Photography

Light Metering

When the camera is set to Auto, Program, Aperture Priority or Shutter Priority Mode it checks on certain parts of the image and calculates the best ISO, aperture value, or shutter speed based on the amount of light in these reference areas. This process is called Light Metering or simply Metering. Most camera metering method gets the average luminance level of the images within the reference area. If the result does not match the luminance level of a reference "Middle Gray" (approx 18% gray), then the appropriate exposure adjustments are made. There are three common metering modes used by many digital cameras today, each with its specific advantages and disadvantages.

Center Weighted Metering - This is the most common method of metering found in most point-and-shoot cameras. It uses the entire screen as a reference for calculating the exposure but puts more weight on the center. This method generally gives a fairly good exposure result for scenes that contain a good balance between light and dark areas. However, some scenes may be dominated by light images, for example, a white bird on a white sand in a sunny day - this gives an inaccurate result as it will tend to lower the exposure setting thinking that the scene is too bright. The same problem may also happen if the scene is generally dark such as a black pig wallowing in the mud.

Partial or Spot Metering - Similar to Center Weighted Metering but the reference area is much smaller and most of the areas away from the center are ignored. Partial Metering uses only about 13.5% of the screen area while Spot Metering uses 3.5%. This allows the light measurement to be confined to a more specific area and put more priority on proper exposure on that area. A typical example would be if your subject has a very bright background. Using Center Weighted Metering would tend to balance the exposure such that the details will not be flushed out by the bright background lighting. As a result, your subject, having less illumination than the background will appear very dark, almost like a sillhouette. With Partial or Spot Metering, the balancing of the exposure is done based on a smaller area. If you choose to target the face of your subject then your subject will have proper exposure while the background will be overexposed, which is a more useful compromise.

Evaluative or Matrix Metering - A more sophisticated algorithm is used in Evaluative or Matrix Metering. The screen is divided into several parts, each part is calculated separately and the camera decides which area has the most significant reading and sets the exposure based on the reading on this area. The Auto-focus area sometimes plays a part in the way the camera decides which area to use and most of the time the resulting exposures are accurate. Most cameras today uses Evaluative or Matrix metering as a default for its Auto setting since it is the most reliable for most situations. It is also useful if you are unsure which Metering to use.