There are prisms located inside binoculars that function to flip an inverted image upright. There are two common styles of prisms used in binoculars: the BK-7 and the BAK-4. The BAK-4 prism is made of a higher-density glass and can produce sharper images than a BK-7 prism can. If you are unsure as to which prism is being used, hold the binoculars out in front of you and look through the eyepiece. If you see a square shaped beam of light, chances are a BK-7 prism is being used. A round beam of light indicates the use of a BAK-4 prism.
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