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How Center-Weighted Metering calculates Exposure?

Discussion in 'Nikon DX DSLR Forum' started by clickmepp, Sep 17, 2008.

  1. clickmepp


    Oct 1, 2007
    :frown: :frown:
    I am bit confused to be honest. Can any one explain how Center-Weighted Metering calculates the exposure?

    I understood that unlike Matrix Metering, SPOT Metering set the tonal value of the metered area to Middle Tone ( 18% approx ). It is our responsibility to compensate the exposure to match the Tonal value we like. If I am not wrong Center-Weighted Metering also uses the " Middle Tone ( 18% approx )" exposure calculation method. But how the final exposure is arrived? since "Camera meters entire frame but assigns greatest weight to area in center of frame (Default 8mm)" as stated in D300 User Guide?

    Thom's Guide talks about 75% & 25% and gave an example- I couldn't clearly understand what is happening really in calculation of exposure.

    I would appreciate if any one explain the above, probably with an example?
  2. gvk


    Jun 17, 2005
    Mystic, CT
    All exposure meters average luminance over part or all of an image to arrive at a recommendation for "correct" exposure. The three Nikon metering methods, spot, center-weighted and matrix basicly use different averaging techniques. Spot metering simply averages luminance uniformly over only a small portion of the frame, and leaves it up to the photographer to point the camera at a suitable midtone, or compensate exposure for the relative brightness of the metered area. Center-weighted metering averages luminance over the entire frame, but considers brightness in the center region more significant than in the outer part of the frame (i.e. luminance in the center is considered 3X more important than the rest of the frame with the usual 75%/25% weighting). CW metering then adjusts exposure such that this weighted average corresponds to a midtone. Matrix metering divides the image into subsections, measures luminances of each piece, and analyzes differences between them using proprietary alogorithms. Thus matrix metering essentially uses a variable averaging technique that takes into account many factors such as, overall scene brightness, the selected focus point (and subject distance for 3D matrix), brightness of the background or other uniform areas such as sky, distribution and size of bright highlights, etc. The specific average chosen by the matrix metering algorithm is selected to provide correct exposure corresponding to a matching scene from a large database of images.

    Spot metering is the most consistent and predictable, but places the burden on the photographer to interpret the meter reading and adjust exposure to produce the desired results. Center-weighted metering also behaves relatively predictably, but assumes that the subject is centered, and needs some compensation with subjects lighter or darker than average. Matrix metering provides the exposure it considers "correct," but is much less predictable, especially in difficult, contrasty lighting where subject brightness range exceeds the dynamic range of the sensor. Matrix metering's exposure choice in such conditions varies from camera to camera, depending on how Nikon tweaked its algorithms for that particular model. It can also be quite sensitive to small changes in composition. However, it does provide acceptable exposures for many situations, particularly in moderate to low contrast situations or with mixed flash and ambient lighting.
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