From a viewer's email question:

I was wondering if you could help me understand what a zone ruler is for, i know its to measure dynamic range and to see what your camera is capable of, but im so confused as to why we do the grey card test to find this out?? Why do i need to know the results of this test and then make a zone ruler?  

Ok I'm not sure where this Zone Ruler idea has come from - I had to look it up! I found a good page on the subject at http://thephotostudent.com/blogpi/?tag=zone-ruler. From what I can tell it's simply a way of figuring out what the dynamic range of your camera is at a certain ISO. By Dynamic Range I mean the range of light to dark it can capture without over or under exposing. I suppose the idea is that there might be some ISO settings where your camera has a larger dynamic range.