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Japanese 33 x 200 mm Folded Binocular System


The binocular 33 x 200 is designed unlike any of the previously discussed Japanese instruments, see fig. 189. The optical system is a folded one, where light from the objective lens is directed downward by a large right- angle prism into another of slightly smaller dimensions, which in turn directs the beam forward against two smaller 45 degree prism surfaces, returning the image rearward to the eyepiece. The objectives are triplets of the Cooke type. The entire 1400 mm focal length is compressed into an instrument with a total length of 787 mm, or 31 inches. The eyepiece design is again a variation of the classic Erfle with a real field of 1.8 degrees and an apparent field of 60 degrees. This binocular, manufactured of bronze casting, and weighs over 250 pounds without the mounting. It is suited primarily for placement aboard a large Naval vessel or on a coastal installation. The optical system is not coated, hence there is a significant loss of light (approximately 48%) due to reflection. Surprisingly, the optical performance is quite satisfactory.

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