Zeiss Jena Deltarem 8x40
Mfr: Carl Zeiss Jena, Germany
Date: circa 1934-35(?)
AFOV: ~90 degrees
TFOV: 198m at 1000m (~11.2 degrees)
Eye Relief: ~11-12mm
Prisms: high index
Serial Number: 1667986
The Deltarem (and the individual focus version, Deltar) was perhaps the first ultra wide angle binocular. It has a distinctive squat body enclosing its large prisms. The Deltarem is said to have an aspheric Abbe orthoscopic (3-1 elements) type of eyepiece. Normally, an orthoscopic 4 element eyepiece is only good for about 45 degrees. Zeiss designers stretched this to 90 degrees by including an aspheric surface. Similar eyepieces were used on later wide angle Zeiss binoculars such as the blc 25x100.
According to Hans Seeger, Zeiss first produced an ultra wide angle 8x40 in 1933 and delivered units for German military use (probably Luftwaffe) in 1935. See section 2.7 of Seeger's gray book (1st edition). If the serial number on the unit shown above is correct, it appears to be a very early sample of the Deltarem, though it has no indications of being a military glass. Note that this particular unit has light blue anti-reflection coatings. It is believed that a previous owner had the coatings applied sometime after World War II (coatings were not used on commercial binoculars until after the war).
Optically, the Deltarem appears to be fairly well corrected for rectilinear distortion (straight lines). However, with such an enormous field, the resulting image has a strange "warping" (for ultra wide angles, it may be better to correct for angular distortion). In addition, there is a large amount of astigmatism towards the edge of field. Despite these quirks, this binocular is a very interesting early example of wide angle optics.