The designations BK7 and BaK4 when used above refer to glass designations used by Schott AG, a German manufacturer of optical glass.
A lot of the advertising hype on budget Chinese binoculars has been their "BaK4 prisms". This morning, I was looking up the data on these when I realised that the data I was seeing for BaK4 was not the same as for the data I remembered for BaK4 -- specifically, the critical angle was larger. So, a little bit later, this is what I have:
The Chinese designation "BaK4" is an entirely different glass to the Schott BaK4 -- BaK stands for Baritleichkron (Barium Crown); the Chinese BaK4 is actually Schott PSK3, which is not a Barium Crown at all: it is a phosphate crown. PSK3 is much cheaper to make than BaK4; it also has a lower refractive index.
Here are the important data:
Not only this, but the tolerances for bubble density is much laxer for Chinese BaK4 (PSK3); it permits twice the bubble density that is permitted in Schott BaK4.
What this means is that, whilst Chinese BaK4 gives you a smaller critical angle than Schott BK7, it is not as small as for Schott BaK4. For wide angle binoculars, this means that there is a greater likelihood of non-total internal reflection.
It also means that I have been entirely wrong when I have dismissed Chinese BaK6-prismed binoculars as "hype" -- the specs are actually closer to Schott BaK4 than is the Chinese BaK4.
It's not all down-side; the dispersion is lower than in Schott BaK4; this is relevant in binoculars that are collimated by prism-tilt because, if the "wrong" side is adjusted (P=0.5 for amateur collimation ), the amount of false colour that is introduces will be less. Of course, the difference will probably be disguised by lax optical quality elsewhere in the system, but....
The daft thing is that you can buy (from China) the Chinese equivalent to Schott BaK4 for exactly the same price as Chinese BaK4! Why the hell don't they just use the "proper" stuff!?!?!?
Edit: There's a useful chart at: http://upload.wikime...e-diagram_2.svg
You can see the relative positions of Chinese BaK4 (PSK3), BaK4 and BK7. The Abbe number gives a clue about dispersion -- the higher it is, the less dispersion you will get. This is why, for example, BK7 is to be preferred to BaK4 for binoviewers.
Edited by tetenterre, 06 February 2012 - 01:20 PM.