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Prism glass: BaK-4 vs BK-7

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#51 pat in los angeles basin

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Posted 14 July 2021 - 07:50 PM

SMark; I've  only been "under the hood" of one nikon ( 8x30 8.5) and was impressed with the prisms- largish back prisms to catch the large portion of the cone  and smaller forward prisms, both slotted. I've been inside  kowas and not  found the slotting on any other Japanese made binos. Are you aware of any other that use that particular  tech? And does the slot end  up being a stress raiser? Also found the Nikons had the element edges blacked- another first in the Japanese binos I've examined.  Pat



#52 Grimnir

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Posted 15 July 2021 - 03:21 AM

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

 

Not me. My guess - and it is only a guess - is that there have been none post-war but maybe there were some in the early days of Nippon Kogaku.

 

As others have said you're the guy to ask!

 

Graham



#53 Grimnir

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Posted 15 July 2021 - 03:23 AM

SMark; I've  only been "under the hood" of one nikon ( 8x30 8.5) and was impressed with the prisms- largish back prisms to catch the large portion of the cone  and smaller forward prisms, both slotted. I've been inside  kowas and not  found the slotting on any other Japanese made binos. Are you aware of any other that use that particular  tech? And does the slot end  up being a stress raiser? Also found the Nikons had the element edges blacked- another first in the Japanese binos I've examined.  Pat

 

Some of the early up-market Swift porros had slotted prisms.

 

Graham


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#54 pat in los angeles basin

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Posted 15 July 2021 - 10:48 AM

What does the slot contribute to the throughput? Is there some interference that happens without it that is usually tolerated?    Pat                                                                                                                           



#55 Grimnir

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Posted 15 July 2021 - 11:35 AM

What does the slot contribute to the throughput? Is there some interference that happens without it that is usually tolerated?    Pat                                                                                                                           

Yes, reduces stray light and thus enhances contrast.

 

Graham


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#56 SMark

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Posted 15 July 2021 - 12:01 PM

Look for them in Tamron made binoculars. As Grimnir mentions, those really nice early Swift models as well as the Tamron Rangemaster.


Edited by SMark, 15 July 2021 - 01:12 PM.

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#57 pat in los angeles basin

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Posted 15 July 2021 - 12:34 PM

DO the German/Austrian makers typically use slotted prisms ( never had the opportunity to look inside one!) in their classic porro  bino's? Curiosity run amok...   Pat



#58 FrankL

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Posted 15 July 2021 - 01:25 PM

The purpose of slotting Porro I prisms was to reduce stray light inside the prism, and was generally speaking a characteristic of higher quality binoculars although lots of high quality binoculars were made without them. Zeiss often used them, for example. They are not uncommon. To what extent they actually reduce stray light is a good question.


Edited by FrankL, 15 July 2021 - 01:28 PM.

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#59 j.gardavsky

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Posted 15 July 2021 - 01:25 PM

DO the German/Austrian makers typically use slotted prisms ( never had the opportunity to look inside one!) in their classic porro  bino's? Curiosity run amok...   Pat

Here is a pair of ZEISS Porros prisms

 

 

Zeiss Porro prisms pair.jpg

 

 

https://www.ebay.de/...z8AAOSwCypWnf0B

likely for the tactical binoculars.

 

Best,

JG


Edited by j.gardavsky, 15 July 2021 - 01:28 PM.

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#60 pat in los angeles basin

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Posted 15 July 2021 - 01:36 PM

I appreciate the information and the pics. Regarding the Nikon slot- square slot . The Zeiss appears to be semicircular. I wonder if the half circle slot conveys any further advantage vs the hard angle slot. Back to the question of stress raiser? I found the corners of a BaK 4 prisms chipped more often than BK-7 prisms. But that's from the very limited sampling I've experienced. Regards, Pat



#61 ECP M42

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Posted 15 July 2021 - 07:07 PM

DO the German/Austrian makers typically use slotted prisms ... in their classic porro  bino's?

Certainly, also in Leitz (Campar-it and Binux-it). And very good craftsmanship and baffling for the Swaro Habicht SL prisms. Here you can find the PDF of the article which includes the glued SL prisms. Other links

 

https://www.army-sto...ary-binoculars 

https://www.birdforu...0-black.387173/

 

 

I found the corners of a BaK 4 prisms chipped more often than BK-7 prisms.

Generally: denser = stiffer = more brittle.


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