Two decades ago (1998 or thereabouts) I spent some time looking for the British Patent covering the Depth of Field Scale. I presumed such a patent existed based on information found in two books. But I failed to find the patent at that time. Less than a year ago, I found the Patent while searching for radio and sonar patents! This is the story.
The initial 'breakthrough' came about when I found the book Photographic Lenses A Simple Treatise by Conrad Beck and Herbert Andrews. Towards the back of this book are advertisements for various lenses and cameras being sold by R. & J. Beck of 68 Cornhill, London E.C. England. One of the cameras, the "Focussing Cornex" carried "The CORNEX (PATENT) INDEX". This is pretty much the depth-of-field scale as we know it today. In my opinion there are a couple of errors in the accompanying drawing. This might have been done deliberately to mislead other camera designers. There is no date in my 6th edition copy of the book, but the "Cornex" series of cameras looked to date from about 1910. This date would fit as an approximate patent date. Given the 20-year 'term' of a patent, this would imply the patent would run out in 1930. And, indeed, that is about when the depth-of-field scale starts to appear on Leicas and many other cameras.
The second book is McKeown's Cameras, 12th edition from 2005. This lists two "Cornex" models from 1903 and 1905 but makes no mention of the "Cornex Index". Also listed, however, is the "Zambex" of 1911 and the existence of a "patented depth-of-field scale" is noted. This appeared to support my estimate for a reasonable patent date.
About the year 2000 I spent some time at the British Library in London looking for this (and other) patents, but found nothing relevant. Of course I did not have a patent author or patent title to go by: only an approximate date, and "Beck" as the probable firm to which the patent might have been assigned. So I knew there should be a British Patent but that was it.
About a year ago I found British Patent 23,733 of 1902 by Conrad Beck. The Patent Title is simply Improvements in Photographic Cameras. The patent describes a roll-film folding camera looking rather like the Zambex - but not exactly. Actually the complete camera described in the patent is more sophisticated than either the Cornex or the Zambex. But the Depth-of-Field Scale is listed as one of about 5 improvements and is clearly illustrated in Figure 2 of this document:
This figure also has 'errors' although different ones. For Figure 2 of the patent, the depth-of-field markers for f/8 should be half-way from the focus pointer to the f/16 markers and the markers for f/11 should be a little further out from the focus pointer than shown here.
The Conrad Beck Patent may be downloaded here. And the complete book (Photographic Lenses, 2nd Edition, 1903) can be found at the Open Library.
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