Why Are Prisms Usually Used in Binoculars Instead Of Mirrors

Why Are Prisms Usually Used in Binoculars Instead Of Mirrors?

prism vs mirrorA type of telescope commonly referred to as a “binocular” is a modern optical instrument that can magnify the view of distant objects.

At the front, the objective lens is focused on collecting information. At the other end, the ocular lens produces magnified images of the collected data.

A pair of binoculars provide the user with the best possible optics. However, manufacturers prefer to use prisms over mirrors in modern binoculars. The question is, why are prisms chosen over mirrors in binoculars?

In order to answer the question, we must first understand what a prism is.

What is a Prism?

A prism is a type of optical component commonly referred to as a three-dimensional shape. However, its function is not always clear. In order to better understand it, we should try to separate the multiple facets of its operation from its conventional definition.

Understanding a prism is very important. So, we need to keep three things in mind when it comes to learning about it.

  • A figure should have three rectangular faces. The three sides of the rectangle should be facing each other, while the bottom and back of the rectangle should be facing each other.
  • A three-dimensional figure should be the first thing people think of when they think about a figure.
  • The number of parallel triangles at the ends must be identical. This means that there are two parallel ends.

In short, a prism is a three-dimensional figure consisting of flat faces and two identical ends, simplifying the word’s definition.

Why Are Prisms More Preferable in Binoculars Instead in Mirrors?

Besides being useful for viewing the night sky, prisms are also helpful in binoculars due to their properties. There are also various reasons why people use them instead of mirrors. These types of binoculars are known as Galilean. The distance between the objective and the eyepiece is long.

In fact, in old-fashioned binoculars, the use of mirrors is common. But with the use of prisms, you can now see clearly in modern pairs of binoculars.

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The Reliability

Many types of mirrors come with multiple coatings. Over time, these coatings start to fade and eventually disappear. There are various reasons why these coatings can get removed, such as poor sealing, environmental issues, weather, and so on.

Since you would not like the inverted and reversed images, it is essential to use prisms instead. They are more reliable and do not require special maintenance. Also, replacing the optics at regular intervals can be costly.

Most of the time, hunting binocular manufacturers prefer to use prisms to mirror their optics to provide the best possible image quality. For instance, the BAK4 can still produce better images even in low light conditions.

The Portability

The use of prisms has shortened the size of binoculars. These days, carrying an old-fashioned pair of binoculars seems odd.

Modern binoculars have a compact lens mechanism that includes the main objective and eyepiece lenses. Various types of lenses are also commonly used in the construction of binoculars.

The incident light path of the lens then moves inside the prism, allowing it to travel in a restricted range, making the binoculars more portable. Similar to the light in the mirror, the incident light path of the lens also travels the same distance in the binoculars.

The Quality of the Image and Reflection

The advantages of a prism over mirrors are its higher TIR (total internal reflection). This allows it to provide better optical performance. Unfortunately, mirrors can distort the image quality resulting in poor image quality.

The absorption rate of light for mirrors is 95%, while that for prisms is equal to the incidence of light. This makes them ideal for creating magnified images. They also produce high-quality images with better contrast.

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The Stability and Magnification

The ratio between the object height and the image height is called magnification. It refers to various factors that affect the image’s field of view and stability.

Although mirrors provide less power and magnification than a prism, they still allow one to see more clearly. Through the reflected light, one can actually see an object as if it were more extensive than it is.

A prism bends the light that is going through it. It then breaks the rays of sunshine into various colors to create a closer image of the object.


When using mirrors in binoculars or periscopes, they need to be cleaned regularly to keep them looking their best. Also, the delicate surface of mirrors can get damaged if not handled properly.

Although there are usually noticeable scratches on the surface of mirrors, such as on the corners, they are not unusual for lenses. Compared to other types of lenses, prisms have robust and incredibly thick construction, making them ideal for cleaning and maintenance.

The Types of Prisms That are Usually used in Binoculars

Modern binoculars come in two types: roof prism and Porro prism. Roof prisms are commonly used for viewing images.

The different types of prisms have their own unique features and advantages. They are also different from each other in terms of their light transmission and shape. An in-depth study of these types will help you choose the right one for you. Let’s know more about these two prism types below:

The Roof Prism

The roof prism is commonly used in binoculars that are made for viewing different objects. These are also called Dach prisms. These are made to give the binoculars a more portable and handy shape.

This type of prism is made of a single thick glass, unlike the Porro prisms. The roof prism shape is similar to a hexagon, but not exactly.

The light from the objective lens is collected by the prism and then converted into the ocular lens. This process is a bit more complex compared to the Porro prism process.

The Porro Prism

Since 1960, the Porro prism has been available in the market. The name of the product comes from Ignazio Porro.

The mechanism of the Porro prism is relatively simple. These types of prisms have triangular shapes and remain in pairs.

In binoculars, the two lenses are positioned in the same triangular shape. Through the objective lens, they capture the image of the object in reverse.

The first prism focuses on the image and transfers it to the other prism, which then produces an oriented view, and then delivers it to the ocular lens.

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For optimal optical performance, opt for prism from all angles. This kind of lens is suitable for people who are wanting to enhance their vision. Besides being used for astronomical purposes, binoculars have become a must-have for hunters and marine adventurers. Prisms are also an excellent alternative to mirrors in enhancing one’s vision.

Understanding the various types of prisms used in binoculars will allow you to use them more effectively. Also, having the right knowledge about the different kinds of prisms will help you choose the right one for your needs.

Besides having the proper knowledge about the various types of prisms, you also need to know how they work in a pair of binoculars to ensure that you get the best possible product.

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