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Archives of an email list on the history of binoculars. - page 2 / 150





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  The invention is for an erecting prism of the type commonly known today as the Abbe-Koenig (or Abbe-Konig) roof prism and was used extensively since 1905 by the German firm of Hensoldt in many of their binocular models. The Hastings prism type (aka Abbe-Koenig) is currently used in the Zeiss Victory series of binoculars. It is not clear to me why Zeiss and others refer to this prism type as the Abbe-Koenig prism and whether or not there was a description of this prism in Germany prior to the 1897 US patent. The "Abbe" refers of course to Ernst Abbe at Zeiss and Albert Koenig was an optical scientist at Zeiss. If any list member knows more about the German history of this prism please add to our knowledge. There is reference to a German Patent by Hensoldt: D.R.P. Nr. 180644, dated April 14,1905 in a recent Zeiss binocular brochure (What You Should Know About Binoculars, p.4) and in Seeger's Red Book (p114).

  Hensoldt 1905 Roof Prism Patent:

Text:    www.lightmechanics.com/Patents/DRP_180644_1.pdf    www.lightmechanics.com/Patents/DRP_180644_2.pdf

Drawings:    www.lightmechanics.com/Patents/DRP_180644_3.pdf

  (You will need Acrobat Reader to view the Hensoldt patents, it can be downloaded free from www.Adobe.com )

  It is of course possible that the Germans were not aware of the Hastings Patent since it was not used in any known production binoculars until the 1905 Hensoldt or it is possible that Hastings was not aware of some prior German invention of the prism by Abbe.

  There a construction difference between the Hastings roof prism and the Abbe-Koenig in that the Hastings is built up from 3 prism pieces bonded together while the Abbe-K type is made from 2 prism pieces. However the basic optical form of the reflecting surfaces to accomplish the image erection is the same. This construction difference is sufficient reason to justify separate Patents on the two very similar prisms.

  The Hensoldt form of the prism allows for a more compact prism cluster with larger objectives and larger FOV for a given prism size by moving the roof edge in closer to the optical axis of the objective lens. You can't do that with the Hastings form as the roof edge will start to vignette the rays from the objective.

  A note from Dick Karlson pointed out that some Hensoldt roof prisms used an airspace instead of a cemented interface. Indeed, the Hensoldt Dialyt binoculars that I have disassembled use the roof prism as shown in the 1905 Patent except that they use an airspace between the two prism pieces instead of a cemented interface. This is a further advantage in making the prism compact because the roof edge can be moved in even closer and still maintain total internal reflection (TIR) inside the second prism.

  The Hensoldt (aka Abbe-Koenig) prism can be used with bonded prism parts as indeed is shown in the 1905 Patent. For the on-axis rays shown in the three Patent figures the cement layer is not in the area of the TIR in the second prism so there is no problem having cement. With increasing field of view there would be some vignetting at the cement layer. The addition of the airspace allows for a larger FOV or smaller F-Number objective without increasing the prism sizes.

  The Hastings prism in three pieces must be bonded or it would suffer large chromatic aberration problems with the airspaced surfaces not being perpendicular to the optical axis.

  The Hensoldt Patent shows how the prism arrangement can be optimized for the largest objective lens diameter in Fig.3 vs Fig.1. In Fig. 3 the telescope tube diameter (dashed lines) is the same as in Fig. 1 but the objective lens is much larger in diameter.

  The Hastings prism and the Hensoldt prism can both be used with or without displacement of the optical axis. This is shown in the Hensoldt Patent. Fig. 1 and Fig. 3 have no displacement of the optical axis (labeled 'X') whereas Fig. 2 shows a latereal displacement of the 'X' axis. It all depends on the relative position of the objective lens axis and the prism faces.

  Just to confuse the issue even more for those who have MIL-HDBK-141 (the best english language review of optical prisms IMO) see Fig. 13.37 Abbe Prism, Type A and Fig 13.39 Abbe Prism, Type B. The Abbe Prism, Type A is the Hensoldt configuration and the Type B is a three piece construction but not the Hastings arrangement of parts.

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