Measuring the Light

As you saw in Chapter 4, “Digital Photography Foundations,” the proper exposure for a photograph is achieved through a combination of aperture, shutter speed, and the ISO sensitivity of the image sensor. A light meter, either in the camera or an external, handheld model, is used to determine the optimal exposure settings by evaluating how much light is being reflected back from the scene being photographed. Most cameras have built-in light meters that do a good job of calculating an adequate exposure setting, and higher-end cameras offer more sophisticated meters that produce excellent results. For photographers, this feature is definitely a case of “better living through technology”; you can concentrate more on the composition of the image and changing dynamics within the scene while knowing that you can rely on the camera’s light meter to do a good job in most lighting situations. Although you may trust your camera’s light meter when it comes to exposure decisions, it’s still important to know how the light meter evaluates light, so you can better anticipate how the exposure settings it recommends will affect the image.

How Light Meters See Light

The light meters found in modern cameras are very good at analyzing the light in a scene and selecting an aperture and shutter speed combination that will yield an image that is neither drastically underexposed nor overexposed. In most cases, the exposure may actually be quite good, but it’s not necessarily the best exposure for a given scene. A light meter doesn’t know what you’re photographing, nor can it determine if you’re using it the right way or even pointing it at the right place. A light meter doesn’t give you the correct settings to use; it just gives you the settings to create a certain type of exposure based on its very narrow interpretation of the scene before your lens. This limitation of the device is due to the fact that all light meters can see only luminance (brightness) in the form of how the light is being reflected from a scene. They can’t see color, evaluate contrast, or even tell the difference between black and white, so the reflected light from every scene they analyze is averaged into a shade of medium gray. A light meter’s view of the world is so limited that the gray tone seen by light meters is not just any gray, but a very specific, 18 percent gray. This precise percentage comes from the fact that most scenes tend to reflect approximately 18 percent of the light that falls on them. When you point a light meter at a scene—no matter if it’s a snowy hillside, a dark cave, or a casual portrait—and you take a reading, the meter assumes that it’s pointed at something that is 18 percent gray; the meter is calibrated to calculate an aperture and shutter speed that will record the average reflected luminance of the scene as a middle gray (Figure 5.1).