Whats the difference between BAK4 and BK-7 prisms?

bak4 bk-7 Prisms
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Whats the difference between BAK4 and BK-7 prisms?

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BAK4 prisms (barium crown glass) are the highest quality available. BK-7 prisms (borosilicate glass) are also good quality, but brightness falls off slightly at the edge of the field compared to BAK4. Note: Every pair of general purpose or astronomical binoculars we offer has BAK4 prisms.

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