Shutter speeds, apertures and Warnerke sensitometer values

In my previous blog I mentioned that I did not have the means to measure shutter speeds. Since then I experimented with a fast rotating disc. I put a little bulb on the rim of the disc and one in the center. The rotating light on the edge of the disc is projected on a ground glass in the back of the camera. When I snap the shutter of the old camera, I take a photo of the image on the ground glass with my digital reflex. The degrees of the arc of light on the ground glass is measured and when the rotations of the disc per second are known, I can calculate the speed of the shutter.

My testlab to measure shutter speeds.

To photograph the ground glass I set my camera to 3400 ISO, f/2.8 and 6 seconds exposure. During the exposure the room is dark, so the image on the ground glass can be seen and photographed. With much lower ISO settings the image could not be photographed.

To "calibrate" the speed of the disc I photographed it with my digital camera set on 1/10, 1/20 and so on. Calculations showed how fast the disc had spinned. Also I filmed the rotating disc and viewed it in slow motion, counting the turns per second.

I am still trying to find out the film speed of the early films. There are some original film boxes with the text "30 sensitometer", but I do not know which scale was used. Maybe it is the Warnerke scale, but even if it is, I cannot compare the Warnerke scale to a ASA/ISO or DIN scale. Even a conversion to a Scheiner or H&D scale would be useful, as long as I know which version of these scales are used. There are US, UK and German versions of these scales.

My own conclusion based on shutter speeds and standard aperture settings is that film speed around 1900 was about 25 ISO. 

What else did I do? Well, I bought two nice photographs to go with my cameras. The first one is a scene made with a No. 4A Folding Pocket Kodak. Yes, it says Folding on the back of the card. It must be a mistake of the printer, because the 4A was called Folding Kodak. It was and is way to large for any Pocket.

The second photo is a beautiful shot of an early aeroplane. It was made with a No. 3A Special Kodak. I do have that camera model, but I display the photo with my much more interesting Military version of the No. 3A Autographic Kodak Special. Only 100 of these cameras were made in 1916 for the US Signal Corps.