Photographer's Slide Rule

Before there were lightmeters...

Another interesting Nomographic device.

Like the Airship Engineer's Slide rule I discussed previously, other technically complex disciplines made use of nomographic devices to simplify on the fly calculations.

Photographers developed devices for determining exposure times known as Actinographs.

The name originally meant a device that recorded the amount of sunlight for a day but was adopted for the photographer's slide rule that used that data.

Here is an example from the later 19th C

Since the exposure time depended on several factors these could be set by sliding the scales. The scales were adjusted to account for plate speed, lens type, and other variables.  The curves on the roller indicated the changing light intensity for different times of the year.  The result was an estimate of the exposure time needed.

Here is another Actinograph that used a card instead of a roller for the sunlight data.
This is one of the first commercially produced devices made by Ferdinand Hurter and Vero Charles Driffield and patented in 1888.

Keep your sightglass full, your firebox trimmed and your water iced.
KJ