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Thread: Binoculars: BaK4, BK7, fully multicoated - FAQ

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    Default Binoculars: BaK4, BK7, fully multicoated - FAQ



    Hello friends of binocular astronomy,

    there are lots of questions regarding the prism materials and the coatings, frequently asked.

    BK7 is borosilicate crown glass with low dispersion and low refractive index. As the material for Porro prisms in binoculars, it used to be popular in extremely wide field military binoculars (like Asahi Pentax), and also in some wide field astronomy binoculars (suspected in Binon by Miyauchi).
    The advantage is: no added chromatic aberration, and a very sharp view.
    The drawback is: high cost if specified according to Schott, and moreover, additional mirror coatings are necessary on the Porro prism diagonals, due to the low refractive index of this material. Porro prism binoculars employing this material may suffer from a lower transmittance, and eventually from lower contrast, even if the view is still extremely sharp. The brightness of the view may drop towards the edge of the field of view.

    All these advantages and disandvantages are well recognizable on my 7x35 (11deg) Asahi Pentax vintage binocular. However, the extremely wide field of view enables to fix the very large nebulas.

    BaK4 is Barium crown glass with high dispersion and high refractive index. This is regarded as the preferable material for the Porro prisms.
    The advantage is: clear bright view due to the totally reflecting diagonals in the Porro prisms, additional mirror coatings are not required.
    The drawback is: some added chromatic aberration, readily visible during daylight on the bright illuminated white surfaces against a darker background - as frequently seen in the popular "chimney" test. (The rest of the chromatic aberration is due to the lenses in the binocular.)

    Most of the binocular Porro designs in Japan employ BaK4 for the prisms.

    Some of the binoculars on the market indicate the BaK4 prisms, even if this material has been already replaced by phosphate crown, or replaced in another way. This new material has a high refractive index, nearly as high as BaK4, and a low dispersion, nearly as low as BK7, combining to some extent the advantages of both. Binoculars using such prism materials have often slower optics (f/# larger than f/5), and they have larger sized prisms, even if the field of view is not very wide. The overall impression is that of a "bulky" binocular. Possibly, the BA8 series by United Optics employs such prism material, or anything similar. An added chromatic aberration is as good as not recognizable. The inherent chromatic aberration due to the lenses, is lower when the optics is slower, anayway.

    Fully multicoated is a property important for the clear bright viewing without ghosting. Fully broadband multicoatings yield viewing with a sort of "enhanced" colors, especially in green, yellow and red. This is the typical requirement for birding. In astronomy, the fully broadband multicoatings show the true orange-red color on the Mira-variable stars and on lots of the Carbon stars. Without this "broadband" property of the coatings, the bright orange-red stars may turn into the "brownish" spots, or they show a "pale" color.

    Other questionable specs are the field of view, and the "usable" size of the front lenses. The usable size of the front lens indicates a fraction of its diameter that transmitts the light flux to a fully opened eye. Both these binocular properties depend on the size of the prisms and on the design of the eyepieces, and they may be 10% down if compared with the specifications.

    Best

    JG
    Binoculars: Leica Ultravid 7x42, 8x42HD; Swarovski EL 8.5x42 Swarovision; Nikon 10x70 Astroluxe; Docter Nobilem 7x50 Porro; Jenoptem 7x50W, 10x50W; BA8: 10.5x70, 15x85; 25x100FB, AsahiPentax 8x40, Refractors: Sky-Watcher 150mm/750mm; Leica APO Televid 82mm (25x-50x WW ASPH); EPs:Baader Classic Orthos; Fujiyama ortho, Leica B WW, ultrawide zoom ASPH, Periplan GF, HC Plan S, L; DOCTER UWA; Wild UW mil; Tele Vue Delos, Nagler Zoom, Plössls; Swarovski SW; Pentax XW; ZEISS diascope B WW T*, Carl Zeiss E-Pl; Hensoldt mil; Filters: Astrodon, Astronomik, Baader (CCD), TS; Astrophotography: AstroTrac; Leica R7: Leica 2/50, 2/90mm, 2.8/180mm lenses
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    Default

    Thanks for the information JG.

    There still remains the problem of manufacturers properly stating the specifications of their binoculars, as many of these properties are difficult to test. A binocular may be termed 'fully multi-coated', even though some of the internal surfaces are not multi-coated. Sometimes, you can see reflections from the glass surfaces when illuminating them with a bright light that are not green or red in colour, and this can be an indicator of the binocular having surfaces without full multi-coating. Also, the difference between Bak4 and BK-7 can also be difficult, unless the exit pupil appears as a diamond-like shape (BK-7) compared to the typical round Bak4 appearance.

    I suspect many of the Chinese imports are improperly labeled, as the vendors selling to end-users may not even know themselves what the specifications are! I suppose then it is utterly important to buy from a reputable dealer that might even 'test' the binoculars they import, or better yet, manufacture their own.

    Thanks again JG, this is a very important discussion.

    Jeff
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    Default

    Hello Jeff,

    thanks for the highly agreable comments.

    Some of the dealers, who put their labels on the imported binoculars, state BAK4 prisms, indicating another glass material with some properties close to the classical BaK4.

    The 'diamond shaped' exit pupil indicates BK7 prisms with missing anti-reflex coatings, and prisms with small sizes. My Asahi Pentax with BK7 prisms does not show this phenomenon, but there are some side-by ghosts around the circular exit pupil discs, not necessarily interfering with the view. These side-by ghosting has nowadays emerged on some cheap roof prism binoculars, as well as.

    The binoculars at Garrett's Optical seem to be correctly specified, and tested before delivery. However, you can get theese binoculars under different labels, elsewhere.

    I am regularly reading the test reports at Jülich (Germany),

    Fernglas Spektiv und Teleskop Tests, Erfahrungsberichte und Testberichte der Firmen Zeiss Leica Swarovski Kowa

    you may drop in, and run a German-to-English translator. You would find out, that there is no binocular to be found, world-wide, without having some flaws in the performance, even the Leicas have been criticized. But we must live with it, and find out those ones with the best performance-to-price ratio.

    Thanks for the follow up

    JG
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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    I read somewhere that there's German Bak4 and Chinese Bak4. I don't know what the difference is, what kind of Bak4 is used in high-end Japanese binoculars, or how significant the difference is for somebody handholding 10x50 for backyard astronomy.
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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    Quote Originally Posted by Lou3 View Post
    I read somewhere that there's German Bak4 and Chinese Bak4. I don't know what the difference is, what kind of Bak4 is used in high-end Japanese binoculars, or how significant the difference is for somebody handholding 10x50 for backyard astronomy.
    Hello Lou3

    as in the thread start, the BaK4 is the classical Barium Crown prism material, used in fact world wide, including Japan.
    The advantage of the BaK4 is its high refractive index, resulting into a practically zero transmission loss of the light in the Porro prisms in binoculars. The drawback is the added chromatic aberration due to the high wavelength dispersion, especially visible towards the edge of the field of view in the binoculars.
    The abbreviation BAK4 is used in China for a prism glass material, which has lower refractive index and lower dispersion, resulting into some loss of the light transmission, but having the advantage of less added chromatic aberration.

    I find the BAK4 prisms in the BA8 binoculars series of United Optics (China) as a good choice.

    Best,

    JG
    Binoculars: Leica Ultravid 7x42, 8x42HD; Swarovski EL 8.5x42 Swarovision; Nikon 10x70 Astroluxe; Docter Nobilem 7x50 Porro; Jenoptem 7x50W, 10x50W; BA8: 10.5x70, 15x85; 25x100FB, AsahiPentax 8x40, Refractors: Sky-Watcher 150mm/750mm; Leica APO Televid 82mm (25x-50x WW ASPH); EPs:Baader Classic Orthos; Fujiyama ortho, Leica B WW, ultrawide zoom ASPH, Periplan GF, HC Plan S, L; DOCTER UWA; Wild UW mil; Tele Vue Delos, Nagler Zoom, Plössls; Swarovski SW; Pentax XW; ZEISS diascope B WW T*, Carl Zeiss E-Pl; Hensoldt mil; Filters: Astrodon, Astronomik, Baader (CCD), TS; Astrophotography: AstroTrac; Leica R7: Leica 2/50, 2/90mm, 2.8/180mm lenses
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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    Thank you for the info, JG.

    I hope to try a BA8. They sound like great binoculars.
    telescope: in between telescopes
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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    Just to add a bit: the Chinese BAK-4 (note that they use an upper-case "A") is not even a barium crown, it is a phosphate crown. It has a lower refractive index and lower dispersion than Schott BaK4. Not necessarily a bad thing (I have no complaints about it in my United Optics BA8 15x70), but note that it also has a higher permitted bubble-count, which may result in increased light scatter (can't detect it, though, in my BA8 or the "el cheapo" 15x70 that I use to experiment on.

    If you're not bored already, I say a bit more about it at BinocularSky - The Minefield: Advertising Hype in Binoculars
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    C&DS, Steve

    Binoculars: Opticron 10x42 BGA, United Optics BA8 15x70, Miyauchi Bj100B 20/37x100

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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    Quote Originally Posted by Tetenterre View Post
    Just to add a bit: the Chinese BAK-4 (note that they use an upper-case "A") is not even a barium crown, it is a phosphate crown. It has a lower refractive index and lower dispersion than Schott BaK4. Not necessarily a bad thing (I have no complaints about it in my United Optics BA8 15x70), but note that it also has a higher permitted bubble-count, which may result in increased light scatter (can't detect it, though, in my BA8 or the "el cheapo" 15x70 that I use to experiment on.

    If you're not bored already, I say a bit more about it at BinocularSky - The Minefield: Advertising Hype in Binoculars
    Hello Tetenterre,

    thanks for repeating what has already been written above in the thread.

    Regarding Schott, this company is a sort of offspring of Zeiss. In the meantime, Zeiss, Leica and some others are cooking their OEM glass melts in their ovens, supposed being above and besides (the new glass materials) the Schott standards.
    Regarding the micro inclusions in the glass, that's what makes quite a difference between the Swarovski, Leica, and Zeiss on the one hand, and those others on the other hand.
    The scattering of light due to the micro inclusions in the glass is visually quite noticeable when you take in one hand one of the budget astronomy binoculars, and in the other hand a high quality binocular. Then, aim the binoculars against the skies, and watch their exit pupils. The high-end binoculars will show you a clear exit pupil, the budget astronomy binoculars will show you a sort of 'milky' exit pupil in this comparison.

    Unfortunately, it becomes increasingly difficult to obtain some high quality glass materials from Schott. The last piece I have succeeded to order and to get (10 years ago) has been the BG39, the color glass with the maximum transmissivity around 500nm.

    Best,

    JG
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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    Quote Originally Posted by j.gardavsky View Post
    Hello Tetenterre,

    thanks for repeating what has already been written above in the thread.
    Oops -- my apologies. I entered the thread at Lou3's query on Chinese Bak4, and did not back-track. Lesson learned. (but still can't see how my bit on bubble-count was a repetition)

    The scattering of light due to the micro inclusions in the glass is visually quite noticeable when you take in one hand one of the budget astronomy binoculars, and in the other hand a high quality binocular. Then, aim the binoculars against the skies, and watch their exit pupils. The high-end binoculars will show you a clear exit pupil, the budget astronomy binoculars will show you a sort of 'milky' exit pupil in this comparison.
    I'll try that again, then. I was unable to notice it the last time I tried (a few years ago) with my Opticron BGA and a Kunming (UO) BM2.
    C&DS, Steve

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    Default Re: Binoculars: BaK4, BK7, fully multicoated - FAQ

    Quote Originally Posted by Tetenterre View Post
    ...

    I'll try that again, then. I was unable to notice it the last time I tried (a few years ago) with my Opticron BGA and a Kunming (UO) BM2.
    Hello Steve,

    even the BA8 10.5x70 shows some 'milky' exit pupil if compared with my Leica Ultravid 7x42. Nevertheless, the BA8 10.5x70 is a high quality binocular, and I would not hesitate to recommend it for the astronomy observations.

    Thanks for your input, and looking forward to your binocular tests,

    JG
    Binoculars: Leica Ultravid 7x42, 8x42HD; Swarovski EL 8.5x42 Swarovision; Nikon 10x70 Astroluxe; Docter Nobilem 7x50 Porro; Jenoptem 7x50W, 10x50W; BA8: 10.5x70, 15x85; 25x100FB, AsahiPentax 8x40, Refractors: Sky-Watcher 150mm/750mm; Leica APO Televid 82mm (25x-50x WW ASPH); EPs:Baader Classic Orthos; Fujiyama ortho, Leica B WW, ultrawide zoom ASPH, Periplan GF, HC Plan S, L; DOCTER UWA; Wild UW mil; Tele Vue Delos, Nagler Zoom, Plössls; Swarovski SW; Pentax XW; ZEISS diascope B WW T*, Carl Zeiss E-Pl; Hensoldt mil; Filters: Astrodon, Astronomik, Baader (CCD), TS; Astrophotography: AstroTrac; Leica R7: Leica 2/50, 2/90mm, 2.8/180mm lenses
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