What most people don’t understand about Ansel Adam’s zone system is that it included both exposure and development. He’d expose the negative in order to support the development process. In this way he could place the tones of the image exactly where he envisioned. What’s important to realize is that the Zone System was used to create images that matched Adam’s vision of the scene...not necessarily the way things actually looked.
In the digital era, the Zone System has been largely replaced with different techniques. One specialized technique is called expose-to-the-right, where the driving force is to protect highlights or to maximize exposure. The tool of ETTR is the histogram. ETTR is combined with post-processing to create the final image. A more common technique is to simply expose the subject correctly. That’s where a gray card comes in.
If you take a picture of a black wall, it comes out gray. If you take a picture of a white wall, it comes out gray. The black wall was overexposed and the white wall was underexposed. The reason is that the camera is trying to make everything gray. Since the camera is trying to make everything gray, the best way to achieve “standard” exposure is to let the camera meter something that actually is gray. So you put a gray card in front of the camera, angled to catch the light that’s lighting the scene, and you spot meter the card. Now you have standard exposure.
Whether standard exposure is the exposure you want...that’s another question entirely. You may want to purposely over or under expose the image for esthetic reasons. That’s a decision that only you can make.
The easiest way to use a gray card is with Auto Exposure Lock. You point the camera at the gray card and you press the AE Lock button (I find it easier to configure the camera’s AE Lock to hold the lock after the button is released...some cameras hold the lock by default, so this isn't necessary.) Once the exposure is locked you can now adjust the setting under your control. For example, if you’re in Aperture Priority, you can change the aperture and the camera will change the shutter speed to maintain the locked exposure. That makes it very easy to control the effect of the aperture (or shutter in Shutter Priority) without having to worry about keeping the correct exposure with every change...as you do with Manual mode. If you don’t have a gray card handy, you can use nature’s “gray” cards...blue sky about 45 degrees up or the sunny side of green grass. Just point and press AE Lock.
A gray card is of no use when the light is constantly changing. When light is changing, consistent exposure on a subject is achieved by using auto modes and metering the subject directly, while applying Exposure Compensation. Let’s say your subject is a bride in a white dress. Well, we already know that white objects get underexposed. You can set the camera to Center Weighted or Partial metering (depending on your brand of camera) and apply EC of +1.3 or so to achieve standard exposure. Now, no matter how the light changes, your exposure of the bride will always be the same. With experience you’ll learn how much compensation is needed for various subjects. Light skinned faces usually need +1 EC. Black cats and dogs may need -1.3 EC. It’s a question of how much darker or lighter the tone is from 18% gray.
Here's a chart that gives the EC necessary to capture the color as you see it, when spot metering the color in question. Light colors need positive EC and dark colors need negative EC.
