Optical glass

Three types of glass are currently used for making lenses and prisms: BK-7, BaK-4 and SK15.

SK15 is the highest quality glass available and has only recently been introduced to optics of binoculars (expensive). I found only two brand names using this glass: Kowa, in their Kowa Genesis XD Prominar series and Eschenbach, in their Eschenbach Farlux Selector series. Manufacturers claim “near total reflecting BaK-4 and SK15 prisms”.

BaK-4 (barium crown) is a high density glass (with less air bubbles and glass deformities) used in all quality binoculars for both the lenses and the prisms and is by far the best for making binocular lenses and prisms, since it virtually eliminates light scattering, ensuring sharp, well defined images – clarity and brightness to the very edge of the field of view, as well as high colour fidelity.

BK-7 (borosilicate) used to be used decades ago and is still used in cheap binoculars. In the case of BK-7 optics some light is lost, resulting in slightly dimmer images. Fortunately nowadays even cheap binoculars have Bak-4 lenses and prisms, whereas that would not have been the case a decade or two ago.

Wondering whether your binoculars have BaK-4 or BK-7 prisms? Look down the objective lenses towards the inside, and if you see a perfectly round image, you have prisms made from BaK-4; if the image is squared off at the edges of the image, your binoculars have BK-7 prisms.

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