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.
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