Although not as much used in modern digital photography, the Zone System can still be helpful in some cases, particularly if you are using spot meter mode. Back in the 1930’s, in the days of black and white film photography, Ansel Adams and Fred Archer devised a system for using a light meter to assess a scene and to choose the correct exposure to capture all the detail. This was important in the film era because you had no LCD screen to look at, and didn’t know if you had properly exposed the film until you developed it. A bad exposure was an image lost forever. Adams and Archer assigned numbers to different tonalities. Pure black was 0, pure white was 10, and a midtone was 5. A 1-step increase in number represented a 1-stop increase in brightness. By taking meter readings off different parts of the scene and assigning the proper numbers based on the measured brightness, the proper exposure could be determined. In theory, this might be done from a single object in a pinch. Ansel Adams’ famous photograph “Moonrise, Hernandez, New Mexico” was supposedly taken without the benefit of a light meter because he couldn’t find it in time, but he knew from memory the luminance of the moon and was able to assign the proper zone number.
In color digital photography, a similar idea can be used. In fact, I think it is easier to think of in terms of color than black and white (what is the difference between light grey and light, light grey?). I also think of it in terms of brightness compared to the mid tone rather than the original zone numbers. Here is the system I use and which I originally saw in Outdoor Photography magazine:
The numbers refer to how many stops to adjust if you take a spot meter reading. Since a spot meter reading assigns a midtone value, this is how many stops to adjust to get it to look like what you see in the image. For example, if you take a spot reading off a light pink flower and use this as the exposure, the flower will appear medium pink in the image which will be under exposed. If you considered the flower a pastel pink, take the spot reading, but increase the exposure by 1 stop longer to get the proper exposure.
You will note that a white object with detail (i.e. bright clouds, waterfalls) is 3 stops brighter than a midtone, but technically you can only overexpose a midtone by 2.5 stops. A midtone on your camera is defined as 18% grey which is 2.5 stops below the maximum brightness your camera can record. Therefore, if you take a reading off a white object (which will be assigned a midtone value) and open 3 stops you will over expose it. The solution here would be to only open 2-2.5 stops and recognize that you may be slightly underexposing true midtone objects in the scene in order to fit it all into one exposure. For white objects such as clouds or waterfalls, which may contain some featureless white area, it may be possible to increase exposure 3 stops if you are willing to live with some clipping of the brightest highlights.
I find this particularly helpful taking photos where there is no midtone object in the field of view. An example would be a white flower with dark green leaves. Here, you could take a reading off the dark green leaf (dark color), but reduce the exposure time by 1 stop to obtain the dark green color. You could take an additional reading off the bright white part of the flower and open 2 stops (see above). Then compare the shutter speeds you calculate. If the reading off the flower demands a faster shutter speed, use that to avoid overexposure. As always, you need to check the highlight warning and adjust the exposure if needed.
Taking spot readings and applying the zone system can also help you decide if you need to plan an HDR image. In a bright sky, darker foreground situation, for example, take a reading off a foreground midtone object first and note the shutter speed. Then take a reading off the sky and adjust. For example, with the camera set to aperture mode at f/16, you might get a reading of 1/8 sec off a foreground mid-tone rock. Then take a reading off the sky which you visualize as being a pastel blue in the final image. Imagine you get an exposure of 1/60 sec. This, by the zone system, would be corrected by +1 stop to 1/30 sec for a pastel color. Your proper sky exposure (1/30 sec) is therefore 2 stops darker than the proper foreground exposure (1/8 sec) after correction. You could choose to shoot this at 1/15 sec meaning you underexpose the foreground by 1 stop and overexpose the sky by one stop. In an image processing program, such as Adobe Lightroom, you could lighten the shadows and darken the highlights and still make a picture out of this, albeit the exposure of the foreground might be less than optimal in the shadows. Alternatively, you could take 1/8, 1/15, and 1/30 second exposures and do an HDR blend to get better detail in the shadows. Either way could yield an acceptable image. If, in the above example, you instead had gotten an exposure of 1/125 sec for the sky, you would find the corrected sky exposure to be 3 stops darker than the foreground and would definitely need to do an HDR blend to get good capture of all tonalities.