SIGMA DP3 Merrill - Manual (Page 71)

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SELECTING THE METERING MODE
The camera has three advanced
metering modes.
The Metering Mode can be set in Quick Set Menu (P.35) or [
Capture Settings] (P.28) → [AE Metering Mode]
Evaluative (Default)
Center Weighted Average
Spot
EVALUATIVE METERING
The camera divides its screen into 256 and measures the brightness of
the scene by independently measuring each segment of the focusing
screen and analyzing the correct exposure for the main subject under
any lighting situation. Even under strong back lighting or complex
lighting situations, the camera will give you the correct exposure.
CENTER WEIGHTED AVERAGE METERING
The camera will measure the average brightness of the entire scene
with additional emphasis on the center area. It will determine the most
suitable exposure with respect to lighting conditions.
SPOT METERING
The camera will measure the brightness only inside of the center
focusing point on the color LCD monitor. This mode is suitable when
you wish to set exposure for a portion of the scene and ignore the
influence of rest of the scene.