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Photog’s Antique Mechanical Exposure Time Computer

This beautiful little gadget dates to the 1920s, and is known as Kaufmann’s Posographe:

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)…

The linkages, of course, are hidden inside. The linked site, above, includes a patent illustration that shows how they work. [Thanks, Alan Dove!]

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8 thoughts on “Photog’s Antique Mechanical Exposure Time Computer

  1. The “History of Computing” pages on that site were what kicked my interest in mechanical computers into high gear.  It was the first place I’d ever heard of a pinwheel calculator.  Glad to see they’re getting some press.

  2. Lovely linkage devices like this can be thought of as a function, in this case of six(!) variables, f(u,v,w,x,y,z).  Here’s my simple-minded question:  what limits are there on the functional domain of simple linkages?  i would suppose that all(?) polynomials would be within their ability, and perhaps with some wheels (within wheels?) most of the trig functions, but complex valued Gamma functions..?  (“well, what do you mean by ‘simple linkages’?”)  (i want an Antikythera device that can compute Fourier transforms http://en.wikipedia.org/wiki/Antikythera_mechanism )

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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