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Prism glass: BaK-4 vs BK-7

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#1 ECP M42

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Posted 10 July 2021 - 05:30 AM

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


Edited by ECP M42, 10 July 2021 - 05:32 AM.

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#2 DeanD

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Posted 10 July 2021 - 05:41 AM

Generally the BK-7 prisms in porros (usually older models) are a bit undersized and give a squared off exit pupil, which reduces the contrast: but those with BaK-4 have full, circular exit pupils and are therefore a bit brighter. I am not sure why the BK-7's can't be a bit bigger though...


Edited by DeanD, 10 July 2021 - 05:43 AM.

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#3 ECP M42

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Posted 10 July 2021 - 06:26 AM

In fact, it is sufficient to narrow the field of view or close the aperture, or use larger prisms, to make the squared pupils "disappear".
We were taught that it is the fault of the BK-7 glass, but no one can actually be sure of the type of glass in the prisms (without doing complex technical tests that require expertise).
In my binoculars (wide field), with squared pupils, there are visibly undersized prisms and it is enough to close the aperture from 50 to 44mm, to solve them.


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#4 Rich V.

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Posted 10 July 2021 - 09:49 AM

It's not being undersized that makes BK7 prisms show the "darkened square" in the exit pupil, it's that BK7 doesn't provide as wide an angle of incidence for total internal reflection to take place.  The shorter, steeper angled light cones used in most modern binoculars dictates the use of BaK4 to avoid those four areas (one for each prism face at 90° to each other) of less that total reflection that makes the dark "square" seen in the exit pupil.

 

If applied to longer, narrower angled light cones where TIR can take place for every ray of the light cone, BK7 works just fine.  It's just that BaK4 passes a steeper light cone with TIR.

 

Rich


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#5 Echolight

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Posted 10 July 2021 - 10:10 AM

What I have seen, with my vintage 10 degree Viewlux 7x50, which I assume uses Bak-7 prisms, is that under low light conditions, the exit pupils are diamond shaped. But in bright light, the exit pupils are round.

So... different binoculars obviously change effectiveness under different conditions and for different uses. Like as the sky darkens over the course of the night.



#6 ECP M42

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Posted 10 July 2021 - 06:05 PM

Rich, it's not the shoes that are tight, it's the feet that are too big! 

 

lol.gif lol.gif  ... for a thousand whales, I had forgotten about refraction  foreheadslap.gif  ...  funnypost.gif

 

 

However, narrowing the field of view or the aperture results in BK-7 glass being brighter and more transparent.

And since a higher focal ratio would tend to bring even better vision, I've always wondered why it isn't used more often and with more pride.



#7 Jon Isaacs

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Posted 10 July 2021 - 06:10 PM

What I have seen, with my vintage 10 degree Viewlux 7x50, which I assume uses Bak-7 prisms, is that under low light conditions, the exit pupils are diamond shaped. But in bright light, the exit pupils are round.

So... different binoculars obviously change effectiveness under different conditions and for different uses. Like as the sky darkens over the course of the night.

 

I suspect it's not the binoculars that change but rather your perception of the brightness.

 

Jon


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#8 Echolight

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Posted 10 July 2021 - 06:54 PM

I suspect it's not the binoculars that change but rather your perception of the brightness.

 

Jon

I believe my perception is accurate.

 

Of course the binoculars don’t change. But the appearance/shape of the exit pupil does change, depending on the brightness.

How do you perceive these?

 

Dim light

198ECD66-C483-4C1C-A257-FE41FFE4485A.jpeg

 

Bright light

573D7924-D720-40F7-B9B1-8ACDFE88D4F6.jpeg



#9 gwlee

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Posted 10 July 2021 - 07:11 PM

Regardless of the reason or the wisdom of the manufacturer, the best quality binoculars only come with B4 prisms these days


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#10 Rich V.

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Posted 10 July 2021 - 07:12 PM

 

However, narrowing the field of view or the aperture results in BK-7 glass being brighter and more transparent.

And since a higher focal ratio would tend to bring even better vision, I've always wondered why it isn't used more often and with more pride.

Sure, increasing focal ratio by any means possible narrows the light cone, which in turn narrows the range of reflectance angles at the prisms.  The problem is, most folks like their binos shorter, lighter and easier to handle.  There are some longer focal ratio binos but most users seem to prefer the compromise of the more compact options.

 

Mounted binos can go longer; I've got one bino that's f6 and a BT that's f7.5 so I'm doing my part...

 

Rich


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#11 Rich V.

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Posted 10 July 2021 - 07:18 PM

I believe my perception is accurate.

 

Of course the binoculars don’t change. But the appearance/shape of the exit pupil does change, depending on the brightness.

How do you perceive these?

 

I perceive the brightness/contrast of the second photo as too high and washed out to see the more subtle dimming caused by the prism's reflectance angles.  Like you say, nothing changes at the bino; only your perception of the different brightness levels changes, as Jon points out.

 

Rich


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#12 Echolight

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Posted 10 July 2021 - 07:32 PM

I perceive the brightness/contrast of the second photo as too high and washed out to see the more subtle dimming caused by the prism's reflectance angles.  Like you say, nothing changes at the bino; only your perception of the different brightness levels changes, as Jon points out.

 

Rich

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?  

C9A67C72-48EE-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.


Edited by Echolight, 10 July 2021 - 07:40 PM.

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#13 Echolight

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Posted 10 July 2021 - 07:50 PM

I suspect it's not the binoculars that change but rather your perception of the brightness.

 

Jon

And what I said, was not that the binoculars change themselves, but that they change effectiveness under varying conditions. Of course if you pick a certain word and change the context of the statement, then you can contradict any statement no matter the accuracy of it.

 

I will try to be more clear. 

ALL binoculars have their strengths and weaknesses. Some do better under certain conditions. But say the conditions change. Like as the sky gets darker, in the summer, from late evening, say 10-ish PM, when it is not at it’s darkest, to a time when it is darker at say 2 AM, then two different binocular might be most effective at the different times of the night.

I.e, Eg., or something like that... a smaller exit pupil might be more effective at 10PM, or under bright Moonlight. But a larger or brighter exit pupil might show more at 2AM with no Moon.


Edited by Echolight, 10 July 2021 - 08:01 PM.


#14 SMark

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Posted 10 July 2021 - 08:12 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.

What I perceive is that you probably have BaK4 prisms in that Viewlux.



#15 Echolight

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Posted 10 July 2021 - 08:20 PM

What I perceive is that you probably have BaK4 prisms in that Viewlux.

That’s a possibility. I have no way of knowing. But of the several known BAK-4 prism equipped binoculars that I own, none exhibit a diamond shaped exit pupil in low light like the Viewlux do.



#16 pat in los angeles basin

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Posted 10 July 2021 - 08:43 PM

I just tried the same with several BaK 4 prismed binos and can't duplicate the shaded diamond figure. Brought out the kowa  prominar 10 deg 7x35 with what I know are BK7 's and the diamond is there. I was heavily illuminating a brown  (melamine) surface -  similar in tint to the wood surface you used . The light hitting the wood directly and me viewing through a shaded eyepiece. I think the viewlux is a BK 7 prismed bino.  I 'd  be interested on hearing more.    regards, Pat


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#17 SMark

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Posted 10 July 2021 - 08:54 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

You're not the first person to argue this point here in the forum. The problem is that when you start applying all your physics to vintage porro 1 binoculars, it never seems to work out the way you describe it. You can certainly state your point and show your numbers to "prove" it, but real life has already taught us otherwise, so we choose not to believe you. 

 

And here's why...

 

Go back into the mid to late 1950's and start working your way forward. Look at a few of the best binoculars made, and then look at some of the worst. And look specifically for those with the brightest images, and then look for those with the dullest images. What we have found is that the vast majority of those binoculars with the brightest images just happened to also have BaK4 prisms. And the vast majority of those with the dullest images just happened to have Bk7 prisms.

 

What we have seen is that the best binocular manufactures tended to use BaK4 prisms in their best binocular models. We also have seen that cheaper binoculars were almost universally made with Bk7 prisms. So the argument that BaK4 prisms are brighter, and that Bk7 prisms are not brighter, makes a whole lot more sense in the real world.

 

And I'll go one step further with this. While it's difficult to find the perfect test for this, because only a few vintage binoculars were made exactly the same except with the two different prisms. But there were a few that would allow for direct comparison. One of these is the Kowa wide angle 7x35 binocular, mentioned by Pat above. There is a Bk7 version and a BaK4 version. I've had both out under the stars in direct comparison with one another. The BaK4 version was clearly brighter than the Bk7 version. And I repeated the test multiple times. I also did this test with Fuji and Kowa standard angle Featherlights. The difference was less pronounced, but the BaK4 versions were still clearly brighter.

 

So that's why I will always recommend a BaK4 vintage binocular for the brightest images. Because it's true!  coolnod.gif  


Edited by SMark, 11 July 2021 - 07:28 PM.

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#18 Rich V.

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Posted 10 July 2021 - 11:52 PM

 

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.

Yes, the exit pupil appears round in the image to me.  I don't see the "square" dimming like the low light pic but I wish there was a uniform light colored background, though, so any dimming could be seen more easily.  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.  The BK7 prism never cuts the transmission down to zero so the exit pupil remains round but less than fully illuminated in each of the four prism face's axes. Transmission is decreased but never to zero.  Clearly, though, each of the four faces suffering from this partial dimming lowers the total amount of light available at the exit pupil whether you can see it or not.  That's why BK7 binos are ultimately dimmer than otherwise similar BaK4 binos as SMark points out.

 

Rich


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#19 Echolight

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Posted 11 July 2021 - 01:57 AM

Yes, the exit pupil appears round in the image to me.  I don't see the "square" dimming like the low light pic but I wish there was a uniform light colored background, though, so any dimming could be seen more easily.  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.  The BK7 prism never cuts the transmission down to zero so the exit pupil remains round but less than fully illuminated in each of the four prism face's axes. Transmission is decreased but never to zero.  Clearly, though, each of the four faces suffering from this partial dimming lowers the total amount of light available at the exit pupil whether you can see it or not.  That's why BK7 binos are ultimately dimmer than otherwise similar BaK4 binos as SMark points out.

 

Rich

Now you think it’s not the same binoculars! You think I’d go through the trouble to create some kind of exit pupil farce? And then need an explanation or lecture from you. Ha! Y’all are way too full of yourselves.

Of course it’s the same binocular. 

Anyway, I would always prefer Bak4. And am not proposing that BK7 is equal. I wasn’t the one who started this thread suggesting that.
I just thought it was interesting that the exit pupils on this particular binocular looked different depending on the level of light that was available.


Edited by Echolight, 11 July 2021 - 01:59 AM.


#20 Neil Sanford

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Posted 11 July 2021 - 07:41 AM

As to Echolight's above post #19, I propose that anyone inclined to offense by his argumentative language let it be, and that we return to this worthwhile (though not new) topic.  Certainly in his second paragraph, all 4 sentences seem quite reasonable. 

 

I have a BK7 in work by Cory.  It is somewhat unusual and I am excited by its prospects.  Will report. 


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#21 Pinac

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Posted 11 July 2021 - 07:49 AM

Thanks for this, Neil (I was in fact „inclined to offense“) …😄


Edited by Pinac, 11 July 2021 - 07:49 AM.

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#22 Fiske

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Posted 11 July 2021 - 08:19 AM

Sure, increasing focal ratio by any means possible narrows the light cone, which in turn narrows the range of reflectance angles at the prisms.  The problem is, most folks like their binos shorter, lighter and easier to handle.  There are some longer focal ratio binos but most users seem to prefer the compromise of the more compact options.

 

Mounted binos can go longer; I've got one bino that's f6 and a BT that's f7.5 so I'm doing my part...

 

Rich

Rich,

 

What BT is F7.5?

 

Fine conversation here with lots of good info. Bak7 versus Bak4 is certainly on the top 10 list for endless discussion in the Bino Forum. Along with Exit Pupil versus Pupil Dilation. lol.gif Forty-five versus 90 degree bino eyepieces too. Oh, and lets not forget roof versus porro prism...

 

Anyway, great job everyone. wink.gif

 

Fiske


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#23 Echolight

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Posted 11 July 2021 - 08:55 AM

As to Echolight's above post #19, I propose that anyone inclined to offense by his argumentative language let it be, and that we return to this worthwhile (though not new) topic.  Certainly in his second paragraph, all 4 sentences seem quite reasonable. 

 

I have a BK7 in work by Cory.  It is somewhat unusual and I am excited by its prospects.  Will report. 

I apologize if my language seemed argumentative. I did not intend to offend anyone.

 

I just thought the condescending tone of the replies against my shared observation were unfounded.

 

And then, from the part of Rich V’s comment which I have attached below, it appears that my integrity was in question. 

 

If it's the same bino in both dim and brighter pics, 

 

Rich



#24 Rich V.

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Posted 11 July 2021 - 09:39 AM

Now you think it’s not the same binoculars! You think I’d go through the trouble to create some kind of exit pupil farce? And then need an explanation or lecture from you. Ha! Y’all are way too full of yourselves.

Of course it’s the same binocular. 
 

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


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#25 Rich V.

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Posted 11 July 2021 - 09:46 AM

Rich,

 

What BT is F7.5?

 

It's the Miyauchi Saturns. 100mm f7.5 wide air spaced achromat.  It would likely work fine with BK7 or BaK4 prisms, though Miya says BaK4.  I'm sure they're the same as the ones used in their 100mm f5 BTs.

 

Rich


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