Prism glass

Type of glass the prisms are made of: Bak-4 is a high density glass, of better optical quality than BK-7 and by far the best for making binocular prisms. Fortunately nowadays even cheap binoculars have Bak-4 prisms, whereas that would not have been the case a decade or two ago. Really cheap binoculars all have BK-7 glass prisms.

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