Know the Prisms BAK4 or BK7?

Shopping for Binoculars, rifle scopes, etc?  Are you struggling to get a clear and crispy view?  Prisms used in your optical product is everything that matters to the performance.  If you have not heard of BAK4, BK7, or other names, you need to read this article to better understand what glass is used on your optical products.

When the first time I showed a Midas G2 UHD 12x50 Binocular to one of my coworkers, his expression was unreal.  He said "Wow, this is SO clear compared to the one that I have been using for years!"  This is a common reaction first-time Athlon product users.  The secret ingredient is the prisms made of a premium type of glass called BAK4.

BAK4 (Barium Crown) is a special glass that originates from German and is made by the prestigious glass maker Schott.  BAK4 is recognized as the premium material to make high end optical products due to its high Refractive Index.

Most of the binoculars are made with BK7 (Borosilicate crown) glass, which is also a high quality glass made by Schott.  Compared with BAK4, BK7 has a lower Refractive Index (1.516 vs, 1.569 for BaK7).

Well, what does that 0.053 delta do for you?  It means that much more light can go through the optical product and eventually reach your retina.  Typically, you won't notice too much of a difference at the center of the view.  However, you will see a clear distinction at the edge of the view, meaning that a binocular with BAK4 glass will have a sharp and crispy view at the edge while a BK7 binocular will have a faded view at the edge.

As you may have learned from many websites, a common approach to differentiate a BAK4 and a BK7 prism is to point the binocular to bright light and then view the exit pupil.  A binocular made with BAK4 prism will show a round, evenly distributed clear circle (picture on the left). For BK7 prism, you can see the grey “cut-off” edges as a result of less light went through (picture on the right).


There has been a lot of controversy around BAK4 on internet, largely due to BaK4 is also a code name for glass made in China.  The Chinese Standard BaK4 has a Refractive Index of 1.55248, slightly lower than the BAK4 made by Schott.  Unfortunately most manufacturers do not disclose what type of glass they are using, or who actually made the glass under what standards.

If you like the sharp and clear view, we would like to recommend our collection from Athlon Optics which are proudly made by Schott BAK4 glass.

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