In a properly calibrated system, these zones should have a one stop exposure difference between each zone. The middle zone, zone V, is the value to which our meters are calibrated. If we point our meter at a bright white object and make an exposure using the given information we will record an exposure on film that would change this white object into an middle gray. The same is true with dark objects, taking a meter reading and make that exposure will render a middle gray image. So it is important to realize that we do not want to take this meter reading literally. To look at the chart above we see that to keep that white object white it is necessary to give an additional three or four stops of exposure to raise this object to a value of zone VIII or zone IX. Also to make the dark object render as a dark object in our photograph we will need to reduce our exposure by one or two stops to have an exposure value of zone III or zone IV.
If I were reading this for the first time I would be asking how do I deal with all of this information? This is what the zone system is, a way of dealing with the various luminances in an image and determining how to expose and develop the film to make the image that we envision.
Expose for the shadows and develop for the highlights, I don't know the origin of this idea but it may be the most true statement ever made about we need to do to make a high quality image in photography. This idea is also the basis of the zone system. First we look at our important shadow area and meter to determine our exposure. Lets say that we meter a shadow that we want to show full detail and we get a meter reading of 1/125 at f/11. We choose to give one stop less exposure, PLACING our shadow in zone IV. So now our base exposure is 1/125 at f/16.
PLACING an important zone, usually a shadow area, is the first step in our previsualization. From here we find out where the other luminances FALL. We now meter what we believe to be an important highlight, our meter reads 1/125 at f/45. This would translate to a zone VIII exposure on our exposure scale, three stops up from our base exposure or zone V. If this is an acceptable vision of the image that we want to make, we would make the exposure of 1/125 at f/16 and develop as normal. That is all there is to it! The zone system at its most basic, PLACE the shadow area and see where the highlights FALL and adjust the exposure accordingly.
Now you are probably saying to yourself, if that is all there is to the zone system, what is so special about that? Well, for one thing, you have corrected the exposure error that you would have had if you had read your meter literally and maybe chose the wrong areas to base your exposure. What has been explained above is the zone system at its most basic. If, using the example above, we had found that our highlight had read 1/125 at f/32 which would translate to a zone VII exposure but we wanted it to be a zone VIII value to give a more full tonal range, we could expand the contrast range using a "N+" or "normal plus" development. This is another step in the visualization process.
What is this "normal" or "normal plus" stuff? In order for this visualization process to work accurately, we must calibrate the whole process and how it works together. We calibrate exposure using our meter and our cameras aperture and shutter speeds. We also calibrate our development process for the film, developer and processing technique. With this calibration process we will learn how to make the whole process work for us to make the images the way we want.
"Normal" is the term given to the development that gives the zones pictured above a one stop spacing between zones. Most beginners with the zone system feel that they want to make normal photographs so the PLACE their shadow area, accept their highlights where ever they FALL and develop everything "normally". This is fine in the beginning when you are getting use to the whole visualization process. The real power of the ZONE SYSTEM is when you learn to PLACE your exposure zone and then make your highlight zone FALL where you want it by altering your development. At this point you are in control, making photographs the way that you want and not at the mercy of Kodak or Ilford recommendations. I have gone through this process, during my first couple of years using the zone system I rarely ever deviated from normal zone placements and development. Today, probably less than 20% of my images would be considered as normal placement and development. That means that 80% of my images have been changed from what is called "Normal".
I hope that you are beginning to see the doors that the zone system can open for you in photography. It is a personal approach to photography by design. It helps you to take control of the technical steps that are available in photography, exposure and development, and use them to create the images that you envision.