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Binoculars

Binocular telescopes, or binoculars, are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, one to be viewed through each of the user's eyes to present the viewer with a greater sense of depth and distance than a single-lens telescope allows.

The most common binoculars are of a size adequate to be held using both hands, and contain optical elements to fold the optical path so that the physical length of the binoculars is less than the focal length of the lenses. The folding of the optical path allows the separation between the objective lenses to be increased, allowing larger lenses to be used and giving a better sensation of depth. All practical binoculars display an erect image, obtained either by using simple Galilean optics ("field glasses", "opera glasses") or by using optical prisms to both erect the image and fold the optical path.

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.

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.

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