Why Are Prisms Used In Binoculars Instead Of Mirrors?

Binoculars are tools used to facilitate viewing of objects at a far distance clearly and reliably. They are designed to use a technology known as optics. Binoculars are made as an advanced entry of telescopes as they view objects through both eyes. the question may always be why are prisms used in binoculars instead of mirrors?

The device has lenses on both sides, which is used to focus light and magnify images through the magnification process. This helps in bringing the object closer to your eye when you view through the lenses.

However, the process of magnifying is not that simple.

The objective lens picks objects in an inverted shape or reverse form. As the light rays fall on the lens, the image is transmitted to the prism where it is flipped and sent to the eyepiece in a 3D for you to see it.

With that information shared, it forms the basis of an argument why use prism instead of mirrors.

For starters, it’s good to note that mirrors absorb a portion of light hence reducing the quality of images through the lens compared to normal appearance.

Here are a few other reasons why binoculars use prisms instead of mirrors.


When it comes to binoculars, this is an investment that you don’t keep changing with seasons.

Therefore, one would want to buy a device that will last longer and still maintain the quality of performance. Mirrors use a coating that often gets oxidized depending on the sealing design.

This is to say, if your pair of binoculars use mirrors, it has to be placed in an argon or nitrogen seal to be effective.

In other words, if the binoculars are not in either seal, the results you get from the device will not be convenient.

Additionally, if the coating on the mirror is removed, the image that will get to the eyepiece will still be in an inverted or reverse formation.

Therefore the binoculars will not serve the purpose of ensuring that you get clear and quality images from a distance.

On the other hand, prisms don’t require any special conditions for it to perform. Therefore, you will not be worried about where you store your binoculars between uses for it to keep performing the same.

Prisms are also known to stay collimated even when the device is subjected to numerous abuse. You don’t expect your binoculars to flip images or shift focus by simple mismanagement as compared to mirrors.

The fact that mirrors also absorb a considerable amount of light transmitted is also something to be concerned about.

Prisms reflect light effectively as compared to mirrors. They are internally reflective, meaning there is little light lose when transmitting it from one surface to the other.


The size and structure of mirrors are heavier and bigger compared to the prism. Therefore, when binoculars are designed using mirrors, then it gives the user a hard time carrying the device on various adventures.

Comparing between the two binoculars, one made of mirrors while the other prism, one with a mirror, will have a bigger design.

Additionally, the quality of image projection will also vary hence making the device unfit for extreme adventures.

Therefore, carrying binoculars made of mirrors will be not only stressful to use but also unworthy.

The experience of having a heavy device hanging on your shoulders to result in low-quality performance can be frustrating.

However, with the use of a prism, such challenges will not be an issue.

Prisms are lighter, and the quality of image projection is not compromised. Therefore, it ensures that you get to view objects and get precise details in a 3d setting.

Types Of Prism Used In Binocular

There are two common prisms that are used in designing binoculars the roof prism and Porro prism.

The two prisms in their unique ways lengthen the distance between the objective lens and the eyepiece for a better image projection.

Therefore, whichever style you use, you are guaranteed that the performance is extremely incredible, and all your images will be projected in a 3D setting.

However, the design of each prism differs from the other hence making them different.

However, this difference is only visible in the glass setting, but the functionality is much the same.

The two prisms are placed in straight tubes, which makes the size of the binoculars to be smaller and compact.

Porro Prism

In this prism, light is transmitted through an optical path from the objective lens to the second focal plane.

It is then reflected in the first focal plane, which then reflects it to the eyepiece. In this reflection process, a considerable amount of light is lost from the entry point through to the exit point.

However, this issue can be resolved by subjecting the prism to an anti-reflective coating to minimize light loss.

Additionally, the coating guarantees to enhance image transmission for better viewing.

The distance between the second focal plane and the eyepiece is minimal.

This guarantees that there is sufficient room for inverting the image before reaching the eye.

Therefore, up to hitting the first focal plane, the image will be formed in an upright position allowing you to see it as it appears on natural.

Roof Prism

Roof prism, on the other hand, is designed in a different style from the Porro prism.

In this category, the optical path appears to be on a straight line from the objective lens to the ocular lens. One prism, in this case, has no internal reflective properties so, the light reflects on one prism.

However, with roof prism, you are guaranteed to reflect up to 99% of the light that goes through the objective lens.

The prisms are located in an adjacent style, allowing them to meet at the roof of the binoculars tune.

When light enters the tube through the objective lens, it is transmitted straight to where the two prisms meet (at the roof) and reflect back toward the eyepiece.

Compared to the Porros prism, the reflection distance in roof prism is direct and maintains the same line of the path.

Additionally, the prism doesn’t require any special coating for it to function as compared to Porro prism. With this, one can be able to choose the best porro prism binoculars

What Makes Prism Better Than Mirrors

Prisms have an internal reflective mechanism allowing sufficient light transmission for easy observation.

Therefore, the performance of your binoculars will not be subjected to any other operating mechanism.

Also, even without upgrading the glasses with either dielectric, silver, or aluminum coating as in mirrors, you are still guaranteed of high light transmission.

On the other hand, mirrors will still lose considerable light even with extreme coating.

Therefore, the quality of your image will always be compromised, especially when there is a limitation of natural lighting.

Also, your binoculars have higher chances of losing focus when exposed to a complicated situation,


Whether you have binoculars with prism glasses or uses mirrors, the bottom line is in how you operate it.

However, one made of prism glasses has higher chances of giving accurate results compared to the other. Taking care of the optical device also plays a major role in how the device will serve you and maintain quality performance.

Therefore, if you are thinking of getting a pair of binoculars or upgrading your current one, ensure to enquire about the glass used in manufacturing.

This way, you are guaranteed that you will invest in a reliable product that will serve you for a considerable time.

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