I've been having a really good close look at this Kaufman Posographe.

Printing off the pictures as A4 and then googling some of the French into English helped a lot

There must have been a whole technology of mechanics years ago that, if not forgotten, is now ignored by most of us. I did a bit about first, second and third class levers when I was at school - that was it. There is obviously a lot more to it than that. Fascinating to sit for a few minutes and work out how this works. It obviously uses well understood principles and I guess it probably wasn't too difficult a task to design at the time.

From a photographic point of view, it calculates exposure from Month, Time of day, Scene, weather and colour & lighting of the subject. Interior views use colour of light and colour of walls, I think (I haven't translated that bit, yet) and the 'zones' are the distance from the window.

There seems to be an emphasis on colour presumably because this would have made a big different in the days of 'ordinary' and 'orthochromatic' emulsions.

One thing that is missing is film speed! This is accounted for by the output display unit (the oblong brass widget) having four pointers on it. these are designated Ordinary, rapid, extra rapid and ultra rapid. I think they refer to specific plates and the spacing between them isn't even. They could be the equivalent of something like 2, 8,16,32 ISO, maybe? (Would Ultra Rapid have reached ISO 32 in the 1920?)

It seems to be it is a pretty comprehensive calculator that takes into account most factors and I reckon would have worked well, with the four types of plates it was intended for in the 1920s.

Most modern exposure calculators use only a scene description with a sliding scale to calculate different aperture / shutter combinations for a variety of film speeds.

The idea of a modern version of this more comprehensive calculator, which puts film speed into the equation as an input variable and perhaps places less emphasis on colour, sounds intriguing