Photography with slow emulsions
Captured on glass Part 2
This is probably in part due to the slow emulsions on the dry plates used to produce these photographs.
The part of the negative that holds the photographic image is called the emulsion. The emulsion has a number of components that includes a light sensitive compound – commonly silver. The emulsion is exposed to light in the camera.The emulsion sensitivity is the starting point for determining an accurate exposure.The exposure is worked out using the sensitivity of the emulsion (expressed today as an ISO value), the amount of light passing through the lens (controlled by the aperture) and the amount of time the lens is open (controlled by the shutter speed). The slower the emulsion, the greater the amount of light and/or time required to correctly expose the negative.
Controlling the light falling on the scene in front of the camera is near impossible when photographing with sunlight. The photographer needs to choose the best combination of shutter speed and aperture for a correct exposure based on the available light level.
Although it was the middle of a gloriously sunny day in summer using handmade dry plates in a workshop at Gold Street Studios in Trentham, Victoria, my exposures were more than a second and I was unable to remain motionless, so I appear blurry in the final photographs. The relatively slow exposure times were due to the liquid light emulsion we used to create the dry plates. This emulsion is about 3 ISO according to workshop notes. This is very slow especially compared to modern silver gelatin film emulsions (usually from 100 ISO), or the split-second exposures taken by sensors inside digital cameras.
At about 6 ISO, the dry plates used by Sydney Police in the 1920s were a similar light sensitivity to the liquid emulsion.
The slow emulsion helps to explain the long exposures required when photographing with available light in the holding yard at Central Police Station, and also explains why some of the subjects appear unsharp or blurry in the photographs.