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The Binocular Advisor


Birding Binoculars

Part 3

through a glass darkly

Each time light enters or leaves a piece of glass, about 5% is reflected back. Binoculars may have 16 air-to-glass surfaces, with light lost at every surface. In early binoculars, less than half the light got through to the eye. The rest bounced around inside the binoculars, making the image hazy and hard to see, like a movie in a theater with the lights on.

Bigger objective lenses can compensate for the lost light, but they result in heavier binoculars. In the 1940's, it was discovered that coating the glass with magnesium fluoride would let more light through. The original coating technology was a single layer, which reduced reflections to 1-1/2% per surface, instead of 5%. More recently, advanced multi-layered coatings have reduced reflections to as little as 0.25% per surface. Today, in the best binoculars, 95% of the light gets transmitted to the eye.

Coating technology depends on applying perfectly uniform thicknesses, a few millionths of an inch thin. A little too thick or thin, and the coatings won't work. As you might imagine, the better the coatings, the more expensive the binoculars.

With expensive roof prism binoculars, a special feature to look for is anti-phase shifting coatings. Roof prism binoculars with these special coatings on the roof surface will deliver higher-contrast images.

Here are some symbols that are used to describe binocular coatings:

(C), coated optics: one or more surfaces coated.

(FC), fully coated: all air-to-glass surfaces coated. But if any plastic lenses are used, they may not be coated.

(MC), multi-coated: one or more surfaces are multi-layer coated.

(FMC), fully multi-coated: all air-to-glass surfaces are multi-layer coated.

—Michael and Diane Porter

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