Roof Prism vs Porro Prism? BK7 vs BAK4? Binocular Glass 101

If you’re a guy or gal who has a “type”, you might be perceived as being shallow. But, in the optics world, that preference will certainly help you in the buying process.

And, when you’re right in the middle of shopping for that one, perfect binocular, the first part of the elimination process will have you asking yourself something like “roof prism or porro prism?”.

If you can arm yourself with everything there is to know about the two prisms, you’ll be able to make the best binocular buying decision of your life… And, perhaps it’ll be the one bino that you can really settle down with for the rest of your hunting days.

In this article we’ll help you determine the differences between the two prisms and which one could potentially be for you. And, since you can’t really talk about prisms without talking about glass, we’ll get into glass types too.

But, first off…

Why Prisms At All?

You’d think that when you looked through binoculars, you’d be able to see your image without anything too fancy going on behind the lenses right? I mean, binoculars are also kids toys, there can’t be much to it? Well…

Without prisms, you would have an image that would be backwards and upside down. I don’t even know what that would look like – a whole mess of a view if you asked me.

Tip
Prism assemblies correct the orientation of the image so that you view it the right way up and facing towards you. This is why technically they’re called “erecting prisms”.

Now, how the prisms are put together are where roof and porro come into play.

Porro Prism Binoculars

These types of binoculars have the more iconic and traditional binocular design, and they’ve been named after the Italian inventor Ignazio Porro. Additionally, there are actually two types of porro prisms: the standard porro and the reverse porro. The Redfield Renegade 10X50 binocular is an example of a Porro Prism binocular.

Standard Porro Prism
The standard porro prism system has offset eyepieces to objective lenses. The eyepieces are closer together while the objective lenses are further apart.

Reverse Porro Prism
The reverse porro prism system also has offset eyepieces to objective lenses. Instead, the eyepieces are further apart while the objective lenses are closer together.

How do Porro Prisms Work?

porro_prism_binoculars
Image Source: HowStuffWorks.com

Porro prisms incorporate an optical path that directs light in a Z or zig zag-like pattern.

All surfaces in a porro prism bino are completely internally reflective. This means if optical coatings and light transmission are the best, there will be no light loss.

Light-waves in a porro prism are only reflected off surfaces a total of four times.

To sum it up – light comes in through the objective bell, is directed through the reflective porro prism system in a Z pattern with four reflections to bring the image upright and forward to reach your eye.

Since light-waves are “folded”, this explains why there needs to be that wide and offset construction in a porro prism bino.

Tip
The wider and set-apart objective lenses are actually what helps with creating that stereoscopic image where you have a 3D-like, rich depth of field.

Benefits of Porro Prism Binoculars:

  • All internal surfaces are reflective – no light loss due to prism system
  • Only four reflections in porro prism limiting further likelihoods of light loss
  • 3D-like and rich depth of field
  • Cheaper to manufacture and therefore, lower cost for consumer
  • Can spend the same amount on a mediocre roof prism bino for a high-quality porro prism
  • Basic optical quality is superior to roof prism bino

Drawbacks of Porro Prism Binoculars:

  • More difficult to weatherproof
  • Typically bigger and bulkier than roof prisms

Porro Prism User Recommendations:

nikon-aculon-a211-8x42-binocular-review
The Nikon Aculon A211 8X42 Bino Utilizes a Porro Prism System.
  • Hunting
  • Bird Watching
  • Wildlife Observing
  • Sporting Events
  • Hiking

Porro prism binos are excellent tools for all outdoor activities, but just beware, you won’t want to be caught out in less than ideal weather.

Tip
Hunters on a budget might want to upgrade to a high-quality porro prism binocular rather than spend the same amount on a mediocre roof prism bino.

Roof Prism Binoculars

victory-ht-8x42
The Zeiss Victory HT 8X42 Bino features an Abbe-Koenig Prism System.

Roof prisms are named for the assembly’s roof shape. You may have heard roof prisms also called as Dach prisms.

Here’s a little language tidbit for you – Dach means roof in German.

And, if you know anything about optics and glass, it’s that Germans certainly have a solid footing in the optics industry.

Now, there are a few variations of the roof prism design. There is the:

  • Abbe-Koenig
  • Schmidt-Pechan
  • Amici

Essentially, they all maintain the same basic function – to keep light-waves entering and exiting the binocular in a straight line.

This is why roof prism binos have an aligned eyepiece-to-objective lens construction.

But, light-waves may take a little bit of a different route within the optical path of the roof prism assembly with the different roof prism variations.

How do Roof Prisms Work?

roof_prism_binoculars
Image Source: HowStuffWorks.com

Light-waves reflect off the roof surfaces within the assembly a total of six times.

There are two prisms in a roof prism assembly.

The first prism has one surface that has no internal reflective qualities. To fix this, it needs to be applied with a special coating such as a mirror coating in order to raise its reflectivity to limit light loss.

The second prism has a point where the light reflects off an edge that requires manufacturers to use advanced technology to reduce chromatic aberrations such as color fringing and double vision. To fix this, usually a phase-corrected coating is used to keep light-waves in-phase.

To sum it up – light enters in the objective bell through to the two prisms in the roof assembly. The light path is leads through a course of six reflections to produce a horizontally and vertically erect image. Light then exits the binocular through the eyepiece in the same straight line that it entered the objective bell.

Since all of this happens in a straight line, this explains why you can have a more compact, streamlined, and weatherproof binocular.

cl-pocket-10x25
The Swarovski CL Pocket 10X25 features the Schmidt-Pechan Prism System.

Benefits of Roof Prism Binoculars:

  • Can be optically superior to porro prisms with quality phase-corrected and mirror coatings
  • Streamlined and sleek look is more attractive than porro prisms
  • Lighter weight and more compact build
  • Easier to weatherproof

Drawbacks of Roof Prism Binoculars:

  • Requires advanced technology to manufacture
  • Is more expensive than porro prism binoculars
  • Requires more internal reflections with higher chance of light loss
  • The best roof prism binos will be very pricey to get the best technology and coatings

outland-x-10x42
The Celestron Outland X 10X42 is a popular low-budget Roof Prism bino.

Roof Prism User Recommendations:

  • Hunting
  • Bird Watching
  • Wildlife Observing
  • Sporting Events
  • Hiking

They’re ideal for almost all types of outdoor enthusiasts because they’re often compact, lightweight, and fully weatherproof.

Tip
To really get the optical benefits of roof prism binoculars, you’ll want to spend on the pricier end for quality coatings and the technology used to ensure in-phase light-waves to reduce optical aberrations.

Glass 101: BK7 vs BAK4

There’s a lot of confusion when it comes to glass, so let’s clear up the scene with a basic glass 101 lesson. Why?

Just as important as prisms are in binocular function, so is the type of glass that the prisms are made out of. The glass has to go through processes such as grinding and polishing to ensure a sharp, clear, and bright image.

There’s a lot of pressure on glass to perform optimally. They need to:

nikon-monarch-5-10x42-binocular-review
The Nikon Monarch 5 10X42 bino features both ED and EcoGlass.
  • produce a correct oriented, erect image
  • allow as much light in as possible
  • maintain in-phase light-waves
  • be as reflective as possible
  • prevent as much light loss with each reflection as possible
  • transmit as much light to the edges of the field of view as it can

All of this just so that you can see that once in a lifetime trophy or the rare bird that you traveled miles away for at 4 A.M. in the morning.

This brings us to types of glass elements that are sometimes included in making your image perfect.

ED Glass
Extra-Low Dispersion or low dispersion glass provides High Density and High Definition images because it keeps light-waves in-phase, from bending, and from producing various types of aberrations.

Eco-Glass
This is when the actual glass is made without toxic chemicals such as lead and arsenic. When you go to toss your binos, if you ever have to, you know that you’re not contributing to chemical pollution.

Now, back to BK7 & BAK4…

Too many times you will find misinformed buyers that think BK7 and BAK4 are prism assemblies instead of the glass that the prisms are made out of.

So, let’s go over what exactly these mysterious elements seem to be.

BK7 Glass

BK7 is a term designated by Schott AG (a German authority on glass worldwide). It’s made out of borosilicate (crown) glass, and it’s highly reflective with a refractive index rate of 1.5168.

Myth: BK7 glass is only found on porro prism binoculars.

Truth: BK7 is found on both porro prism and roof prism binoculars. They also help to keep costs low.

How to Identify BK7 Glass

Step 1
Hold up your binos to a bright background or light to view the exit pupil.

Step 2
Tilt the bino on angles, in circles, in any direction to get a full view of the exit pupil.

Step 3
Identify the squarish, clear zone within the center of the exit pupil. You should also be able to notice the shadowy or gray cut-off edges of the exit pupil.

But, how will the cut-off zone of the square exit pupil affect you?

Now, you’re asking the good questions! You’re going to have a vignetting effect with darker viewing and color loss around the periphery of the edge of the field of view. This can get worse the wider field of view you have. But…

colorado-10x25
The Barska Colorado 10X25 is a low-budget bino that utilizes BK7 Glass.

Pay attention to the exit pupil size. If your BK7 prism glass binocular has an exit pupil of 4 mm or larger, and you’re primarily using it for daytime glassing, your pupils are only going to be open to really no more than 3 mm.

If your pupils are only taking in light between the clear and open part of the exit pupil, you’re not going to be affected too much by the cut-offs of the BK7 exit pupil shape anyway.

If your pupils are dilated much wider than the exit pupil, say during low light conditions, the cut-offs and the vignetting effect will be an issue for you.

Tip
With all of this in mind, if you’re considering a BK7 prism glass bino, shop for one with a larger exit pupil.

Benefits of BK7 Glass:

  • High refractive index rate
  • Most commonly used type of crown glass on binoculars
  • More affordable than BAK4 prism glass binoculars

Drawbacks of BK7 Glass:

  • Squarish exit pupil with gray cut-off edges
  • Slightly lower refractive index rate compared to BAK4

BAK4 Glass

meopro-hd-10x42
The Meopta MeoPro HD 10X42 features Schott BaK4 Glass.

BAK4 is another term coined by Schott, and it’s supposed to stand for BaritleichKron which is Barium Crown in German. Barium is another type of crown glass, and it’s highly reflective with a refractive index rate of 1.5688. As you can see, this is barely higher than the BK7 glass.

Myth: All binoculars with BAK4 prism glass is made from barium crown and is considered high-end.

Truth: Not all BAK4 prism glass is made equal.

Schott BAK4 is made from barium crown and has the refractive index rate mentioned above.

But, most of the time when you see BAK4 prism glass, the manufacturer doesn’t tell you that it’s Chinese-made BAK4 glass that’s actually made out of phosphate crown glass.

Tip
Chinese-made BAK4 prism glass has a refractive index rate of 1.5525 – lower than the real BAK4 but still higher than BK7.

But, why is everyone so hyped up about BAK4?

The higher refractive index rate and the complete, circular shape of the exit pupil will mean more light will be transmitted to the entire field of view. This will mean brighter, color-rich, and sharper edges of your periphery.

Benefits of BAK4 Glass:

  • Higher refractive index rate
  • Brighter edges of the field of view
  • Circular shape exit pupil

Drawbacks of BAK4 Glass:

  • Can increase overall cost of the binocular
  • Manufacturers don’t offer distinctions between Schott BAK4 and Chinese BAK4

To sum things up – BK7 has acquired an undeserved rep of being only a low-budget glass source since BAK4 doesn’t want to share the spotlight. But, the real differences between the types of glass used isn’t a big deal unless you’re using wide field of view binoculars with small exit pupils where the BK7 glass may not cut it.

Bonus Info: Bino Glass Coatings

Not only is prism type, glass elements, and type of glass important about a binocular, the coatings can make all the difference. None of the information we’ve just covered would matter if you don’t have the coatings to match the prisms or the glass.

What are Lens Coatings?

bx-1-yosemite-6x30
The Leupold BX-1 Yosemite 6X30 has 64 layers of coatings!

This is usually a proprietary formula that enables the lenses to allow a higher transmission of light-waves to reach your eyes. The way light behaves when it makes contact with the lenses is largely dependent on whether or not they’ve been treated with coatings and how many times it’s been layered.

How to Define Binocular Coatings

  • Coated – one single layer coating to at least one air-to-glass surface
  • Fully-Coated – one single layer on all air-to-glass surfaces
  • Multi-Coated – multiple layers to one air-to-glass surface
  • Fully Multi-Coated – multiple layers to all air-to-glass coatings

But beware...

There isn’t a current industry standard to define multi-coated and fully multi-coated coatings.

You can have one manufacturer boast of having fully multi-coated coatings on a binocular but only apply it two to three times. Whereas, you can have another manufacturer applies 64 coatings to the lenses.

And, density and evenness of the layers are important in ensuring you have bright and clear views. Unless the manufacturer specially tells you about their coating process, you really can’t distinguish between the quality of the coatings.

Don't Forget!
Roof prism binos need additional, specialized coatings and mirror coatings to be functional and optically on par with porro prism binos.

6 Final Tips to Choosing a Prism and Glass Type

With all of this information on what to look for in a binocular, it’s hard to not feel overwhelmed. So, I’ve taken the initiative to act as match-maker. I’ve put together a few bonus tips to give you an idea of what to look for when scoping out your next beauty.

Tip 1
Don’t fall for the hype of BAK4 glass based on that alone. Although it has superior reflective and dispersion qualities versus BK7, you need to take into consideration coating quality, prism security, prism size, and other factors. You could do better with a BK7 that’s securely housed with the right size prisms and excellent coatings. My point? – Consider the other quality factors also.

Tip 2
If you want to spend the minimum amount of money possible on a binocular, a porro prism with BK7 glass is usually going to be the most economic buy.

Tip 3
If you’re convinced of a roof prism bino, plan to spend some decent cash for quality phase-corrected coatings. Without them, you may be able to get more bang for your buck with a porro prism binocular.

Tip 4
Do you want a compact and lightweight bino? – Roof prism. Do you not care much for size and weight? – Porro prism. Make sure to look at the specs – sometimes porro prisms aren’t that much bigger or heavier than roof binos.

Tip 5
When going with BAK4 glass binos and you want the best there is, double check that your optics manufacturer isn’t using the Chinese-made BAK4. Check for European glass optics such as Carl Zeiss and Swarovski.

Tip 6
The Chinese-made glass isn’t bad. Optical quality is still excellent and you can get a more affordable price than Schott BAK4 binos. Often times, highly reputable optics companies outsource their bino manufacturing but they still require very strict quality control measures to be met by the foreign contractors.

Now that you know everything there is to know, it shouldn’t be too long now before there’s a “save the date” event for your online purchase or your in-store buy. There’s nothing more titillating than gearing up for your big day. Happy bino hunting!

  • Elvis

    Thanks for the well documented information. I’ll be bino hunting for a holiday present and now I won’t look too dumb or uninformed. Great article.