Binocular prisms – why are they so weird and different?

Bill, Optics Central’s resident birdwatcher and all-round optics nerd, explains prisms in binoculars. He starts with how Porro prisms which are found in the traditional shaped binoculars work. Then, he goes into more complex roof-type prisms. These roof prisms are the ones you’ll find in the more modern “straight through” design of binoculars.

Binoculars, including their prisms, are a surprisingly complicated beast. When designing a pair, manufacturers have a lot of choices. These will determine the cost of the product, the quality of the image they produce, as well as other attributes such as weight and ergonomics. In this blog I talk about just one of these things: prisms. I also show off some of my more complicated photos. I love this job!

What’s inside those binoculars?

We sell lots of binoculars at Optics Central. Most of our customers are bird watchers or people who are travelling and want to see the view. We also get sport fans, hunters, and others. It’s a pretty diverse bunch.

But one thing that everyone asks is why some binoculars are better than others. What goes inside them to get the best image?

The answer is complicated, because there are so many things that go together to determine how well a binocular performs.

However, one of the most important choices binocular makers have is what type of prism to use. Prisms are essential (unless you happen to be standing on your head). They affect a pile of different things, not just the image brightness and sharpness, but also the weight, balance or length of the binocular.

Personally, I’ve always found prisms to be utterly fascinating. When you look into one, it’s like looking into the TARDIS. There are any number of rooms inside that little box, and the light bounces around any number of times. But I had no idea they were so complicated until I started looking into the subject.

Porro and roof binoculars

Birdos talk about binoculars a lot, and they use the terms “Porro” and “roof” to describe the basic types. These terms describe the two basic types of prisms inside the binoculars. The traditional type of binocular – the one that has a zig-zag barrel – has a “Porro” prism. The newer, “roof prism” binocular simply has two straight barrels.

Here are some examples, side-by-side. You can see the zig-zag in the elderly Zeiss, built to house the Porro prism. The Nikon at the top just looks like a tube.

But the image is upside-down!

Binoculars are essentially two Kepler-type refractor telescopes side-by-side. There’s a problem with this, though. Kepler’s telescope produces an upside-down image. If you want to design a useful binocular, you need a way to flip the image right way up.

(As an aside, the original Galilean refractor had a convex objective lens at the front and – weirdly – a concave eyepiece lens. You can’t get much magnification without making the binoculars too large to hold. But at least the image is the right way up. This is how they make modern opera glasses. Here’s a quick and dirty explanation of telescope types.)

Prisms

Back to the problem of how to flip an image so it’s the right way up. You can do this with a prism. In fact, it’s mind-boggling what you can do with light rays and a prism.

…and a mathematics degree.

Put simply, a prism is a chunk of glass (or similar transparent material) that has nicely polished faces. These faces are arranged so that light moving into the prism gets bounced around inside the prism. Eventually, it comes out in the direction you want it to.

How does a prism work?

(Warning: science content!)

The concept of “total internal reflection” is complicated. It’s got something to do with the density of the glass affecting its “refractive index”. It gets complicated,but in short, there are two rules we need to know about. (I’ve simplified it a bit).

  • If light is approaching the back wall of the prism dead on, most of the light goes right out of the prism. It leaves without changing direction at all. It’s like the wall of the prism is a window.
  • If light is coming up to the back wall at more than a particular angle, then none of the light gets out of the prism. It’s like the wall of the prism is a mirror. This is the “total internal reflection” I mentioned.

Armed with these rules, cleverer people than me can design prisms that do pretty much anything.

Prisms in binoculars

Prisms in binoculars have to do one critical thing – put an upside-down image the right way up.

As a plus, they can lengthen the distance the light has to travel between the front (objective) lens and the back (eyepiece) lens. This means the binocular maker can get more magnification without making the binocular too big to handle.

A prism can do both of these by bouncing the light around inside itself a few times. There are two main types of prisms that do this: Porro prisms and roof prisms.

Porro prisms

A Porro prism is about as simple as you can get. This is good, because it makes them cheap to manufacture. It’s simply a right-angled isosceles triangle in three dimensions (simple?). Here’s a diagram. It’s just a kid’s building block made of glass.

A simple Porro (right angled triangular) prism

Where the magic happens is when you look in left or right half of the bottom. Here’s a photo. By the way, when you look at these photos, think about where the light that’s hitting the camera is coming from.

Single Porro prism and ray diagram

In this case, we’ll follow a light ray that shines off the letter F I’ve drawn on paper and stuck on a bit of Blu-tac. The F is backwards in the diagram because we’re looking at the back of the piece of paper.

Looking at the diagram, the light goes away from us and hits the front wall of the prism pretty much dead on (at point A in the diagram). Because of the “dead on” rule above, the light goes straight into the prism without changing direction. Then, (at B in the diagram) it hits the left back wall of the prism, which is angled at 45°. Now, the light ray follows the total internal reflection rule, and bounces off to the right. It crosses to the other side of the prism and hits the right back wall (at C) and bounces towards the camera. Still inside the prism, it then meets the front wall of the prism (at D) dead on, so it goes straight out to the camera.

Phew, it’s like a pin ball machine!

Sharp-eyed readers will notice how the F is readable, not a mirror image. But that’s not good enough. It’d be a pretty useless pair of binoculars if it only showed you what was behind you!

If we put in a second Porro prism, we can turn the light ray around again so you aren’t looking over your shoulder. Here it is on my desk.

By the way, the photo on the right shows the same as the one on the left. It just shows the position of the little F sign, and more of how the Porro prisms sit together.

A pair of Porro prisms lying flat

But it’s still not right. While the image is readable, it’s not upside down like we need it to be to counteract the lenses in the binocular.

So we rotate the second prism through 90° to get the right effect. And here it is.

A pair of Porro prisms

Pardon the Blu-tac, the fingerprints and the general messiness – it was hard to set up the shot. However, the F sign is in front of us, it’s readable, and upside-down. Phew, finally!

Porros in a binocular (or monocular)

Using this arrangement, clever optical engineers designed the Porro style binocular, like this monocular. (This is the one that I pulled the Porro prisms out of for the photos above.) I’ve drawn the light rays in located the lenses, but it’s pretty much as in the shot above.

Light path through a monocular with Porro prisms

Roof prisms

Roof prisms are more complicated. I’ll try to explain what’s going on with some simplified diagrams.

There are two common types of roof prisms. Both have a “roof” section, which has the effect of reflecting the image left-to-right. In the diagrams I’ve made below the roof sections aren’t all that obvious, but they’re above a thin blue line.

Abbe-König roof prism

The simpler type is called the Abbe-König roof prism. All these prisms are named after their developers, by the way.

Light enters the prism and is internally reflected upwards before being flipped in a roof section. It then moves down into the second element and reflects to be flat again.

Abbe-König prism

This is an elegant design, and light only bounces four times. (The diagram only has two dimensions. You can’t see the bounce that pushes light from one side of the roof to the other). Every time the light reflects, the image gets a little duller and less sharp. So the image that gets all the way through an Abbe-König prism is sharp and bright.

However, there are problems with constructing these prisms. They need to be long, because at no stage does the light bounce backwards. So they take up more space inside the binocular barrel, and this makes the bins larger and heavier. For the same reason, they also tend to make the bins front-heavy, which a lot of users don’t like. Very few companies make this type of prism, and this makes them rarer and more expensive.

Schmidt-Pechan roof prism

This is a more complicated design of roof prism. Light entering the prism bounces six times (twice more than in the Abbe-König prism). So the images produced by these prisms aren’t quite as bright or crisp.

Schmidt-Pechan prism

However, apart from having the phase-coating requirement in the roof section, there a heap of advantages to the Schmidt-Pechan prism. They’re cheaper, easier to produce, small and lighter than Abbe-König prisms. As a result, binoculars with Schmidt-Pechan prisms can be smaller and cheaper.

In the photos below I’ve pulled a Schmidt-Pechan prism out of a pair of old binoculars, so you can see how it works. The prism is in a chassis, and the right-way-up F appears upside-down when viewed through the prism. The magic of science!

Schmidt-Pechan prism in a chassis showing how the image is flipped

So what’s best?

The more times it bounces, the dimmer and degraded the image gets. Have a look back at the Porro prism diagram – the light only bounces four times. This is the same as in the Abbe-König prism.

Roof prisms need to have phase coatings as a result of image dimming, but Porro prisms don’t need these because they don’t have the roof-shaped sections.

So why aren’t we all using Porro binoculars? Well, some of us are. Roof prism binoculars, because of their simpler shape, are more robust.

Also because of the shape, roof prism binoculars are easier to waterproof. That’s because there are way fewer corners you have to seal.

Ultimately, it’s going to be up to you to decide which is best.

If you want the absolute best, you’ll end up with a super-premium pair with Abbe-König prisms. These will have all-total internal reflection configuration which needs no dielectric mirrors, but will still need phase coatings. It’s going to be big and heavy. As far as I know, the Gold Standard binocular is the Nikon WX. You can’t get these any more, but they went for around $11,000 and weighed about 2.5 kilos.

If you’re on a budget, you’ll be looking at a non-waterproof Porro or a Schmidt-Pechan without phase coatings. Inexpensive, and in bright sunshine, the difference might not be all that bad.

My current pair is an old Nikon Monarch ATB 8.5×56, which has a Schmidt-Pechan prism with phase correction. Not too heavy, good image, waterproof and it doesn’t break when you drop it.

What’s your opinion?

Further reading

https://baltimorebirdclub.org/choosing_optics.pdf

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, how to take photos of the sky, or even how to see some rare birds.

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