By using our own "best of" I anwsered at least one of my own questions. Rboe quotes a source saying that older f5 binoculars used BK7 prisms and f4 or faster use BaK4 prisms. This doesn't prove the f5 designs were perfect, but provides a "historic fence" between workable BK7 and Bak4 designs.
Another post does state a higher refractive index for Bak4. So BaK4 bends light more at transistions and BK7 less so. So which glass needs bigger prisms? Looks like I have some more thinking to do.
If anyone is still interested, I reviewed all of Rolands posts on astromart that have anything to do with binoculars / binoviewers. The thread called "bino wars" is the most applicable. The message is the same but he never gets into the nitty-gritty of why Bak4 is better BK7 with fast optics. Perhaps it's like my old physics text says, The rest of the proof is left to the reader as an exercise.
Never one to beat a dead horse,
Peter
-------------------- Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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