Nixterdemus
Well-known member
I've been looking at the discontinued, 2009-2014, Weaver Super Slam 8.5X45. I suppose the most intriguing aspect, more so than the focus wheel in front of the rear bridge, is the claim of a SK, no mention of number, sub-prism combined w/Bak-4. As well the discontinued Grand Slam 8.5X45 claimed sub-prism of BK-7 combined w/Bak-4.
The differing cost of the various prisms isn't much, so I wonder if potential advantage in mix-n-match, due to each glass having its own Abbe & refraction number, could make enough difference beyond marketing spiel.
Also, wouldn't the Bak-4 be placed betwixt the objective and the alleged complimenting prism?
To add to me confusion, not that it's much of a reach, I found this inside of a lengthy post.
"My understanding of dispersion, which is the cause of colour fringing, is that it occurs where light traverses a boundary from one material to another of a different refractive index. Therefore it stands to reason that dispersion would not occur within the prisms, at the surfaces of total internal reflection, but only at the entry and exit faces of the prisms. Further, if the incident rays of light are limited to those close to the normal of the prism (perpendicular to the prism's entry face), as would be the case in a binocular of narrow field of view and/or large f-number (or long focal length, when talking about binoculars of a given objective diameter), then the dispersion caused by the prism would be minimised.
Interestingly, Cabela's website states that Kowa use SK15 glass for the prisms of the Genesis/Prominar XD 44. Here is how that compares to common prism glasses, according to Schott's 2007 optical glass catalogue (PDF file):
Refractive index:
N-BK7 - 1.51680
N-BAK4 - 1.56883
N-SK15 - 1.62296
Abbe number:
N-BK7 - 64.17
N-BAK4 - 55.98
N-SK15 - 58.02
(I think the "N" means the glasses are environmentally friendly replacements for the old leaded versions.)
Cabela's says that the "SK15 roof prisms have a high refractive index". Now, I can't speak for anyone else, but I'm not terribly fussy about the refractive index of my prisms! As long as the refractive index is sufficient to cause total internal reflection for all light within the light-cone from the objective, as will be the case in any competently designed binocular, the refractive index doesn't matter. BAK4 glass allows a steeper light cone (i.e. a faster objective, which in turn permits a more compact binocular) than BK7, but otherwise BK7 is superior due to its higher Abbe number (lower dispersion). SK15 seems to strike a decent balance: a higher refractive index than BK7 or even BAK4, permitting a more compact binocular, but lower dispersion than BAK4. I suspect the high Abbe number is why Kowa chose to use it, because the Kowa Prominar XD isn't unusually compact (though compact enough to prevent the use of BK7)."
http://www.birdforum.net/showpost.php?p=1106753&postcount=69
It seems to say that Bak-4 is superior to BK-7 due to higher refractive value if the optical train was too fast/lower focal ratio for BK-7; also allowing for a more compact bin.
Then a SK-15 could help, due to higher Abbe # /lower dispersion/better contrast than Bak-4?
I'm enjoying a headache attempting to explain what I do not understand.
ETA: I'm more curious if the argument has validity than the particular instrument in question. Could there be a noticeable advantage to mixing prisms? It would seem if such were the case that it would be somewhat prevalent as opposed to obscure. Likewise, if SK-15 is the cat's
meow why not use it exclusively or is the sum of the twain mixed, Bak-4/SK-15, greater than either alone?
More snooping on various aspects:
https://www.bestbinocularsreviews.com/glossary-binoculars.php
"Abbe Number
Also known as Abbe constant, the Abbe number of a glass and other materials quantifies the amount of dispersion (variations in index) for a specific spectral range. A high Abbe number means that different wavelengths will have nearly the same index of refraction in that medium, yielding less separation between wavelengths of light and thus generally gives less color dispersion and reduces color aberration. Also, certain glass types have different transmission wavelength regions. The Abbe Number of a glass and the index of refraction are used by designers as degrees of freedom when designing systems.
Index Of Refraction
The index of refraction or refractive index of a substance in optics is a number that does not contain any dimensions, but which describes how light (or indeed any other radiation), goes through that substance.
Thus, the index of refraction refers to the ratio of the speed of light in a vacuum to the speed of light through a given material at a given wavelength.
A higher index of refraction generally bends light more efficiently so there is less of a need of curvature in the lens. Spherical aberration is less present in lenses with higher indices of refraction, while light travels faster through materials with lower indices of refraction. The index of refraction and Abbe number of a glass are typically used by designers as degrees of freedom when designing systems.
BaK-4 prisms
Standing for Baritleichkron which is a Barium Crown glass, BaK-4 prisms are in most cases considered superior to BK-7 prisms as they have a higher refractive index (1.569) and so has a lower critical angle. The result is that that less peripheral light is "lost" through the non-total internal reflection.
BK-7 prisms are usually found in lower priced binoculars and are made from a very fine borosilicate crown glass. These are satisfactory, but for the most part are said to be inferior (for binoculars) to the BaK-4 prisms made from barium crown glass, a finer and higher density glass that can reduce internal light scattering and so has the potential to deliver sharper images.
SK15 prisms: Prisms of very high quality SK15 glass that enable minimization of undesirable internal reflections and thus provide a crystal clear image with the best contrast.
According to Schott's 2007 optical glass catalogue here are how the different prism glasses compare to each other:
Refractive index Abbe number
BK7 1.51680 64.17
BaK-4 1.56883 55.98
SK-15 1.62296 58.02
So as you can see from the table above, BaK-4 glass has a higher refractive index than BK7 and thus has a steeper light cone, which in turn enables the manufacturer to make a shorter, more compact binocular than those that use BK7 glass.
BK7 however has a higher Abbe number and therefore has a lower dispersion.
SK15 glass strikes a good balance balance between the two with a higher refractive index than BK7 and BaK-4, permitting a more compact binocular, but still has lower dispersion than BaK-4".
There isn't an abundance of information on the Super Slam/Grand Slam bins. Allbinos has a review on the GS in 2010, but that was the older pre-2009 GS. Whether Weaver did anything other than spruce up the outside w/new two tone body/dropping the ribs I do not know.
Their customer service has been most congenial to me numerous inquiries on a bin three years since discontinued. Still, some data is no longer available. Howevah, as I understand some of the new model GS came out w/BK-7/Bak-4 combo the same time, 2009, the Flagship SS appeared w/SK something/Bak-4.
I see talk of the SS being a clone/cousin of the Vixen Artes 8.5X45 save the forward placement of the focus wheel. I've been researching on/off for a couple of weeks though new information is a trickle at best.
The differing cost of the various prisms isn't much, so I wonder if potential advantage in mix-n-match, due to each glass having its own Abbe & refraction number, could make enough difference beyond marketing spiel.
Also, wouldn't the Bak-4 be placed betwixt the objective and the alleged complimenting prism?
To add to me confusion, not that it's much of a reach, I found this inside of a lengthy post.
"My understanding of dispersion, which is the cause of colour fringing, is that it occurs where light traverses a boundary from one material to another of a different refractive index. Therefore it stands to reason that dispersion would not occur within the prisms, at the surfaces of total internal reflection, but only at the entry and exit faces of the prisms. Further, if the incident rays of light are limited to those close to the normal of the prism (perpendicular to the prism's entry face), as would be the case in a binocular of narrow field of view and/or large f-number (or long focal length, when talking about binoculars of a given objective diameter), then the dispersion caused by the prism would be minimised.
Interestingly, Cabela's website states that Kowa use SK15 glass for the prisms of the Genesis/Prominar XD 44. Here is how that compares to common prism glasses, according to Schott's 2007 optical glass catalogue (PDF file):
Refractive index:
N-BK7 - 1.51680
N-BAK4 - 1.56883
N-SK15 - 1.62296
Abbe number:
N-BK7 - 64.17
N-BAK4 - 55.98
N-SK15 - 58.02
(I think the "N" means the glasses are environmentally friendly replacements for the old leaded versions.)
Cabela's says that the "SK15 roof prisms have a high refractive index". Now, I can't speak for anyone else, but I'm not terribly fussy about the refractive index of my prisms! As long as the refractive index is sufficient to cause total internal reflection for all light within the light-cone from the objective, as will be the case in any competently designed binocular, the refractive index doesn't matter. BAK4 glass allows a steeper light cone (i.e. a faster objective, which in turn permits a more compact binocular) than BK7, but otherwise BK7 is superior due to its higher Abbe number (lower dispersion). SK15 seems to strike a decent balance: a higher refractive index than BK7 or even BAK4, permitting a more compact binocular, but lower dispersion than BAK4. I suspect the high Abbe number is why Kowa chose to use it, because the Kowa Prominar XD isn't unusually compact (though compact enough to prevent the use of BK7)."
http://www.birdforum.net/showpost.php?p=1106753&postcount=69
It seems to say that Bak-4 is superior to BK-7 due to higher refractive value if the optical train was too fast/lower focal ratio for BK-7; also allowing for a more compact bin.
Then a SK-15 could help, due to higher Abbe # /lower dispersion/better contrast than Bak-4?
I'm enjoying a headache attempting to explain what I do not understand.
ETA: I'm more curious if the argument has validity than the particular instrument in question. Could there be a noticeable advantage to mixing prisms? It would seem if such were the case that it would be somewhat prevalent as opposed to obscure. Likewise, if SK-15 is the cat's
meow why not use it exclusively or is the sum of the twain mixed, Bak-4/SK-15, greater than either alone?
More snooping on various aspects:
https://www.bestbinocularsreviews.com/glossary-binoculars.php
"Abbe Number
Also known as Abbe constant, the Abbe number of a glass and other materials quantifies the amount of dispersion (variations in index) for a specific spectral range. A high Abbe number means that different wavelengths will have nearly the same index of refraction in that medium, yielding less separation between wavelengths of light and thus generally gives less color dispersion and reduces color aberration. Also, certain glass types have different transmission wavelength regions. The Abbe Number of a glass and the index of refraction are used by designers as degrees of freedom when designing systems.
Index Of Refraction
The index of refraction or refractive index of a substance in optics is a number that does not contain any dimensions, but which describes how light (or indeed any other radiation), goes through that substance.
Thus, the index of refraction refers to the ratio of the speed of light in a vacuum to the speed of light through a given material at a given wavelength.
A higher index of refraction generally bends light more efficiently so there is less of a need of curvature in the lens. Spherical aberration is less present in lenses with higher indices of refraction, while light travels faster through materials with lower indices of refraction. The index of refraction and Abbe number of a glass are typically used by designers as degrees of freedom when designing systems.
BaK-4 prisms
Standing for Baritleichkron which is a Barium Crown glass, BaK-4 prisms are in most cases considered superior to BK-7 prisms as they have a higher refractive index (1.569) and so has a lower critical angle. The result is that that less peripheral light is "lost" through the non-total internal reflection.
BK-7 prisms are usually found in lower priced binoculars and are made from a very fine borosilicate crown glass. These are satisfactory, but for the most part are said to be inferior (for binoculars) to the BaK-4 prisms made from barium crown glass, a finer and higher density glass that can reduce internal light scattering and so has the potential to deliver sharper images.
SK15 prisms: Prisms of very high quality SK15 glass that enable minimization of undesirable internal reflections and thus provide a crystal clear image with the best contrast.
According to Schott's 2007 optical glass catalogue here are how the different prism glasses compare to each other:
Refractive index Abbe number
BK7 1.51680 64.17
BaK-4 1.56883 55.98
SK-15 1.62296 58.02
So as you can see from the table above, BaK-4 glass has a higher refractive index than BK7 and thus has a steeper light cone, which in turn enables the manufacturer to make a shorter, more compact binocular than those that use BK7 glass.
BK7 however has a higher Abbe number and therefore has a lower dispersion.
SK15 glass strikes a good balance balance between the two with a higher refractive index than BK7 and BaK-4, permitting a more compact binocular, but still has lower dispersion than BaK-4".
There isn't an abundance of information on the Super Slam/Grand Slam bins. Allbinos has a review on the GS in 2010, but that was the older pre-2009 GS. Whether Weaver did anything other than spruce up the outside w/new two tone body/dropping the ribs I do not know.
Their customer service has been most congenial to me numerous inquiries on a bin three years since discontinued. Still, some data is no longer available. Howevah, as I understand some of the new model GS came out w/BK-7/Bak-4 combo the same time, 2009, the Flagship SS appeared w/SK something/Bak-4.
I see talk of the SS being a clone/cousin of the Vixen Artes 8.5X45 save the forward placement of the focus wheel. I've been researching on/off for a couple of weeks though new information is a trickle at best.
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