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Prism glass: BaK-4 vs BK-7

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#26 Echolight

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Posted 11 July 2021 - 10:24 AM

Well, apparently I have to pick my words very carefully to keep from insulting you, Echolight.  Perhaps if I word that sentence "Since (not if) it's the same bino in both dim and brighter pics, we know the dimming is still there even if it's more difficult to see under brighter conditions." you'll accept that as not being an insult?  I never intended your taking the wording as meaning you're deliberately switching binos to try to fool us.  I'm just trying to make sense of photos that appear contradictory at first look.  It's hard to put words together perfectly when trying to express abstract thoughts rather than a visual representation.

 

Rich

The photos, and my initial statement, were just to show something that seemed out of the ordinary. Something that I found interesting and thought I would share. And they are what is seen. Now, apparently by anyone who cares to see. And could even give the impression to some that the binoculars contain Bak-4 prisms, which I never fully believed or intended to imply.
 

When what I was trying to share was challenged several times as merely my own delusion, and then finally qualified with an “if”, well....

 

Sometimes you look and don’t see. And just because you can’t see it doesn’t mean it’s not there. 

Sometimes things that you do see aren’t really there lol.gif


Edited by Echolight, 11 July 2021 - 10:25 AM.

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#27 Erik Bakker

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Posted 11 July 2021 - 10:26 AM

Perhaps it is the summer heat and nobody meant any harm, but let me remind everybody to keep it friendly, both on the sending and receiving end of communications flowerred.gif


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#28 Fiske

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Posted 11 July 2021 - 10:27 AM

A little OT but I'll say it anyway. I used to be quite a bit more confrontational in discussion on CloudyNights. Long time participants can attest to that. wink.gif

 

But through various experiences and helpful advice about online media, I have come to understand how easily things can be misunderstood and how easy it is for people to feel hurt or insulted when such was never the intention of the message. Everyone here loves binoculars and talking about them and for the most part, whether they agree or not with another person's point of view, accepts that they sincerely believe whatever they are asserting.

 

When something that has been written seems offensive to us, or seems confusing, a good technique is to ask for clarification. Maybe in the topic or possibly in a private message. In a friendly way. Many misunderstandings and disputes can be avoided with this practice.

 

Just saying. grin.gif

 

Fiske


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#29 Profguy

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Posted 11 July 2021 - 03:54 PM

When people speak face to face, there’s a lot of contextual information that communicates nuances of meaning: facial expression, body language, intonation, pace of speech, among others. All that is absent in a post, which increases the likelihood of misinterpretation. I heartily concur with Fiske’s suggestions above!

 

Gary


Edited by Profguy, 11 July 2021 - 03:54 PM.

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#30 MeridianStarGazer

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Posted 11 July 2021 - 04:54 PM

Cheap binos use BK7 because it costs less. It happens to be clearer too, but not enough to compensate for the poorer execution of the cheap binos.

 

The premium binos use BK4 because everyone expects it on premium binos. Why spend all your money on perfect execution just to lose a sale because people insist BK4 is better. Shifting the market is not easy.

 

So even though BK7 can be made better, customers will always pay more for BK4, and high end bino makers will stick to BK4 to get the sales, and low quality bino makers will stick to low priced BK7 or make up their own naming convention to sound like BK4. And if you are hunting for vintage binos, you should set aside your engineering skills and just get BK4, since those tended to be made better. There are a few cheapos though that also use BK4, so watch out and read the reviews.


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#31 DeanD

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Posted 11 July 2021 - 08:44 PM

... and then there is Chinese "BAK4" vs Schott "BaK4"... 

 

See: https://topbinocular...est-binoculars/

 

in particular this paragraph:

 

"There are international industrial optics standards that determine what type of BaK4 glass that should be used. For example, the international BaK4 standard designation is 569561 with the first three digits telling the refractive index (1.569) and the remaining three telling you the Abbé number (56.1).

Glass Type Refractive Index Critical Angle Dispersion
Schott BaK4 1.5688 39.6° – 0.0523μm -1
Chinese BAK4 1.5525 40.1° – 0.0452 μm -1
Scott BK7 1.5168 41.2° – 0.0418 μm -1
The Abbe indicates the amount of light that will disperse into its constituent colors. The higher your Abbé number the lesser the dispersion.

Even so, the “BAK4” glass on the Chinese optics is not the original Schott BaK4. The Chinese binoculars use phosphate crown glass and not the Barium Crown. Phosphate crown has lower refractive index, dispersion, and abilities compared to the Schott BaK4."

 

​It's a minefield out there! (Says he as he heads off to the dealer to take back his new Steiner Wildlife XP 10.5x28's, which have completely different sized "sweet spots" in the left (~30° in the centre) vs the right (~60°) views... frown.gif  )


Edited by DeanD, 11 July 2021 - 10:39 PM.

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#32 jimhoward999

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Posted 11 July 2021 - 10:12 PM

I often read in various forums (fora) that ordinary people attribute greater brightness to binoculars with BaK-4 (Ba = Barium) prisms, compared to those with BK-7 (B = Boron). 

 

But in reality, BK-7 glass is more transparent than BaK-4 glass and we can take an example from SCHOTT's list of new HT and Ultra-HT glasses. 

 

9-ht-and-htultra-glasses-en_759px.png?re

 

These "T**" transmittance data, are valid for 400nm and 10mm thickness, but by averaging the data from the complete tables and adjusting the thickness for ~ 100mm of glass (more typical of a Porro-prism system), the values of transmittance they become ~ 98% for BK7 and ~ 93% for BaK-4. 

 

Obviously, the BaK-4 glass is the most used in prisms, because it facilitates the reduction of weights and dimensions, in the construction of binoculars of the same size.

 

But it is certainly not the brightest!  smirk.gif

I think you are being way to pessimistic on the internal transmittance of both N-BK7 and N-Bak4.  While the transmittance may be attenuated in the deep blue (you can't see 400nm and the sky scatters it significantly) over the photopic and scotopic visual ranges both glasses will transmit more than 98% through 100mm.

 

I dont think you just do a straight average.  You usually spectral weight the average using the eye's response.

 

I am not sure that binoculars in my price range are even using HT glass.  So I have attached the data sheets for the garden variety materials.    Maybe Zeiss uses HT material, but I figure my binos probably have CDGM equivalents in them, which likely wouldn't be HT.   But even so, they likely internally transmit more than 98%.

 

As many people are quick to point out, the advantage of Bak4 is the higher index and therefore lower critical angle for TIR.   

 

Moreover, if BK7 is used in an application where the F/# is too fast so the critical angle is not hit, then you have to either accept some light leak and a funky shaped pupil, or aluminize them (which as far as I know no one does) and that certainly knocks the brightness down

 

 

 

 

 

 

 

 

 

 

bak4.jpg bk7.jpg


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#33 ECP M42

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Posted 12 July 2021 - 11:36 AM

Here, the main point is that which demonstrates the higher brightness of the BK-7 glass compared to the BaK-4 and the difference that is seen in the numbers is to be applied to the total surface reflectance of each binoculars (I counted from 400 to 700nm).
For example, modern binoculars best coated and built with 10 elements in 4 groups + glued prisms (such as Steiner NightHunter 8x56), could achieve a total transmission of 95.7%, if using the BK-7 HT prisms, but a total transmission of 92.8%, if using BaK-4 HT prisms.
Here it is all very theoretical, but the difference could also be quite realistic. 

 

The other, rather important, point is the question of the square pupil and the "legend" who wants to divide the BK-7 glass from BaK-4.
We have now become accustomed to signing the glasses of Porro prisms in that way. And, as we see from Smark's statements, it will be difficult for our mind to open (or reopen) under this new reality. Like a religion, that law has already been absorbed into the cells and the consequence is already evident. 

 

I am very sorry for the harassment that Echolight had to endure. Because, he was making a good contribution to the discussion. However, peace is made, and that phenomenon remains very interesting. I had never seen this before and perhaps I dare to say that those binoculars may actually have BK-7 prisms and not the usual undersized prisms or eyepieces that are too wide for the project. I don't know, but I'm curious.

The point is that none of us (intervening here) could put our hand on the fire, stating with certainty that the glass of his binoculars is BK-7 or BaK-4 (assuming that one of these two materials was used - but let's overlook this trifle).
And only those who have the appropriate skills, doing accurate tests, could say it with more certainty. 

 

So, the second point is: since it is possible to build perfectly circular pupil binoculars, even using BK-7 glass, how can we be sure that all binoculars with round pupils, have BaK-4 prisms?


Edited by ECP M42, 12 July 2021 - 12:23 PM.


#34 SMark

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Posted 12 July 2021 - 03:17 PM

That is an extreme example in the bright picture. And again, of course the physical make-up of the binoculars have not changed.
 

But you are saying that the exit pupil doesn’t look more round to you in the bright light picture , without the diamond shaped shadows in the dim light picture?

 

Here is a “less bright” image, with a less washed out and with obvious fine detail of the lunar type surface seen through the what appears to me as a mostly round (except the top right corner) exit pupil. What do you think now? Only my own perception? Or do you see it too?

 

Forget the argument against my perception for a moment.

Is it not round?  

attachicon.gifC9A67C72-48EE-4C83-9EF6-9827AE057437.jpeg

What I perceive, is an almost large enough Bak-7 prism to eliminate the diamond shape exit pupil. Even in the dim photo, if you look closely, the diamond shape is a shadow that can be seen through to reveal the round field stop.

Interestingly, I just happened to find that same J-B191 Viewlux 10° 7x50 in my collection. Studying it, I will reverse my suggestion that it might have BaK4 prisms. But I have been able to replicate what you are demonstrating. If I keep my eyes perfectly on axis and back off the eyecups just to the point where I can barely see the field stops, the Bk7 diamond shaped cutoffs seem to just fall outside the field stops. Once I go slightly off axis though, the cutoffs on one side or another will come into view. So it's really close. 

 

And then I also found (I keep finding things I forgot that I had...confused1.gif ) another very similar looking J-B191 wide angle 7x50 that does have BaK4 prisms, and it isn't nearly as fussy about eye placement. And this one also boasts 11° rather than 10° of the Viewlux.


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#35 Echolight

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Posted 12 July 2021 - 03:47 PM

Interestingly, I just happened to find that same J-B191 Viewlux 10° 7x50 in my collection. Studying it, I will reverse my suggestion that it might have BaK4 prisms. But I have been able to replicate what you are demonstrating. If I keep my eyes perfectly on axis and back off the eyecups just to the point where I can barely see the field stops, the Bk7 diamond shaped cutoffs seem to just fall outside the field stops. Once I go slightly off axis though, the cutoffs on one side or another will come into view. So it's really close. 

 

And then I also found (I keep finding things I forgot that I had...confused1.gif ) another very similar looking J-B191 wide angle 7x50 that does have BaK4 prisms, and it isn't nearly as fussy about eye placement. And this one also boasts 11° rather than 10° of the Viewlux.

I bought it from SGW, hoping it was BAK-4 because in their random picture the exit pupil looked round. So I haven’t had to be right on axis to see a round exit pupil. But since I got it, the diamond shape exit pupil was there in lower light.
 

Anyway, after using it a bit, I noticed that it exhibits a bit of rolling ball effect. And the field stop is fuzzy. So I don’t know that the 10 degrees is actually as useful as you’d think. I prefer the similarly sized, but more obviously BK-7 equipped 8 degree 8x50 Sears 2515, J-B46 I believe, which has a clearer image and sharp field stop. So obviously a round exit pupil isn’t the end all be all.

2206074F-D2C8-4ED7-9A09-E53F3B4BAB2E.jpeg


Edited by Echolight, 12 July 2021 - 03:55 PM.

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#36 Rich V.

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Posted 12 July 2021 - 03:51 PM

Interestingly, I just happened to find that same J-B191 Viewlux 10° 7x50 in my collection. Studying it, I will reverse my suggestion that it might have BaK4 prisms.

Thanks, Mark, for shedding some light on the BK7 situation.


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#37 ECP M42

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Posted 12 July 2021 - 04:40 PM

Checking all my binoculars, today I could see that if I look very obliquely at the Leitz Camparit 10x40 (7.3 °), I can see (on the edge) a beginning of the squaring of the prisms.
I can't see this in the Binux 8x30 (9°) model, which has the same AFOV and smaller prisms.

 

The funny thing is that these terms "BK-7 and BaK-4" are mentioned almost exclusively for Asian binoculars (Chinese, Japanese, Thai, etc.) and never for European binoculars (Leica, Swarovski and Zeiss).

In fact, if I haven't missed something, to date they are mentioned only for low and very low cost binoculars and for those of medium-low quality, always of oriental production.


Edited by ECP M42, 12 July 2021 - 04:41 PM.


#38 pat in los angeles basin

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Posted 12 July 2021 - 04:41 PM

Echolight: I got a sears 8x50 (circa late 60's guessing) recently, cleaned up nice  thought the focus shaft might have a kink- fortunately only shows up at the end of the out focus of the  eyepieces so it only impact the close focus. They are a great bino!   Good FOV reasonably sharp most of the field . Reasonable field without the eyecups for use with glasses. Really  worth the price of admission thanks to the biggish prisms despite the BK7 specs.  Regards, Pat


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#39 Echolight

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Posted 12 July 2021 - 04:58 PM

Echolight: I got a sears 8x50 (circa late 60's guessing) recently, cleaned up nice  thought the focus shaft might have a kink- fortunately only shows up at the end of the out focus of the  eyepieces so it only impact the close focus. They are a great bino!   Good FOV reasonably sharp most of the field . Reasonable field without the eyecups for use with glasses. Really  worth the price of admission thanks to the biggish prisms despite the BK7 specs.  Regards, Pat

Yep. I wanted some of the 60’s models. I like the look of the eyepieces better on those. I think they had a model number beginning with 6. But these mid-70’s looked barely if ever used. Pretty nice on the stars. They seem plenty wide enough to me. I don’t mind the BK-7.



#40 Binobumpkin

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Posted 13 July 2021 - 03:57 PM

be a barium not a boro

 

rockon.gif


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#41 ECP M42

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Posted 14 July 2021 - 07:10 AM

Barium (BaK-4) = is used to objectives with focal-ratio lower than f/4 and for eyepieces with a large visual, which, however, overall generate an image of lower optical quality and make the transmitted brightness worse.

 

Boron (BK-7) = works for lenses with focal-ratio greater than f/4, and this improves the containment of aberrations, increases the sharpness and depth of field, and the total brightness.

 

Who knows, most likely those typical binoculars built to be maximally bright (7x50, 8x56, 10x70, etc) almost always have narrow field eyepieces, because they use BK-7 prisms (?).

 

I prefer Boron! smirk.gif


Edited by ECP M42, 14 July 2021 - 10:20 AM.


#42 Rich V.

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Posted 14 July 2021 - 09:37 AM

 

Who knows, most likely those typical binoculars built to be maximally bright (7x50, 8x56, 10x70, etc) almost always have narrow field eyepieces, because they use BK-7 prisms (?).

 

I prefer Boron! smirk.gif

 

Those "maximally bright" big exit pupil binos use BaK4 prisms if they are current designs.  The narrower AFOV eyepieces aren't there to accommodate BK7 prism's shortcomings; they're used to avoid giant sized eyepiece field stops that would require giant, oversized prisms as well.  

 

It seems to me that the opposite opinion to yours prevails, though; Bak4 prisms are preferred here on CN and in the binocular marketplace as well, clearly.

 

I know I prefer the wide AFOVs with the increased illumination of the exit pupil that a BaK4 bino can provide. What's not to like about that?  Lower optical quality, how?  question.gif  


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#43 ECP M42

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Posted 14 July 2021 - 10:52 AM

Those "maximally bright" big exit pupil binos use BaK4 prisms if they are current designs.  

And who told you that to you?  question.gif  Just because they write it down somewhere? Do you know how much nonsense they can write to sell you binoculars?

Are you so sure they are BaK-4 prisms that you bet a hand? Or have you checked all of them yourself with your own equipment?  

 

I too prefer wide AFOVs, but I am aware that there is a price to be paid for that. A further drop in brightness due to the enormous amount of glass used in the eyepieces and the greater number of air-glass surfaces (in addition to weight), without considering the field flatteners. And if we also add the use of short focal lenses, we increase the optical aberrations and it becomes mandatory to increase also the volume of the prisms with higher density glasses (therefore with more weight), which further decrease the brightness. 

 

If I need a good 8x56 binoculars (for example), I very much hope they are built with BK-7 prisms, long focal length lenses and Kellner or Otoscopic eyepieces, in a reinforced and lighter weight polymer or composite frame.

 

But I, it's me!  get-em.gif


Edited by ECP M42, 14 July 2021 - 10:55 AM.


#44 Rich V.

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Posted 14 July 2021 - 12:32 PM

fingertap.gif   Good luck with that.


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#45 sneeds

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Posted 14 July 2021 - 03:30 PM

I would think there might be more problems confounding the BK7 vs Bak4 discussion then pure quality.  For example, let's suppose that BK7 glass, for whatever reason, is cheaper than Bak4.  Then by default, it automatically gets put into the lower-cost binocular.  With a lower-cost bino comes many shortcuts and compromises that we can all attest to, particularly coatings.  The lower-cost BK7 bino would likely get worse coatings than the Bak4 further compounding the problem.

 

Anyway, these conversations are fun to think about - thanks all for the discussion.

 

P.S.: I wonder what glass is in the Nikon WX lol.gif


Edited by sneeds, 14 July 2021 - 03:30 PM.

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#46 SMark

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Posted 14 July 2021 - 04:02 PM

I would think there might be more problems confounding the BK7 vs Bak4 discussion then pure quality.  For example, let's suppose that BK7 glass, for whatever reason, is cheaper than Bak4.  Then by default, it automatically gets put into the lower-cost binocular.  With a lower-cost bino comes many shortcuts and compromises that we can all attest to, particularly coatings.  The lower-cost BK7 bino would likely get worse coatings than the Bak4 further compounding the problem.

 

Anyway, these conversations are fun to think about - thanks all for the discussion.

 

P.S.: I wonder what glass is in the Nikon WX lol.gif

I don't know of any Nikon binocular that was made with Bk7 prisms. Has anyone seen a Nikon binocular with Bk7 prisms?


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#47 Jon Isaacs

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Posted 14 July 2021 - 04:29 PM

I don't know of any Nikon binocular that was made with Bk7 prisms. Has anyone seen a Nikon binocular with Bk7 prisms?

 

Mark:

 

You'd be the one to know.. :)

 

How about the 10 x 50 Lookout ll's, I think they were labeled Actions.

 

(Mark digs out 5 examples of the Lookout ll's to check for sure.)

 

Jon


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#48 Fiske

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Posted 14 July 2021 - 05:18 PM

Mark:

 

You'd be the one to know.. smile.gif

 

How about the 10 x 50 Lookout ll's, I think they were labeled Actions.

 

(Mark digs out 5 examples of the Lookout ll's to check for sure.)

 

Jon

lol.gif lol.gif lol.gif


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#49 SMark

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Posted 14 July 2021 - 05:19 PM

The Lookouts and Actions that I have had were all made with BaK4 prisms. I’ve had numerous samples from the Nikon A series that all had BaK4 prisms as well. Wayne, a former member here, indicates that he had an old Novar 7x50 that was made in Occupied Japan that had Bk7 prisms. So that’s going back a ways.
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#50 ECP M42

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Posted 14 July 2021 - 05:29 PM

fingertap.gif   Good luck with that.

Rich, sometimes I don't understand you ... only you know what "luck" has to do with it 

 

 

I wonder what glass is in the Nikon WX lol.gif

For those, they absolutely didn't care, neither weight nor brightness ... they certainly used the thicker, heavier glass they had in the closet (BaPbK-444?) lol.gif


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