When using either AUTO or Program (P) mode, your camera automatically determines the shutter speed and aperture. In Shutter Priority (S or Tv) shooting mode, you manually set the shutter speed and the camera automatically sets the corresponding aperture value. In Aperture Priority (A or Av) shooting mode, you manually set the aperture and the camera sets the shutter speed. You should know by now that the combination of shutter speed and aperture determines the exposure.
The camera uses its metering system which analyzes the image framed in your camera's view-finder or LCD screen, and based on this analysis, sets the shutter speed and/or aperture accordingly. The metering works when you point your camera to the subject before fully pressing the shutter release button.
There are basically three metering modes that are common in most digital cameras.
1. Matrix or Evaluative Metering
The camera analyzes each portion of the segment and determines the appropriate exposure according to the entire scene. This is probably the most complex metering mode, offering the best exposure in most circumstances. Essentially, the scene is split up into a matrix of metering zones which are evaluated individually. The overall exposure is based on an algorithm specific to that camera, the details of which are closely guarded by the manufacturer. Often they are based on comparing the measurements to the exposure of typical scenes.
If you are a beginner, Matrix is where you should start. As your skills grow you will acquire a better understand of when it might be beneficial to use other metering modes.
2. Center-weighted Metering
The camera analyzes the entire scene with an emphasis towards the center and determines the appropriate exposure according to the entire scene.
Probably the most common metering method implemented in nearly every digital camera and the default for those digital cameras which don't offer metering mode selection. This method averages the exposure of the entire frame but gives extra weight to the center and is ideal for portraits.
This method gives weight to the overall exposure calculation. Meaning the circular area in the middle of the frame averages everything it finds there, giving that calculation between 60% to 80% weight in the overall computation for exposure. Brightnesses outside the circle are still considered however their influence is minor.
3. Spot Metering
The camera analyzes only the center of the scene and determines the appropriate exposure.
Spot metering allows you to meter the subject in the center of the frame (or on some cameras at the selected AF point). Only a small area of the whole frame is metered and the exposure of the rest of the frame is ignored. This type of metering is useful for brightly backlit, macro, and moon shots.
To learn more about other metering modes, click here and here.
Keep on shooting!
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