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The Binocular Advisor

Birding Binoculars

Part 2

Roof prisms vs Porro prisms

Porro prism binoculars were standard until the 1960's, when the Zeiss and Leitz companies introduced roof prism binoculars, whose objective lenses were straight in line with the eyepieces. Roof prism binoculars were compact, light, and comfortable to hold. They made the offset, zig-zag shape of the Porro prism design look as old fashioned as propeller-driven aircraft.

Porro prism design has a jog in the light path through each barrel.

Roof prism design looks as if the light goes straight through.

Roof prism binoculars appeared simpler than Porro prism binoculars. But inside, they had a more complex light path and required much greater optical precision in manufacturing. As a result, they cost more to make. The Porro prism design was simpler and more light efficient, and its images showed better contrast. Nevertheless, the roof prism design's appeal was so great that manufacturers went all out to perfect it.

They succeeded. Today, roof prisms dominate the top-end birding binocular market. Porro prism binoculars are not obsolete, however. Dollar for dollar, a Porro prism design will give better performance for the money, especially in medium or low priced binoculars.

Better Porro prisms binoculars are made from a high density glass, BAK-4. If you hold binoculars away from your eyes and up to the light, you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms will have squared-off, non-circular exit pupils.

Michael and Diane Porter

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