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          Kaufmann's wonderful Posographe


[dot_clr]       *    And now, Ladies and Gentlemen, I give you (drum roll...) Kaufmann's
                    Posographe!

[dot_clr]       *   Kaufmann's Posographe  Kaufmann's Posographe - mechanism
                                                                         Click photo to
                    enlarge

[dot_clr]       *       At first glance this is just a small rectangular plate, about 13
                    x 8 cm, covered with dense scribbles, with seven pointers fixed to
                    its frame. Then you realize that the pointers are not fixed, but can
                    slide on the frame... and then you note that they are somehow
                    interconnected -- moving any of the small ones will move the larger
                    one this way or that. Strange. But when you see the diagram of the
                    inner mechanism you realize what this is, and it can take your breath
                    away (well, if you're a techie like me it sure can).
                        Kaufmann's Posographe is nothing less than an analog mechanical
                    computer for calculating six-variable functions. Specifically, it
                    computes the exposure time (Temps de Pose) for taking photographs
                    indoors or out (depending on which side you use). The input variables
                    are set up on the six small pointers; the large pointer then gives
                    you the correct time. The variables are very detailed, yet
                    endearingly colloquial. For outdoors, they include the setting --
                    with values like "Snowy scene", "Greenery with expanse of water", or
                    "Very narrow old street"; the state of the sky -- including "Cloudy
                    and somber", "Blue with white clouds", or "Purest blue"; The month of
                    the year and hour of the day; the illumination of the subject; and of
                    course the aperture (f-number).

[dot_clr]       *       For indoor photos, we have    Kaufmann's Posographe - Indoor side  Kaufmann's
                    the colors of the walls and       Posographe - Outdoor side
                    floor; the location of the        Click a photo to enlarge
                    subject relative to the windows
                    (depending also on the number of
                    windows, and indicated by the
                    little diagrams); the extent of
                    sky in the window, as seen from
                    the location of the subject
                    (again illustrated in little
                    pictures); the sunlight level
                    outside, and how much of it, if
                    any, enters the room; and the
                    aperture.
                        The output indicator
                    actually has four points,
                    designed to show the respective
                    exposure times for different
                    emulsion types.

[dot_clr]       *       For example, the photo at the top of this page shows the outdoors
                    side of the Posographe executing the following calculation: What
                    exposure time should one use for a camera with an f/4.5 lens taking a
                    picture of a subject in the shade on a narrow city street under a
                    cloudy grey sky at 9AM (or 3PM) of a February (or November) day?

[dot_clr]       *       The diagram, from the instruction manual, shows how   Kaufmann's Posographe and
                    the many pointers are all interconnected in just the      leather pocket
                    right way to provide the appropriate computation. One     Click photo to enlarge
                    wonders whether M. Kaufmann achieved this design
                    empirically by trial and error, or by working out the
                    math (my bet would be a combination of both). The
                    result, at any rate, is a true work of art, and comes in
                    a lovely leather pocket embossed with the device's name
                    and an ornate  floral design.

                    Exhibit provenance:
                        eBay, from a gentleman in Quebec.

[dot_clr]       *   More info:
                        More information about Kaufmann himself and his inventions can be
                    found in my article in the Journal of the Oughtred Society.
                        Ace Hoffman was inspired to develop a n interactive software
                    simulation of the Posographe, available here for Windows, Android and
                    iOS devices.
                        A transcription of the French instruction manual is available at
                    http://www.brocantina.com/posogr.pdf.
                       
                     

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