Lighting often changes, and there are many circumstances in which a change in light metering will help to gain a better quality photo.Most cameras feature three light metering options. Each option works by assigning a weighting to different light regions; areas of the image that are weighted higher contribute more to the final exposure calculation.
The three typical light metering modes are illustrated above; they are EVALUATIVE, PARTIAL, and SPOT. A fourth, Center-weighted Average, is available on some digital camera models. The whitest regions in the illustrations above are those which contribute most towards the exposure calculation, whereas black areas contribute less, or are ignored. The image below is another way of understanding metering. Note that the darker the color, or higher the graph lines represent more emphasis being placed on that portion of the image to be light-metered than the lighter colored, or shorter graph lines. This is especially in the “center-weighted average” metering mode that some Canon cameras use.
LIGHT METERING USES
Lighting often changes, and there are many circumstances in which a change in light metering will help to gain a better quality photo.
EVALUATIVE: This is the default setting for most camera, and is best used for even lighting across the scene such as in landscape photography. This is slight emphasis on the area around the focal point(s) used for focusing on the subject.
CENTER WEIGHTED AVERAGE: Places most emphasis on the area around the auto-focus point used, and places less emphasis on the first tier area around that focal spot, and even less emphasis on the next outer-tier. But it does evaluate light from nearly the same total area as in Evaluative mode. I like to use this in low-light situations when light falls rapidly away from the subject.
PARTIAL: Include only about 25% of the area, with most of the emphasis on the focal point. This setting is useful when your subject is strongly or overly backlit by the sun, or other lighting to reduce flare.
SPOT: Very similar to Partial metering, Spot metering only includes a very small portion of the image immediately around the spot of focus. Useful for high contrast between subject and background such as dark subject in the snow, or white bird on the water.
THE CAMERA CONNECTION Most digital cameras have a method for selecting the light metering mode. Point and Shoot cameras usually do this through a menu system, while more advanced cameras often have a dedicated button (such as imaged below) . One point to note is that if you choose an auto-focus point (REFER TO AF-POINT SELECTION) the light metering is based on that same auto-focus point.