Kaufmann’s Posographe - Exposure Calculator from the 1920s

Kaufmann’s Posographe is an intricate pocket-sized mechanical calculator invented back in the 1920s. Measuring 13x8cm and filled with tiny scribblings, the device allowed photographers to approximate the exposure values they needed by simply sliding around six small pointers.

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).

For indoor photos, we have the colors of the walls and floor; the location of the 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.

Here’s a look at the front and back of the Posographe, along with a diagram of its innards: