Why binoculars use prisms?

why binoculars use prisms?

Probably at least once in your lifetime, you’ve seen how a prism looks like. Binoculars use prisms, that’s for sure. But why? What is the role of a prism in the binoculars?

Why binoculars use prisms? When light passes through the objective lens of the binoculars, the image is inverted. The prism main role is to erect the image, where another role of the prisms is to redirect the image in case the objective lens is offset from the optical path to the eyepiece.

Why binoculars use prisms? How are prisms used in binoculars?

The prisms in binoculars are in pair, each of the prism having the role to reflect the light which gets through the objective lens, correct the image and further send it to the oculars where you can see the image magnified, clear and not flipped.

If there were no prisms in binoculars, the image would be flipped upside down. Did you ever tried or experiment with a convex and a concave lens to align them in front of your eyes? You obtain magnification but the image is flipped upside down. I remember these experiments a lifetime back when I was a kid and I was dissembling binoculars.

The prisms are assembled into the binoculars in a specific angle to allow the light to be reflected as best as possible. The position of the prisms is different in every binoculars depending on the body shape and length.

What types of prisms do binoculars use?

There are two main categories of prisms used in binoculars: Porro Prisms and Roof Prisms.

The Porro Prism

This is the most common type of prism found in binoculars and its main role is not only to flip the image but for this to be redirected depending on the shape and size of the binoculars. Because of the Porro prisms, binoculars can have large objective lens diameters and be either bulky or small in size where the objective lens is offset from the optical path to the eyepiece.

The Roof Prism

The role of the roof prisms are only to erect the image and fold the optical path and these are used in binoculars where the objective lens are in line with the eyepiece. The disadvantage of roof prisms is that the image resulted is less bright due to the fact that a part of the light is lost. This is related mostly to the roof prism design, which makes the Porro prisms to be superior to roof prisms.

How the prisms can affect the quality of the image?

The prisms, either Roof or Porro can affect the quality of the image and plays a major role. I mentioned above that the roof prism’s disadvantage is that the light transmission is reduced due to the design, but what about the glass quality?

Glass quality of the prism and the manufacturing is crucial for the image to be as less as possible distorted and to be as bright as possible for the given objective lens diameter.

There are multi-coatings or fully-coatings which are applied to the prisms in order to improve brightness and the contrast. In general, this can affect the price of the binoculars, as more expensive binoculars may benefit from the coatings on the prisms.

Is it possible for the binoculars to work without prisms?

Trying to make some binoculars work without prisms is like jumping centuries ago when we first advanced in optics. It is relatively possible for the binoculars to work without the prisms if the objective lens is in direct line with the eyepiece. But the image would be flipped upside down.

In telescopes, this is another story as some of the giant refractors does not benefit from any prisms but mirrors to reflect the image and this does not erect the image. But think about, in astronomy, you don’t need too much to rely on the image to be seen exactly as with the bare eyes. When observing constellations, galaxies or any deep-sky objects, those are okay to be observed in any way.

Additional information

If you ask me which prisms to focus I would definitely say Porro prisms for regular binoculars but this is not always the case when aiming for high-end expensive binoculars where roof prism is phase-corrected and coated.

Bak4 vs BK7 glass

Two main types of glass in manufacturing prisms. Bak4 is superior to BK7 but is also found in more expensive binoculars. The Bak4 advantages denote that it has circular shape exit pupils, higher refractive index and have brighter edges on the field of view.

BK7 may not be as good quality as the BAK4 as when you observe through the binoculars with a BK7 glass prism, you may notice the edges on the field of view to be greyish and not as bright as the centre of it. If you hold the binoculars in front of your eyes leaving some distance, you probably would notice the square shape of the BK7. The BK7 is usually found in low-end binoculars and this is one of the main reasons I would never recommend for anyone to buy any low-budget binoculars.

Related questions:

Q1: What type of prisms do monoculars use?

A1: Monoculars uses roof prisms as the objective lens is in direct line with the eyepiece. There are monoculars using BAK4 glass prisms and BK7

Q2: Do telescopes use prisms?

A2: Yes, in general telescopes use prisms the same way as the binoculars, in special the models where the eyepiece is on a 45 or 90-degree angle from the telescope tube for easier observations. But keep in mind that not all telescope models are using prisms, and this occurs in special to some refractors.

Q3: Can binoculars affect your eyesight?

A3: If the binoculars are used incorrectly for a longer period of time, they are not collimated or calibrated this can affect your eyesight in time. I would recommend you to read our other article covering this subject here.

Q4: Can prism get condensation?

A4: Prisms can get condensation the same way as the glass inside the binoculars, if the binocular is not waterproof, weatherproof or fog-proof. The differences of temperature and humidity can cause condensation and in time, mould or fungus can be forming inside the binoculars and on the prisms. The binoculars with nitrogen-gas purged inside are made to be resistant to any type of condensation and for any fungus to be forming inside the binoculars.


Thank you for reading this article and I hope to see you around. Please check our other articles if you are interested. More information about binoculars prisms can be found on Wikipedia (external link). Take care for now!

About Gabriel Mihalcea

Passionate about binoculars, photography and blogging with years experience behind, I love to split my time and observe the beauty of this world with different eyes.

Check Also

Should I buy solar binoculars? – Eclipse and solar observations

Solar binoculars are somehow similar to normal binoculars but the front glass elements are acting …

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.