Choosing BaK4 vs BK7 binoculars is more complicated than good glass vs bad glass
BaK4 and BK7 are prism-glass labels, not complete quality grades for a finished binocular. BaK4 can make total internal reflection easier to maintain in demanding prism geometry because a representative BaK4 formulation has a higher refractive index than a representative BK7 formulation. A diamond-edged exit pupil can be a useful clue that part of the