magnification and Stability

The ratio between the image height and object height is called magnification. Magnification relates to various essential factors such as field of view and stability.

Mirrors, without a doubt, provide less magnification or range of power than a prism does. The mirror creates a virtual view of the object through the reflected light. In other words, it makes the object bigger than its actual size.

On the other hand, a prism bends the light that passes through it. Also, it breaks the ray of sunshine into multiple colors like a rainbow to form a closer image of the distant object.


Mirrors, when used in periscope or binoculars, need extra care as they get blurry and dirty quickly. Also, the fragile surface of mirrors often becomes challenging to clean without damage.

Also, having some surprising and shocking scratches, cracks on the surface are not unusual with mirror lenses.

Prism, on the other hand, possesses an inherent thickness and robustness. Thus it offers hassle-free cleaning and maintenance. 

What Are The Types of Prism Used In Binoculars

The modern binoculars come in two prism types; Porro prism binoculars and roof prism binoculars.

types of prism used in binoculars

Each of these prism types has its own advantage and uniqueness. They differ from each other in terms of shape, size, and light transmission. An overall study on them may help you while choosing the right binocular for you.

Porro Prism

Porro prism has been in the market since 1960. These prisms are named after their inventor Ignazio Porro.

The mechanism of the Porro prism is not so complicated. However, these kinds of prisms are triangular in shape and remain in pairs.

In binoculars, both the lenses stay in the right triangular position. Through the objective lens, the image of the object is captured in reverse form.

The first prism gets the image in a 90-degree reverse position and then transfers it to the other prism to form the oriented view and delivers it to the ocular lens.

Roof Prism

The Roof Prism is commonly used in keplerian binoculars. They are also known as Dach Prism. These prisms are made to give the binocular a more handy and portable shape.

In this kind of prism type, one thick glass is used opposite from the Porro prisms. The shape of the roof prism is quite like a hexagon, but not exactly.

The light collected by the objective lens passes directly through the prism and gets delivered in the ocular lens. This image processing of roof prisms is a bit more complicated than the Porro prisms.

At a Glance:

Porro PrismsRoof Prisms
Triangular in shape.Comes in different shapes.
Less waterproof.More waterproof.
Bigger in size and weighs more.Compact in size and lightweight.
Almost all are mountable on tripods.Not all mountable on tripods.
Not so expensive.Higher in price than Porro prisms.
Light transmission rate is high in all.High light transmission only in the expensive ones.


Prism from all angles is an excellent replacement for mirrors when it comes to optical performance. The use of binoculars these days is no longer restricted to astronomical fields. Besides, it has become a mandatory tool for marine adventurers, hunting, photography and so on. However, understanding why prism is used in binoculars will facilitate you to use it in a more effective way. Also, knowing how different types of prisms work in a binocular is always an advantage in selecting the right one for you.

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