When BaK4 is not BaK4: Glass-types for Binocular Prisms

[BinocularSky]

The glass that is most commonly used for for binocular prisms is BK7. This is a Borosilicate Crown glass. Higher quality binoculars have used BaK4 glass; this is a Barium Crown glass. It has a higher refractive index than BK7; this means that its critical angle is lower and that less peripheral light will be "lost" through non-total internal reflection. When light is lost through non-total internal reflection, it gives rise to blue-grey segments ("cut-offs" in the exit pupil):

The designations BK7 and BaK4 when used above refer to glass designations used by Schott AG, a German manufacturer of optical glass.

A lot of the advertising hype on budget Chinese binoculars has been their "BaK4 prisms". This morning, I was looking up the data on these when I realised that the data I was seeing for BaK4 was not the same as for the data I remembered for BaK4 -- specifically, the critical angle was larger. So, a little bit later, this is what I have:

The Chinese designation "BaK4" is an entirely different glass to the Schott BaK4 -- BaK stands for Baritleichkron (Barium Crown); the Chinese BaK4 is actually Schott PSK3, which is not a Barium Crown at all: it is a phosphate crown. PSK3 is much cheaper to make than BaK4; it also has a lower refractive index.

Here are the important data:

Not only this, but the tolerances for bubble density is much laxer for Chinese BaK4 (PSK3); it permits twice the bubble density that is permitted in Schott BaK4.

What this means is that, whilst Chinese BaK4 gives you a smaller critical angle than Schott BK7, it is not as small as for Schott BaK4. For wide angle binoculars, this means that there is a greater likelihood of non-total internal reflection.

It also means that I have been entirely wrong when I have dismissed Chinese BaK6-prismed binoculars as "hype" -- the specs are actually closer to Schott BaK4 than is the Chinese BaK4.

It's not all down-side; the dispersion is lower than in Schott BaK4; this is relevant in binoculars that are collimated by prism-tilt because, if the "wrong" side is adjusted (P=0.5 for amateur collimation ), the amount of false colour that is introduces will be less. Of course, the difference will probably be disguised by lax optical quality elsewhere in the system, but....

The daft thing is that you can buy (from China) the Chinese equivalent to Schott BaK4 for exactly the same price as Chinese BaK4! Why the hell don't they just use the "proper" stuff!?!?!?

Edit: There's a useful chart at: http://upload.wikimedia.org/wikipedia/commons/b/bd/Abbe-diagram_2.svg

You can see the relative positions of Chinese BaK4 (PSK3), BaK4 and BK7. The Abbe number gives a clue about dispersion -- the higher it is, the less dispersion you will get. This is why, for example, BK7 is to be preferred to BaK4 for binoviewers.

[RogerTheDodger]

I'm always amazed at the knowledge on this forum, thanks and keep it up

[John]

Interesting, if a little alarming, stuff.

I wonder if the Chinese will re-invent the glass types used in refractor objectives in the same way

Up to now we thought that FPL-53 was supplied by Ohara but what is to stop someone producing another glass with inferior properties and calling it FPL-53 - or is that product name registered by Ohara ?

[BinocularSky]

I wonder if the Chinese will re-invent the glass types used in refractor objectives in the same way

There's actually a very simple solution: there is an "international standard" 6-digit designation for optical glass. It takes the first 3 (corrected) digits after the decimal point of the refractive index and appends the first 3 significant figures of the Abbe number. e.g. Schott BaK4 has a refractive index of 1.569 and an Abbe number of 56.1 -- its standard designation is 569561. This tells you all you need to know about its refractive and dispersive qualities (but nothing about bubble count, or QC in the factory).

Further Edit: Your FPL-53 is 439950

[Flying Eye]

I suspect this is going to be the bit which is going to make my ears start to bleed!

Nice work though!

It'll be great work if I ever get my head around it, it's like a hideous hybrid cross breed between the periodic table and whack'a'mole.

[Wobbly Bob]

I was doing OK until the BAK7,s for Binoviewers

Still, in general Bak 4 is the prefered choice for binoculars, YES ?

[Photosbykev]

Does anyone actually think that China will comply with any international rules, guidelines or specifications? They would call an orange an apple if there was a profit in it. They have stolen virtually every idea/invention and copied it, totally disregarded every copyright and patent and done it with the full approval of their own government.

[Grunthos]

Does anyone actually think that China will comply with any international rules, guidelines or specifications? They would call an orange an apple if there was a profit in it. They have stolen virtually every idea/invention and copied it, totally disregarded every copyright and patent and done it with the full approval of their own government.

So (to paraphrase Python), apart from multistage rocketry, papermaking, the compass, silk, sundials, kites, abacus, matches, wheelbarrows and printing, what have the Chinese ever done for us!

[Photosbykev]

So (to paraphrase Python), apart from multistage rocketry, papermaking, the compass, silk, sundials, kites, abacus, matches, wheelbarrows and printing, what have the Chinese ever done for us!

Very little of anything really significant in the current era except steal the work of others.

[/back on topic]

[E621Keith]

Come on guys, lets stop the politics and racism

Chinese has designed plenty of astro stuff. ES's 100 deg are not Ethos clone, and ES 9mm 120 deg are the first 120 deg on the market. Both are original Chinese designs. Then there are IEQ45, EQ8, GSO's RC ...

It's interesting to find there are two types of Bak4, but then it's the final image quality that matters. LZOS, LOMO scope uses Russian OK4 glass, which is suppose to be inferior to FPL53, but their scopes are still first class.

I think FPL51's Chinese equivalent is FK61

[Veracocha]

Image quality is everything I agree. It's surprising how many of my birding friends put up arguments around ergonomics/weight etc when they compare their alpha binoculars with a £300 pair of chinese ED's.

[John]

Chinese has designed plenty of astro stuff. ES's 100 deg are not Ethos clone, .....

Not an exact clone maybe, but there are similarities. Have you seen this ?:

ImageShack® - Online Photo and Video Hosting

[E621Keith]

Right, I will withdraw the comment on ES and Ethos.

However, we are starting to see more and more original Chinese designs, so saying they never innovate and only copy other people are just wrong on so many levels.

Didn't Nikon and Canon started off by cloning German cameras and optics. Look at them now. I would be surprise if I don't see a few premium Chinese manufacturers that can rival the like of Tak and Televue in 10 - 20 yrs.

[Veracocha]

A shameless copy from a birding forum I mentioned earlier:-

The fact that more and more binoculars are made in China is a function more of economics than of quality. Americans, of which I am one, have difficulty comprehending that more people in China speak English than live in the USA. Out of 1.4 billiion people in China there are an extraordinary number of highly inteligent people of industry and talent who are figuring out ways to move in the world economy. Do not be surprised if within the next several decades, China will produce binoculars to rival any of the alphas.

There are several advantages to the purchasing of alphas that are not readily apparent to first time purchasers, viz., mechanical reliability, state of the art materials, and whether guarantees are nothing more than word puffery. The alphas have reputations to protect, but whether their marketing experts can convince the buying public their astronomical prices are worth paying, only time will tell.

IMO China will produce a total range of optics from so-so to superior. I will not live long enough to see how all this plays out, but China will become a major world player in producing binoculars.

[Flying Eye]

Right, I will withdraw the comment on ES and Ethos.

However, we are starting to see more and more original Chinese designs, so saying they never innovate and only copy other people are just wrong on so many levels.

Didn't Nikon and Canon started off by cloning German cameras and optics. Look at them now. I would be surprise if I don't see a few premium Chinese manufacturers that can rival the like of Tak and Televue in 10 - 20 yrs.

I'm intrigued; I had always understood Nikon and Canon to be Japanese in origin. Are they in fact Chinese manufacturers? I don't own any examples from either to check for a made in China label. Very interesting.

[RogerTheDodger]

I think he was drawing attention to copying other peoples work, they are Japanese companies.

BTW, my Canon 1100d says "Made in Taiwan" on the bottom...

[E621Keith]

I think he was drawing attention to copying other peoples work, they are Japanese companies.

That's what I meant. Thanks for clarifying. The Japanese were cloning German designs in the beginning, but when they gained enough experience and knowledge, they started to create their own designs. Many Chinese manufacturers are still in the cloning phase, but some are starting to emerge with their own designs.

PS, some of my Nikon lenses are labelled 'Made in China', my DSLR was made in Thailand. Funny enough, my Sigma and Tokina lenses were made in Japan.

[Flying Eye]

Right, thanks guys, I had a comparison feeling then got an Apples and Oranges feeling, thought it best to explore.