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.
So the basic idea is that you take a mid grey card and shoot it at a correct exposure. Then increase your shutter speed by a stop (usually 3 clicks on the wheel on your camera) and shoot again. Repeat the process for 5 shots total. Then go back and do the same, starting again from the correct exposure but this time decrease your shutter speed by a stop each time and shoot 5 shots. This gives you 11 shots total with your "correct" exposure in the middle.
You then take the range of exposures you made and examine them in something like photoshop to see if they're over or under exposed. Put them all side by side and mark the ones where the RGB values went to 0,0,0 (under exposure) or 255,255,255 (over exposure) and that's your zone ruler. It shows you how many stops of light your camera can capture.
So that's the theory but I see a few problems with this. First of all there's the question of RAW. RAW files can capture more dynamic range than 8 bit JPGs but which bits of the dynamic range you get will depend on how you process the RAWs. That makes the business of judging the ends of your ruler rather more tricky.
And then the biggest problem I have with this idea is the question of how you use this information once you've got it. It's not many people that can look at a scene with their eyes and tell you how many stops of light difference there is between the brightest and darkest areas. So I'd say that knowing the dynamic range is interesting but hard to actually use in practice. It might perhaps make you favour one ISO over another but to me ISO is a tool to be used and should be set appropriate to the scene - not something you simply leave set to your favourite setting.
Some people have more of a technical mind than others. For most people Zone Rulers are more of a scientific experiment than a useful artistic tool.
Having said all of that I'll probably give it a try with my 5D mk2. If I do I'll post the results!