Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

bak 4 prism

  • Please log in to reply
35 replies to this topic

#1 publin

publin

    Vostok 1

  • -----
  • topic starter
  • Posts: 168
  • Joined: 14 Dec 2019

Posted 14 December 2019 - 11:50 AM

HI  ALL!    as  an  new    member   and  a  vintage    bino's    collector   I   have   a  question   about  bak4  prisms  on   vitage  bino's  .   when   start  to  have  bak4   on bino's   and   in particular   in   russian /  german  items?  I  have   bpo5  8x30 [russian]   and  steiner  bayreuth  8x30  and  japan made   sears  10x50  all of   them   good  and crispy   emage   optic  but   bk7  prisms!   thanks for   answers!   



#2 asphericalaberration

asphericalaberration

    Explorer 1

  • -----
  • Posts: 94
  • Joined: 30 Jan 2019

Posted 14 December 2019 - 12:01 PM

Can't speak to the German/Russian timeline, but nearly all of my vintage Japanese bins have Bk7. A few upper-end bins had Bak4 prisms going back into the 1940s (probably further). Bak4 has been marketed as "better" for a long time, so consumers think of it as "better." It's more accurate to view Bk7 and Bak4 as different types of glass with different properties. Which of the two is "better" for prisms will depend on the overall design. As you've noticed, there's nothing wrong with Bk7 prisms, especially at a standard-width FOV. It will have the benefits of higher transparency and lower cost. Bak4 does tend to suggest a higher manufacturing budget, however.


Edited by asphericalaberration, 14 December 2019 - 12:47 PM.

  • Cory Suddarth likes this

#3 sg6

sg6

    Cosmos

  • *****
  • Posts: 8,727
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 14 December 2019 - 12:18 PM

You could find that your BaK4 prisms are not BaK4 - there is no standard for BaK4 and it seems some BaK4 is not even Barium, lots are a Phospherous crown glass.

 

One question is: Is there a specific spelling for the BaK4 bit - which are upper and lower case?

 

BK7 is somewhat odd, it would seem as being the standard glass used for optical benches and components. So if BaK4 is so good I would have expected most glass to have been BaK4 not BK7. Cost difference is irrelevant when the optical table is $20,000.

 

Doubt it is supplied but the correct designation of what you expect as BaK4 is 569561

 

Bigger aspect is the prism size. They can be small, especially if BaK4 is classed as greater cost then BK7 and that the 10x50's really more like 10x40's.

 

Binoculars are an area where it seems liberties can be taken, and are. Everything is locked away inside that body and you have little idea what the real situation is.

 

I was once "preached" to by someone at an RSPB binocular day, all he knew was to repeat BaK4, BaK4, BaK4. He didn't like me at the end. lol.giflol.gif


  • Cory Suddarth and j.gardavsky like this

#4 hallelujah

hallelujah

    Cosmos

  • *****
  • Posts: 7,915
  • Joined: 14 Jul 2006
  • Loc: North Star over Colorado

Posted 14 December 2019 - 12:43 PM

https://www.cloudyni...9-bak4-and-bk7/

 

https://www.cloudyni...84-bk7-vs-bak4/

 

Stan



#5 publin

publin

    Vostok 1

  • -----
  • topic starter
  • Posts: 168
  • Joined: 14 Dec 2019

Posted 14 December 2019 - 02:21 PM

thanks for  your  answer   but   still    bak4   have     higher   index  then  bk7 [ even  the  chines  bak4 ]    and  more the    schmidt - pechan   prisms  [ high end  bak4]   also   bak4 " produce"  round  eye pupil and  sharp  emage   edge to edge  and  much more  light  path   gathering  from the   objectiv  lens   [   reduce  loss  dispersing light ].  so  is it   realy   manufacturer   system  to   increase    bino's  cost   and    theirs profits?   I  have  also      bak4    bino's    and   I can see  the  advantage   of   emage   view   vs   bk7   somehow ....?!      as  I  gase   many  of  the  members  her  have  both   bak4/  bk7    bino's   certainly   not only  the   kind   of  prisms   influence  on   bino  QUALITY    but   yet the are  the   importent   ingredient    in  bino's ?         



#6 pat in los angeles basin

pat in los angeles basin

    Viking 1

  • *****
  • Posts: 582
  • Joined: 20 Nov 2015

Posted 14 December 2019 - 03:05 PM

An ingredient but  not the the sole important ingredient. I've had BaK4 stuff (kmart bins) that were utter crap and BK7 stuff  (kowa prominar) that was fantastic . What does that prove? Besides the fact that I want the next Kowa pair I buy to have the BaK4 glass! The Kowas BTW are great to work on, good mechanicals, great optics and precision fit on internals....   Regards, Pat



#7 Rich V.

Rich V.

    Fly Me to the Moon

  • *****
  • Posts: 7,235
  • Joined: 02 Jan 2005
  • Loc: Lake Tahoe area, Nevada

Posted 14 December 2019 - 03:20 PM

It's not about "quality" of the glass, it's about using the glass that has characteristics best suited for the application.  Shorter focal ratios or wide AFOV eyepieces demand a higher index of refraction.  Bak4 is best suited for those applications.

 

A quote from Jay Reynolds Freeman in a post from another forum, http://www.science-b...1d7ab7f8c9a.htm :

 

"The big deal about BAK-4 has to do with total internal reflection.
The reason why many common prisms do not require metallic coatings in
order to reflect light inside them is a phenomenon called "total
internal reflection", whereby a light beam traveling in glass, that
hits a plane surface (one surface of the glass) at a sufficiently
shallow angle, will reflect with 100 percent coefficient of
reflection.  You can see this phenomenon when you are under water in a
swimming pool, with your eyes open or with a mask on.  Look straight
up, and you can see out of the pool.  Look upward at an angle, and you
see the reflected bottom of the pool.
    The catch is, that the critical angle beyond which total
reflection takes place, varies with index of refraction, and for
ordinary crown glass (like BK-7), it is just a hair less than 45
degrees.  Thus a prism that has to deal only with reflecting rays that
approach at 45 degrees can be made of BK-7, but one that needs to
reflect rays at angles much less requires either a reflective coating
or a material with a higher index of refraction.  The difference is
only a few degrees, but the fall-off from total internal reflection is
fast, and many binoculars with Porro prisms have fields of view wide
enough, and objectives fast enough, that BK-7 prisms will provide
total internal reflection on axis, but off toward the edges of the
field, one of the prism reflections is no longer total.  There are
four reflections in the prisms, and failure of total internal
reflection at one reflection or another causes the familiar shadowed
square that one sees on looking at (not through) the exit pupil of
binoculars that are so equipped.
    Notice that even if you look straight through a binocular with
BK-7 prisms, so that you are seeing the rays that form the image right
on axis, you can sometimes see the shadowed square.  You will be able
to see it if the binocular objective is fast enough so that rays at
the edges of the converging cone of light from the objective, are
sufficiently far from 45 degrees to experience non-total reflection at
one prism.  Some binocular objectives are faster than f/4.  Any
telescope of this speed would suffer some loss of light if used with a
star diagonal made of BK-7 or a similar glass, but I believe that most
objectives are slow enough [not] to make this a problem.  You can check by
looking at the exit pupil of the eyepiece with a star diagonal in place;
if it is shaded on one side, there is a problem."


  • Jon Isaacs, hallelujah, Cory Suddarth and 2 others like this

#8 asphericalaberration

asphericalaberration

    Explorer 1

  • -----
  • Posts: 94
  • Joined: 30 Jan 2019

Posted 14 December 2019 - 07:13 PM

> is it   realy   manufacturer   system  to   increase    bino's  cost   and    theirs profits?

 

I don't think so. It is part of the general goal of selling a range of products at different prices, however. Features help them make distinctions.

 

I have about 25 vintage Japanese porros with Bk7 prisms and another 5 or so with Bak4 prisms. Nearly all are ultra-wides, 10 degrees or higher. In the Bk7 sample, only one exhibits noticeable vignetting in daylight use that I attribute to the Bk7 prisms. (It's a Mayflower 7x35 that claims 12 degrees, for those curious.) I don't recall any issues in the others that I attribute to the prisms. They're generally a bit dimmer than modern bins, and several have a stronger color cast--but that's the coatings, not the prisms.

 

Keep in mind that during daylight, your pupils are contracted. They therefore fit entirely inside the bright, diamond-shaped center area that you see when you hold a Bk7 pair away from your eyes. The dimmer area on the edge is outside your vision. You don't see it. At night, your dilated pupils might reach the dimmer area, but minor dimming in peripheral vision is unnoticeable.

 

I'm delighted to find the occasional Bak4 vintage bin, especially at a wide FOV or short focal ratio. I prefer them, partly because in old bins Bak4 often indicates good quality throughout the instrument (there are exceptions). I have no prejudice against Bk7, however.


Edited by asphericalaberration, 14 December 2019 - 08:13 PM.


#9 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,562
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 December 2019 - 07:34 PM

You don't see it. At night, your dilated pupils might reach the dimmer area, but minor dimming in peripheral vision is unnoticeable.

 

 

The dimming caused by diamond pattern affects the entire field including the center, not just your peripheral vision, its like a smaller exit pupil.  

 

Jon


  • hallelujah and SMark like this

#10 asphericalaberration

asphericalaberration

    Explorer 1

  • -----
  • Posts: 94
  • Joined: 30 Jan 2019

Posted 14 December 2019 - 08:28 PM

The dimming caused by diamond pattern affects the entire field including the center, not just your peripheral vision, its like a smaller exit pupil.  

 

Jon

I'm unsure of that. I've believed that contracted pupils during the daytime are in effect part of the optical train. They literally stop down or truncate the bin's exit pupil to the current diameter of the eye's pupil. If that's true, then in daylight use with contracted pupils the outer portions of the bin's exit pupil don't reach the retina because the iris is in the way. Is your comment directed at night use with dilated pupils? The daytime image defect in the Mayflowers mentioned above is the diamond-shaped vignetting you'd expect from Bk7. I believe I picked up this factoid up from Bill Cook's book (which I might mis-interpret or mis-remember).  Please persuade me.


Edited by asphericalaberration, 14 December 2019 - 08:39 PM.


#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,562
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 December 2019 - 02:22 PM

You wrote this:

 

"At night, your dilated pupils might reach the dimmer area, but minor dimming in peripheral vision is unnoticeable."

 

If you look back at my post, you will see I was responding to you comment about ones dark adapted pupil. If the dilated pupil is large enough to reach the diamond region, the entire image is affected. The center is also dimmer than it would have been,  not just the periphery. 

 

Jon


  • SMark likes this

#12 asphericalaberration

asphericalaberration

    Explorer 1

  • -----
  • Posts: 94
  • Joined: 30 Jan 2019

Posted 15 December 2019 - 02:53 PM

Ah, yes, thank you. I misunderstood. I should read before writing.


  • Jon Isaacs likes this

#13 Rich V.

Rich V.

    Fly Me to the Moon

  • *****
  • Posts: 7,235
  • Joined: 02 Jan 2005
  • Loc: Lake Tahoe area, Nevada

Posted 15 December 2019 - 02:58 PM

You could find that your BaK4 prisms are not BaK4 - there is no standard for BaK4 and it seems some BaK4 is not even Barium, lots are a Phospherous crown glass.

Yes, according to the Binocular Optics and Mechanics chapter of Stephen Tonkin's Binocular Astronomy book, he points out that BaK4 is Schott's barium crown glass designation and has a refractive index of 1.5688 with a critical angle of 39.6°.  Chinese "BaK4" are phosphate crown glass formulations that have an index of 1.5525 and a critical angle of 40.1° which falls between Schott's Bak4 and BK7 glasses.  Schott BK7 has an index of 1.5168 with a critical angle of 41.2°.

 

Rich


  • Mr. Bill and j.gardavsky like this

#14 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,562
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 December 2019 - 03:30 PM

Ah, yes, thank you. I misunderstood. I should read before writing.

 

I think it happens to all of us.

 

The worst is when I write a long response and then, before posting, I go back and read the post I was responding to and realize I'd misunderstood the post and the half hour I'd spent writing had just been wasted. 

 

Jon


  • Albie and j.gardavsky like this

#15 SMark

SMark

    Skylab

  • *****
  • Posts: 4,460
  • Joined: 29 Aug 2011
  • Loc: Atlanta, GA USA

Posted 18 December 2019 - 01:32 AM

There are a few higher quality vintage porro models that can be found with either Bk7 or BaK4 prisms. These include the FPO (Fuji) Bushnell Rangemaster and Featherlight 7x35, as well as the Kowa Wide Angle 7x35. With all of these models I have made night sky image comparisons, and in all cases the BaK4 prism models have offered the brighter images across the entire field of view. While the difference isn't necessarily striking, it is nevertheless obvious. Especially in the wide angle models.

 

For this reason, there are but a very few Bk7 loaded wide angle 7x35 binoculars in my collection. I tend to disqualify any vintage wide angle binocular with Bk7 prisms, unless it offers something otherwise unique.  But as mentioned above, BaK4 prisms were generally only offered in the higher quality models anyway. Whereas today they are used almost universally. 


  • hallelujah and Grimnir like this

#16 Chomatic Aberrant

Chomatic Aberrant

    Explorer 1

  • -----
  • Posts: 74
  • Joined: 15 Sep 2019
  • Loc: Near Chicago

Posted 31 January 2020 - 12:39 PM

I know the standard way of detecting which kind of porros are installed in a pair of Binos by looking through objective to see the diamond pattern..  But when prisms are removed I would think that having a test prism known to be BK7 glass and holding that prism up against another prism of unknown glass so that their hypotenuses are parallel and with their sides touching.....one could eyeball through each and compare the amount of deviation of the views.  The higher Refractive Index of a Bak4 specimen would show more deviation.  Would this be correct?      C.A.



#17 Binojunky

Binojunky

    Fly Me to the Moon

  • *****
  • Posts: 5,788
  • Joined: 25 Dec 2010

Posted 31 January 2020 - 01:40 PM

Heck, if you like the binocular then don,t worry about all the stuff people go on about like BAK 4 or BK 7, ED glass,Chro: Abberation, fully coated or fully multicoated, just enjoy it for what its worth, some people are blind or heading that way,JMTCW, D


  • hallelujah likes this

#18 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 5,554
  • Joined: 16 Sep 2014

Posted 31 January 2020 - 03:20 PM

I am one of those who cannot tell any visual difference,

  other than a little coloration, but for others it's important.     

  That BAK4/BAK7 divide

 has actually    been true for about 70 years now.

  For me, it's like paying extra for 'whitewall' tires.



#19 Chomatic Aberrant

Chomatic Aberrant

    Explorer 1

  • -----
  • Posts: 74
  • Joined: 15 Sep 2019
  • Loc: Near Chicago

Posted 01 February 2020 - 10:39 AM

Is this beginning to sound like audiophiles discussing the virtues of their favorite brand of Speaker Cables?   C. A. 



#20 Rich V.

Rich V.

    Fly Me to the Moon

  • *****
  • Posts: 7,235
  • Joined: 02 Jan 2005
  • Loc: Lake Tahoe area, Nevada

Posted 01 February 2020 - 10:52 AM

Is this beginning to sound like audiophiles discussing the virtues of their favorite brand of Speaker Cables?   C. A. 

 

I hope not; IMO, there's a huge divide between the application of real physics and the superstitious belief in magic.


  • hallelujah and SMark like this

#21 Yarddog

Yarddog

    Ranger 4

  • -----
  • Posts: 379
  • Joined: 20 Aug 2019

Posted 01 February 2020 - 11:09 AM

I have looked through a few binoculars with BK-7 prisms and would have been fooled, but I noticed a long time ago that all the premium ones had bak-4 prisms.

 

Probably the best one was an old Remington 7X35 which was new old stock around 20 years ago. I don't know which Remington brand but still every time I look through it, I get that feeling of quality regardless of it's BK-7 prisms.

 

JB-212, E-22, T.J.K.



#22 hallelujah

hallelujah

    Cosmos

  • *****
  • Posts: 7,915
  • Joined: 14 Jul 2006
  • Loc: North Star over Colorado

Posted 01 February 2020 - 11:13 AM

Is this beginning to sound like audiophiles discussing the virtues of their favorite brand of Speaker Cables?  

Speaker cables appeal to the ears; some ears are more attuned than others. blahblah.gif

 

Binocular prisms appeal to the eyes; some eyes are more sensitive than others. bigshock.gif

 

Stan
 


  • SMark and j.gardavsky like this

#23 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 5,554
  • Joined: 16 Sep 2014

Posted 01 February 2020 - 06:28 PM

The difference, whatever it is, is greater for

  super or extra-wide fields than for regular or wide fields.

The color difference you see Eps-at-1-f t is a reflection

   in the BK-7s not being as total.  There is some 'perfection'

   to the argument...to be fair.



#24 Brian57

Brian57

    Lift Off

  • *****
  • Posts: 6
  • Joined: 20 Oct 2019
  • Loc: Tacoma WA USA

Posted 16 May 2020 - 02:39 PM

Related, I own a pair of Fujinon FMT-SX 10x50s that are wonderful for wide-field nighttime viewing on a mono-pod. Now thinking of buying the Fuji 16x70s--but B&H lists them as having BK7 prisms. This is odd since there are reviews here on CN that rate these binos as preferable to the the Nikon Astroluxe 18x70--which have BK4 prisms.

 

I'm not so snobby that I will pass up the Fujis over the more expensive BK4 Nikons--well-written reviews mean more than specifications to me. Still, I'm wondering if the BK7 print is a mistake, or if there is a technical reason Fuji went with this "lesser" prism.  Any thoughts?


Edited by Brian57, 16 May 2020 - 02:39 PM.


#25 hallelujah

hallelujah

    Cosmos

  • *****
  • Posts: 7,915
  • Joined: 14 Jul 2006
  • Loc: North Star over Colorado

Posted 16 May 2020 - 02:46 PM

 

 

Still, I'm wondering if the BK7 print is a mistake, or if there is a technical reason Fuji went with this "lesser" prism. 

Any thoughts?

It's a typo.

 

http://www.company7..../1670fmtsx.html

 

Stan


  • Grimnir likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics