Then someone please explain
Why the giants from fujinon have BAK4?
Why the giants from oberwerk have BAK4?
And almost any expensive bin from a big brand mentions "BAK4".
Are they all wrong at the same time?
Except for Henry's brief comment, it may not be particularly evident from the discussion that optical glass has
two essential properties in design: an index of refraction and an Abbe V number, the latter of which gives the dispersion
relative to refraction (the amount of bending that a light ray undergoes). For visible-spectrum glass, the V-number is defined as the refractive index at a mean wavelength (helium d line), divided by the difference in refractive indices between short and long wavelengths (hydrogen F and hydrogen C lines). This ratio is referred to as the inverse of dispersive power, but the important point is that dispersion is expressed
relative to refraction. A two dimensional plot with the mean refractive index shown on one axis, and the (non-independent) Abbe V number on the other, locates a particular glass within this two-dimensional conceptual framework.
There are many glass types to pick from that could influence final performance. Frankly, I don't think we consumers really get to know which ones are used in any given instrument, and the dichotomy between BaK-4 vs. BK7 has probably become a convenient but somewhat misleading marketing simplification. For those who build their own telescopes the situation is markedly different, of course, because they get to pick not only the glass for their own applications, but also their own applications.
Several parameters in binocular design can be manipulated to correct (compensate) for the CA/SA contributed by the prisms. In the end a holistic instrument is produced, however, and not a collection of individual pieces. Without complex math models, moreover, no one can readily predict the performance of the total system just by knowing the performance of the individual parts. In such fully-coupled, coherent optical systems, aberrations
interact between the components, sometimes canceling and sometimes reinforcing each other. CA and SA are particularly subject to this complex interaction effect, and, in fact, the optics of the eye itself (exclusive of the retina) also become part of the coupled system. Its aberrations interact with those of the instrument.
Vignetting, unlike optical aberrations, is essentially a geometric issue that imposes a boundary constraint on the refractive intex, and this largely limits the glass types that are appropriate, i.e., the right area of the glass chart mentioned above. Assuming that CA and SA are properly corrected, however, I for one am not persuaded that an instrument with BK7 prisms
intrinsically produces a better quality image at the retina than one using BaK-4 (or 1 or 2). One can not reason from the properties of the prism glass independent of the design of which it is a part.
So, to finally answer your question, BaK-4 glass shows up in high-end products because it best meets the design objectives. Apparently, most companies have come to the same conclusion.
I can't imagine why lower-end products use BK7 prisms if not for the price advantage or some other factor related to manufacturing costs.
Blue skies,
Elk