EI = Exposure index. It's the film speed you've calibrated your whole system to. Everything from the film,camera and processing. The ISO number given by the manufacturer can't take into account your developer choice.
Originally Posted by peter38
I'll try and explain the way I understand it. What you do is take a clear frame that equals fog base. Now take several zone one exposures at zone I. Starting at 1 stop below the ISO and working up by 1/3s of a stop until you hit 1 stop over. I guess that's seven total exposures. Measure the density of those zone I exposures. The one that is about 0.1 higher in density then the clear frame. Well at leasts that's the way I understand it.