From
this post
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). I don't know what glass Leica, Swarovski, etc., use in their binocular prisms.
