Exposure Systems

Matrix

In this system, the camera calculates an optimum exposure based on maximum and minimum brightness (contrast) measured across a a matrix of sensors perhaps 8, 10 or 12 across the area selected in the viewfinder. If the camera is set on an automatic exposure mode, TV, AV or Program, - it automatically sets the camera to give a "correct exposure based on this information. While the system takes account of all areas of the scene it places a greater emphasis on the sensors in the centre.

In the example on the right we see 10 segments- although the area of each is different, the tonal value of each is equal. This means there is a greater weighting placed on the area in the centre.

With some lenses like the D or G type Nikkor lenses, a 3-D matrix metering system also takes into account the aspect of the scene in focus.

Centre weighted

Centre weighted systems assume the center of the area selected in the viewfinder is he most crucial, and calculate the exposure based on a designated circle about 12mm in the centre of the viewfinder. So they ignore tonal information out side this area.

Spot

Spot metering systems calculate the exposure based on a very small area of about 3 or 4mm about 2% of the frame. In some cameras this is always in the centre of the viewfinder, while in other cameras it can be moved around several sensors visible in the viewfinder. In the example on the right we see 5 separate areas the sensor can be moved to. Using this system needs special attention as it only reads the tonal information in this very small area.

With some cameras the ability to move the spot sensor is deactivated when the Dynamic auto focus mode is used and it stays in the centre.