How Do Roof Prism Binoculars Work?

Ever wondered how do roof prism binoculars work?. Well, this is another common question by many binoculars lovers like you and me. Most people prefer roof prism binoculars to Porro prism binoculars but most haven’t really found out how they work apart from the fact that the look more appealing to the eyes, less bulky and are sleeky to use.

The Best Roof Prism Binoculars

After coming across this question a lot of times, I  decided to try my best and help find an answer for many people. But first of all, before we look at how roof prism binoculars work, lets first see why they are used in binos.

Why are Prisms Used in Binoculars?

The primary purpose of prisms in binoculars is to correct image orientation created by the lenses. The other purpose which they also serve is to fold the light path and make binoculars appear shorter than they would otherwise have been. Without prisms in your binoculars, you would have an image that is backward and upside down. If you want to understand prisms in binoculars, check out this article on understanding why prisms are used in binoculars.

As mentioned, because of the prism shape, light waves are folded and reflect off surfaces to produce an erect image. This results in a shorter optical path allowing for a compact binocular design.

What is Roof Prism Binoculars and How Roof Prism Work?

Roof prism binoculars are simply binoculars whose prism type is made of roof prisms. There are two prisms in a roof prism assembled binoculars. This prism type is also called Dach or Dachkanten prism which is from German. Roof prism is a reflective optical prism containing a section where two faces meet at a 90-degree angle. The set two 90° faces resemble the roof of a building hence giving it that name.

Тhе fіrѕt рrіѕm hаѕ оnе ѕurfасе thаt hаѕ nо іntеrnаl rеflесtіvе quаlіtіеѕ. То fіх thіѕ, іt nееdѕ tо bе аррlіеd wіth а ѕресіаl соаtіng ѕuсh аѕ а mіrrоr соаtіng іn оrdеr tо rаіѕе іtѕ rеflесtіvіtу tо lіmіt lіght lоѕѕ. Тhе ѕесоnd рrіѕm hаѕ а роіnt whеrе thе lіght rеflесtѕ оff аn еdgе thаt rеquіrеѕ mаnufасturеrѕ tо uѕе аdvаnсеd tесhnоlоgу tо rеduсе сhrоmаtіс аbеrrаtіоnѕ ѕuсh аѕ соlоr frіngіng аnd dоublе vіѕіоn. То fіх thіѕ, uѕuаllу а рhаѕе-соrrесtеd соаtіng іѕ uѕеd tо kеер lіght-wаvеѕ іn-рhаѕе.

The reflection that comes from the two faces returns an image that is flipped laterally across the axis where the faces meet. One feature of a roof prism is that its beam splits in half, with one of the beam hitting the first one face and then the other face.

High precision is required when setting up roof prisms with focus on the focal planes or the edge of the roof would introduce slight distortions and the angle between the two faces must be close to 90 degrees otherwise the image would reduce in quality.

Types of Roof Prisms

There are variants of roof prisms used in binoculars out there. These three popular ones are:

  • Amici
  • Schmidt-Pechan
  • Abbe-Koenig

The  Amici is the simplest roof prism of them all. But the most popular roof prisms used in binoculars are the Schmidt-Pechan prism and the Abbe-Koenig prism. They all essential perform a basic function, that is to keep light-waves entering and exiting the binocular in a straight line. Hence you will often find roof prism binoculars aligned eyepiece-to-objective lens construction.

Abbe Koenig Roof Prisms VS Schmidt Pechan Roof Prisms

These two variations of roof prisms are the most popular. With roof prisms been the most commonly used prisms in binoculars, you sure need to know these two. Each of these prisms is named after its inventor.

The first main difference is the physical length between the two. The Schmidt-Pechan roof prisms binoculars are shorter compared to the Abbe-Koenig roof prisms binoculars of the same magnification and objective lens size.

Abbe Koenig roof prisms VS Schmidt Pechan roof prisms
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Schmidt Pechan prisms are produced all over the world and are a lot more common, while there are only six manufacturers in Europe that produce Abbe Koenig prisms. These are Zeiss, Docter/Noblex, Swarovski, Leupold, Sig Sauer, and Optolyth. They’re really hard to produce and that’s why they’re a lot more expensive than the Schmidt and Pechan prisms.

If you are used to binoculars, you will agree with me that the Abbe-Koenig roof prism binoculars are more expensive. But the good side is that you gain better light transmission. There are a lot less internal light reflections which mean more quality images.

Schmidt-Pechan roof prism binoculars are meant for everyday use and can be made in very compact forms while the Abbe-Koenig prism binoculars are mostly in the configuration of 8×56 or 10×56 with a big objective lens and are meant for low-light.

Common Roof Prism User Cases.

Roof prism binoculars are stylish and easy to hold. Plus, they are appealing to look at and to have so you will mostly find many manufacturers now make them compared to Porro prism binoculars. Here are some user recommendations for roof prism binoculars.

  • Bird Watching
  • Hunting
  • Sports events
  • Wildlife viewing and Safari
  • Hiking
  • Travel
  • Nature observation
  • They are ideal for almost all outdoor enthusiasts because they are more compact, lightweight and mostly weatherproof.

I hope this was helpful to you.

The Best Roof Prism Binoculars

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