A couple weeks ago I was up in the high country capturing images of mountain goats. I saw something I haven’t seen in a while. It was an 18% grey card. A photographer nearby stopped shooting, reached in his bag, pulled out his card, shot it and then carried on. That photographer was simply trying to zero out his light meter on a known tonal and reflectance value. The scene we were shooting was bright and dominated by white subjects. Our cameras meters are designed to make scenes like this very average and very dull.
There are two types of light meters used in photography; an incident and a reflective meter. An incident meter measures light falling upon it. These are often used in studio settings. While some nature photographers carry them, I find incident meters not all that practical for most outdoor and nature work. Reflective meters measure the light being reflected off a subject. These are the meters built into our cameras. Regardless of the meter used the result is the same. Our cameras light meters are designed to render scenes as middle grey or average tones. Most of the time we go along happily firing off frames and the results we get back are exactly what we see. Every now and then, we encounter a scene which fools our camera and our meter fails us. The mountain goat scene is a great example. If I shot this straight up with a centered or zeroed meter my result will be an underexposed scene. Being dominated by bright tones and white subjects the meter will turn the white colors and light tones to grey, effectively darkening the whole scene. Scenes with dominant dark or black subjects the opposite happens and our images tend to be overexposed.
So do I recommend we all add grey cards to our bulging camera bags? Not necessarily. To our cameras middle grey is not a color, it’s a tone. That means we can find middle grey almost everywhere. We just need to know where to look and where to point our cameras. I have a couple of known targets I go to when I suspect my meter may be leading me down the wrong path. A deciduous tree, green lawn medium blue sky, faded blue jeans and fall color are all examples of middle grey tones. I know if any of these targets are in the same light as the scene I shooting I can meter off them and will get a predictable exposure.
Try it for yourself. Take your camera outside, set it to manual mode and center your meter on medium green grass or leaves of a tree as a baseline. Now explore other subjects and notice how your meter responds. When I first did this I was surprised to see drastically different colors had the same tonal value. Once you have your own list of medium tone targets you’ll find you are much quicker in the field setting your exposure and also more confident in your expected results.