Using the Zone System or some variant thereof is one way to have more control over the exposure of your photos. Here's a simple application of the Zone System.
REVIEW OF BASIC EXPOSURE CONCEPTS:
Let's review some basic concepts of exposure first.
1. Technically speaking, an exposure that renders middle gray as middle gray, white as white, and black as black is the "correct" exposure. However, as photographers we are of course free to choose any other exposure based on our artistic intent.
2. An incident light meter measures the amount of light falling on the subject and uses that information to let you know the "correct" exposure. Without an incident light meter, we're left to use the camera's built-in light meter, which is reflection-based (i.e., it measures the amount of light being reflected from the scene).
3. Light objects reflect more light than darker objects, so knowing the 'correct' amount of reflection from an object is necessary for a reflection-based metering measurement to be accurate. Unfortunately, cameras have no way to determine reliably whether the subject is white or black or gray. As a compromise, a camera generally chooses an exposure that will render the subject as middle gray.
SIMPLIFIED EXPLANATION OF THE ZONE SYSTEM:
The Zone System is a way of analyzing exposure. There is an infinite number of brightness gradations from total darkness to blinding light but the Zone System divides that spectrum into 11 zones (numbered from 0 to X) that are 1 stop apart:
- The middle zone (Zone V) corresponds to middle gray.
- The middle 5 zones (Zone III to VII) is the range that can show full texture in a print.
- The middle 7 zones (Zone II to VIII) is the range that can capture texture. In print, Zone II and VIII won't have visible texture but they will look like they have 'substance'.
- The middle 9 zones (Zone I to IX) is the range that can be captured by negative film (although at this time only a few digital cameras can capture such a wide dynamic range). In print, Zone I and IX look totally black and totally white respectively. While Zone II and VIII have substance, Zone I and IX look empty and featureless.
HOW TO USE A SIMPLIFIED ZONE SYSTEM FOR TONE PLACEMENT:
Knowing the basic characteristics of each zone in terms of its ability to show texture and substance, and perhaps some subjects that typically fall into certain zones (e.g. zone VI for "average" Caucasian skin), we can identify the most important part of the photo we want to capture, and visualize in which zone that part should be placed. For example, if you want something to look white while still having texture, then it should theoretically be exposed at 2 stops* above middle gray, i.e. Zone VII. [*The actual amount can vary by camera.] Similarly, if you want something to look black but still have texture, then expose it at 2 stops below middle gray (Zone III).
What about everything else in the photo? We just let them fall where they may. Let's say there are two objects beside each other -- one that is middle gray and one that is white. Using the approach above, we can spot meter the gray object and render the middle gray one as Zone V or spot meter the white one and render it as Zone VII. Both approaches should produce a similar result -- the gray would be in Zone V, while the white would be in Zone VII.
However, for creative purposes, we can specify the placement of 2 tones. In the example above, we can make the gray object look black while the white object would still look white by altering the contrast. To do this, we would render the white as Zone VII but expand contrast by 2 stops so that the gray object is rendered as Zone III. Similarly, we could render the gray as a light gray Zone VI while keeping the white as Zone VII by decreasing the contrast in post-processing by 1 stop.
Here is a shot with the default exposure. The camera was in aperture priority mode and the exposure was 1600 ISO, f/2.8, 1/10, with no exposure compensation adjustment (btw, no flash - just ambient light):
The swaddle on our daughter is supposed to be white, but looks grayish (around Zone VI). In post-processing, I tried adjusting exposure by +1EV:
The white swaddle now looks more reasonably white, but in my opinion our daughter's complexion looks too light. I adjusted exposure so that it was +0.5EV instead, while also increasing the contrast. With a +0.5EV exposure compensation, our daughter's complexion looks the way I visualized it. However, at +0.5EV, the white swaddle still looks a bit grayish:
By increasing the contrast, our daughter's complexion remained as visualized while the white swaddle looks more white:
For reference, here are comparisons of the different exposures:
Resource used: Zone System for 35mm Photographers (2nd ed.) by Carson Graves.