For those of you who attended one of my beginner digital photography courses, you may recall me harping on about the camera's metering always trying to achieve 18% grey average exposure.
So what does that mean, really? Well, if you were to examine the histogram (that funny looking graph on your camera's LCD screen) for a picture you just took, 18% grey falls smack dab in the middle. When your camera evaluates the light in the scene before you take the shot, it measures the light over several points in the scene and makes a decision about the settings required to make the average value of the exposure fall at the middle of the histogram. This is called evaluative (Canon) or matrix (Nikon) metering.
The reason for this is to try to ensure that the shadow and highlight areas of the scene remain within the left and right ends of the histogram to prevent loss of detail. Remember, the left end of the histogram represents the darkest shadow detail that the camera can capture, while the right end represents the brightest highlight it can handle. If the histogram is not centred, and the graph is rammed against one end or the other, then chances are you've lost some detail. In short, the best histogram is one whose graph trails off smoothly at both ends, barely touching the sides.
There are times when this brainwork by the camera can do a disservice, however. If you're shooting a scene that's 90% snow or white sand, for example, the camera will try to average the exposure to 18% grey. That means your snow or sand comes out a dull grey because it dominates the exposure, and the camera wants to expose it to mid-grey. Not only that, but if there is some deep shadow detail in that remaining 10% of the scene, it may be pushed against the left end of the histogram and be lost.
Knowing this, camera manufacturers provide an exposure compensation button, pictured below. Typically, you push and hold this button while you turn the command wheel and read the exposure compensation on the LCD screen. In the snow scene example, you may need to overexpose (+) by 1 to 2 stops. Now you're ready to take the shot and check the resultant histogram to see if the white end of the graph ends up close to, but not touching, the right end of the histogram. If not, adjust the exposure compensation and try again.
Remember, most cameras do not reset exposure compensation to zero after you turn off the camera. Reset it manually to ensure you don't wreck the next day's shoot.
You can also use the camera's other metering modes, such as partial or spot, to meter the light only in the very centre of the scene. A typical use for these is where you are taking a picture of someone who has a strong backlight directly behind them. Evaluative or matrix metering would read the backlight and most likely make the face very dark. You could use exposure compensation in this scenario, but partial or spot metering will do the same thing by measuring the light reflected from the face (assuming you centre it in the viewfinder) and make it mid-grey. Again, in these metering modes, the camera takes what it measures and tries to average the exposure to 18% (mid) grey.
As with exposure compensation, don't forget to reset the metering to evaluative or matrix when you're finished with partial or spot metering.