Light Metering Methods

Most cameras are capable of measuring the amount of available light in several different ways: averaged, spot, center weighted, and the manufacturer’s matrix/evaluative settings. If averaged or spot metering is selected by the photographer, a single value for the entire image area is being calculated and returned. Matrix/evaluative metering is not so cut and dried, and it is hard to make a definitive statement because methods tend to be proprietary and manufacturers are loathe to share details. Some manufacturers do, however, state that they expose different parts of the image differently using electronic techniques. Darker parts of the image are boosted while very bright parts are underexposed.

Averaged Metering

Averaged metering is when the camera takes measurements across the whole scene and simply comes up with a value based on the medium (average) tone in the scene.

Spot Metering

Spot metering is when a measurement is taken from a single, very small part of the image. The idea is that this part of the image will expose perfectly at the expense of the rest of the image. A couple of examples of when this type of metering may be used are on a face, where exact skin tone is required, or on a candle flame, to not overexpose the flame, which averaged metering would do. Averaging would see mainly black and would therefore interpret a tone as close to black as middle gray, and try to lighten the entire scene accordingly. Spot metering is great for drama, and what we often think of as cinematic-type lighting is often the result of spot metering.

In practical terms, spot metering usually involves pointing the camera at the part of the scene that is to be perfectly exposed, half pressing the shutter-release button to lock the exposure, then reframing the shot and fully pressing the shutter-release button.

Center Weighted Metering

Center weighted metering may be thought of as a compromise between spot and averaged. It gives the highest priority to tones in the center of the scene and the least priority to the tones at the edges, but it does take them all into account.

Matrix/Evaluative Metering and Others

Camera companies often use their own formulas for a metering system that is likely to get the exposure correct in all situations. This often involves a database inside the camera that contains comparative scenes and the best settings for such scenes. Manufacturers tend to keep the details of these systems to themselves, so it is not possible to come up with a definitive description. If your camera has such a setting, it will be good, but that doesn’t mean it’s always the best option. I tend to swap between this setting and spot metering.