BAK-4 Versus BK-7
BAK-4 is a high density glass of better optical quality than BK-7. With BAK-4, if you hold binoculars away from your eyes and up to the light you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms have squared-off, non-circular exit pupils.
The prism is alike: Bak4 prism costs more than Bk7 prism - Bk7 prism is fine to most of the users, but, to some "picky" users, Bak4 prism is a must. The difference is the quality of glass and anti-reflective coatings.
The design of the prisms can affect the quality of the exit
pupil. The BK7 prism typically found in lower priced binoculars produces an exit
pupil with shaded (blurred) edges, whereas the BAK4 prism has a perfectly round
exit pupil. Whilst the BK7 prism design would look no different to the BAK4 in
daylight, as the light levels drop, and the eye pupil expands you start to
observe the effects of the shaded regions as the image quality drops and becomes
prone to chromatic aberration around the periphery of the image.
See the illustrations bellow