Roof prisms

If you are faced with the need to choose binoculars, then you probably already noticed that modern binoculars have either a direct design or offset. In the first case, the binoculars are similar to 2 spotting scopes fastened together. The second type has an offset shape.

A roof prism system is a type of prism constructions in which an optical axis of the lens and eyepiece coincide. A beam of light goes in a straight line from an objective lens into an eyepiece, and this fact allows to create compact binoculars. The binoculars with a roof prism system are easy to recognize: their objective lenses stand straight in line with the eyepieces. These are thin, elegant models.

For binoculars with Porro prisms, the eyepieces are offset inward from the line of the objective lenses. Such an optical type has both advantages and disadvantages. It is impossible to say definitely which type of prism you need; choose based on your needs. We’ll talk more about choosing binoculars on Roof-prisms a little bit later. Now let’s deep learn more about the Roof prism system itself.

Roof prism system

So, you are in a store, and you see compact straight-shaped binoculars. What is inside the pipe?

Inside, between its eyepiece and its objective lens, there a system of prisms. Their reflective surfaces set at an angle of 90 degrees to each other and resemble a roof. All reflective surfaces are placed under each other (not side by side how it would be in a Porro prism system). A beam of light passing through the objective lens hits a prism and refracts. Refraction occurs in different ways, depending on the type of structure.

For now, four types of roof-prism systems exist:

  • Amici roof prism
  • Abbe–Koenig prism
  • Schmidt–Pechan prism
  • A pentaprism

Amici roof prisms

reflect the image 90 degrees and flip it. This type of prism is most commonly used as a diagonal mirror of telescopes to correct the direction of the image. Amichi prisms are the simplest of roof prisms.

A pentaprism

can also be used in making roof prism systems of binoculars, gives a mirrored image. It is often used in mirror cameras.

The following two types of prisms can most often be found in binoculars:

Abbe–Koenig prism

Light is reflected three times in this type of design – less than in the Schmidt-Pechan prism. Such a prism is much more difficult in manufacturing so that binoculars with Abbe-Koenig prisms are more expensive. But the image quality is higher, so it’s worth it. Such binoculars are perfect for observation at low light. In Europe, the companies Zeiss, Docter/Noblex, Swarovski, Leupold, Sig Sauer, and Optolyth produce Abbe-Koenig prisms.

Binoculars with Abbe–Koenig prism are:

  • Expensive;
  • Form a qualitative image with minimal distortion;
  • Often elongated due to prism design;
  • Good for low lighting.

This type of prisms is often used in professional binoculars for birdwatching.

Schmidt–Pechan prism

Binoculars with Schmidt-Pehan prisms are much cheaper than binoculars with Abbe-Koenig prisms and are more compact. But the loss of light in them is significantly greater due to their design. Schmidt–Pechan prisms are used in constructing of compact everyday binoculars for standard tasks. You can often find them in binoculars of the companies Nikon, Leik, Svarovski.

Binoculars with Schmidt-Pechan prisms are:

  • Cheaper than binoculars with Abbe-Koenig prisms;
  • Give an image not perfect, but enough qualitative;
  • Suitable for everyday tasks;
  • Can be as compact as possible.

Roof prism binoculars diagram

Let ‘s look inside and see what these prisms are.

What does roof prism binoculars mean?


The main advantage of roof-prisms binoculars is their compactness. If we compare binoculars on roof prisms and on Porro prisms at the same increase, binoculars on roof prisms will be more compact and thin. But the image quality is likely to be a bit worse. Such binoculars are difficult in manufacturing, so binoculars on roof prisms, equal in image quality to binoculars on Porro prisms, will cost more.

In the middle price segment, Porro binoculars give a brighter and more contrast image.

Quality is expensive

The higher the increase in binoculars with roof prisms, the more the image resolution suffers. After all, in the prism, the beam of light is broken into parts and comes out into the eyepiece already with light losses. Therefore, in expensive binoculars, prisms are provided with a dielectric coating that reduces losses. Enlightenment coating of objective lenses also helps to save brightness and colors.

Image quality also depends on production material. It can be:

  • Glass SK15 – is a glass of the highest quality with a crystal-clear image.
  • Glass BAK-4 – is a Barium Crown Glass. It allows for creating more compact binoculars of the average price segment. There are insignificant light losses.
  • Glass BK-7 – is a Crown glass with Bor. Image quality is worse than that of BAK-4, but it is still a special optical glass. Binoculars with such BK-7 are the cheapest.

BAK-4 and BK-7 are the most popular for making prisms. You can learn more about the glasses of binoculars prisms in our article about BAK-4. Actually, a regular, non-special optical glass, is also used in binoculars. But I don’t recommend you buy them.

Waterproof and durable

Roof prism binoculars are better for sport and activity. They are easier to make sealed, for example, to fill the case with nitrogen. It will protect binoculars not only from moisture but also from fogging. For hunters or sportsmen, poor visibility can cause serious consequences. If it’s important to you too, take a look at roof prism binoculars in a rubberized nitrogen-filled case.

Who are roof-prisms binoculars suitable for?

  • Tourists;
  • Athletes;
  • Hunters, ornithologists;
  • You, if you are looking for expensive quality binoculars.

An interesting fact:

In roof prisms binoculars the distance between lens centers (interpupillary distance) is equal to the distance between the center of their eyepiece. So that you will not see on sale such binoculars with objective lenses more than 65 mm. Such binoculars will not be possible to use: most of us have a smaller distance between the eyes.

Still, sometimes it happens that binoculars are too small for its owner: the maximum interpupillary distance is not enough. You have to pay attention to it, too. It can happen as to Roof Prism Binoculars, as to Porro ones.

Still, the vast majority of modern binoculars can increase the interpupillary distance to 72 mm. This should be enough for everyone because our interpupillary distance is about 66 mm.

After studying all this, I, like many others, had the question: why such difficulties? Why do you even need these prisms? It turns out that without them, the image would be turn upside down and from right to left. I would have to make my observations standing on my head!

Well, hope it was interesting and useful for you!

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