Michael, it's all pretty straight forward, although Schaefer made a wrong assumption. The process is the same as with a sensitometer which is just an accurate exposure device. Opacity is incident light / transmitted light. Density is the log of Opacity. Metered camera exposure is 8 / ISO. Opacity comes from the step tablet and incident light is based on the metered camera exposure. These are the two variables needed to calculate the transmitted light for a given step tablet density. Transmitted light = Incident Light / Opacity. The film speed equation is 0.80 / Hm. What the exposure needs to be for a certain film speed is 0.80 / ISO.
Originally Posted by Michael R 1974
I decide which step I want to be the speed point, then determine the exposure necessary to achieve it. This becomes my standard exposure for a given speed film.