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After some more digging, I was finally able to confirm that the Zhumell 25x100mm Tachyon binocular is in fact made in China. What's more, this bino uses BK-7 prisms. One dealer claimed the BK-7 prisms will perform equally well as the BK-4! I found that claim to be dubious, based on what I have learned here. .....

Finally, I saw a strange pair on binos on eBay. They strongly resembled the Oberwerk 22x100 with the white finish. Only these were 25x100 and "multi-coated" with MgFl only. The seller claimed that the more coatings of MgFl the more light transmits through - in other words, the seller was trying to insinuate that these single-coated binoculars were multi-coated because they had several identical coatings of MgFl. .....
MikeG

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Anyone wishing to understand the applications of BaK7 and BK4 prisms should read everything that Roland Christen has published on the subject.

PRISMS
BK7 glass is probably one of the best glass in the world for prisms, however is not particularly well suited for the light cone from a fast lens. For this BaK4 performs equally well or better.

COATINGS
If a lens has several layers of MgF, then it is multicoated. But if so, it may likely not have the characteristic blue color of an MgF coated lens. A properly multicoated lens surface may reflect only 0.2% to 0.5% of the entering light. A properly single-coated lens surface may reflect only 1.5% of the light. An uncoated lens surface may reflect 4% of the light. Considering a binocular might have 12 to 16 surfaces, the importance of multicoating soon becomes evident.

Probably just as important is the precise control of the thickness of the coating process. An improperly controlled multi-coating process can produce a lens coating that reflects more light than a single coating. Although you can't truely determine anything about the precision of the coating application by the color of the coating, in general, poorly coated lens tend to look bright green. The bright green color could result from coatings being applied too thick. Improper thickness will reflect more light than it should. However, a properly multi-coated lens can give off a faint green color or a faint purple/pink color.

You'd be better off with a properly applied single coating than an improperly applied multi-coating.

edz

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