Typical humans see their world in 16 “f-stops”, or measurements of light reflecting off their environment. Cameras see no more than 5 stops at once. So the more contrast there is in the scene, the more stops of light there are to BE seen. This concept means that correct exposure on your camera is critical to avoiding under- or over-exposure. To understand how a camera measures this light, lets start with the proper definitions of Exposure and Metering.
Exposure
Exposure is the quantity of light allowed to act on a photographic material. It is a product of the intensity, which is controlled by the lens opening (aperture), and the duration (shutter speed) of light striking the film or paper. Your ISO setting, your aperture size, and your shutter speed directly impact the exposure of an image.
Metering
Metering is the process of calculating the best exposure from the existing light conditions.
When your digital camera meters a scene, it measures the amount of light in the scene and calculates the best-fit exposure value based on the metering mode (see below for details). All meters are designed to produce midtone results – neither black nor white, neither light nor dark, but somewhere in between. Metering systems are typically calibrated to a value of 18% gray because a typical scene reflects the same amount of light as this gray value. As a result, scenes with high contrast can give the automatic exposure a difficult time.
The metering mode on your camera defines which information in the scene is used to calculate the exposure value. Metering modes depend on the camera and the brand, but are mostly variations of the following three types:
1) Spot (Partial) Metering
Spot metering is a method of metering that only uses a small spot in the centre of the composed scene. The size of the spot varies with the brand of camera, but typically ranges from 1% to 3.5% of the image area. Partial metering covers about 9.5%. Spot metering allows you to meter the subject in the center of the frame (or on some cameras at the selected AF point). This type of metering is useful for brightly backlit, macro, and moon shots. Use this metering method when your scene has significant differences in brightness (e.g. between foreground and background) or for subjects that require precise measurement, such as close-up photography.
2) Center-weighted Average Metering
Center-weighted metering is averaged over the entire scene with emphasis placed on the center area (typically 75% based on lighting conditions at the center and 25% outside). It assumes that you will be composing with the subject in the middle of the frame and most of us know this is not always the case. Center-weighted metering is probably the most common metering method implemented in nearly every digital camera and the default for those digital cameras which don’t offer metering mode selection. Most centre weighted systems have greater sensitivity in the bottom half of the frame, which, when used in landscape format, cut down the influence of the bright sky on the exposure. Be careful when using this mode for portrait photography when the camera is turned on its side.
3) Matrix or Evaluative Metering
Matrix was introduced to the world in 1988 with the Nikon F4. This is probably the most complex metering mode, offering the best exposure in most circumstances. Essentially, the scene is split up into a matrix of 3 to 16 or more, typically 6 metering zones which are evaluated individually, taking into account such factors as the focusing point in use, subject size, position, distance, overall lighting level, front and back lighting and color. The overall exposure is based on an algorithm specific to that camera, the details of which are closely guarded by the manufacturer. Often they are based on comparing the measurements to an on-board database of images. Matrix metering uses a microchip that has been exposed to literally thousands of picture-taking situations. As you point the camera towards your subject, matrix metering recognizes its light/dark pattern and reads the light accordingly.
If you test your spot meter on various parts of a scene youve composed, youll no doubt prove to yourself that there is a vast range of light and shadow, but since the film or digital card cannot record more than a five stop range, what difference does it make? You will still end up keeping the same two or three exposures that were created using center-weighted or matrix metering.
Both center-weighted and matrix metering prove accurate in 90% of one’s picture taking efforts. That should boost your confidence in choosing them when you realize that nine out of ten pictures will be a correct exposure! In either center-weighted or matrix metering modes, you can aim, meter, compose, and shoot when your subject is frontlit, sidelit or under an overcast sky.