A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment

Metering





 
Metering patterns include matrix (top), center-weighted (middle) and spot (bottom).
One of the most important aspects of photography is getting the exposure right because it determines how light or dark an image is and what mood it conveys. The two most important exposure controls are the shutter speed and aperture because both affect the total amount of light reaching the image sensor. However, they do more than just control the exposure. They are also the most creative controls you have.
  • The shutter opens to begin an exposure and closes to end it. The shutter speed setting determines how long the shutter opens to expose the image sensor.
  • The aperture is the hole through which light enters the camera. The size of the hole can be changed to control the brightness of the light that reaches the image sensor.
How does the camera know what shutter speed or aperture to use? It uses its exposure system to measure the average light reflecting from the scene and selects settings that will capture that average as middle gray.
 
All parts of a scene are usually not equally important when determining the best exposure to use. In a landscape, for instance, the exposure of the foreground is usually more important than the exposure of the sky. For this reason some cameras offer more than one metering method. The most common choices include the following:
  • Matrix, sometimes called evaluative, metering divides the image area into a grid and compares the measurements made of each individual area against a library of typical scenes to select the best possible exposure for the scene. The system then selects the best possible exposure for the entire scene.
  • Center-weighted averaging meters the entire scene but assigns the most importance to the center of the frame where the most important objects are usually located. This is a good mode to use when shooting a large main subject against a very bright or very dark background.
  • Spot, or slightly larger partial metering, evaluates only a small area of the scene. Some cameras fix the metered spot in the center of the viewfinder while others let you move it about or link it to the active focus area so you focus on and meter the same area. Spot metering lets you meter just a specific part of the scene instead of relying on an average reading and is ideal when photographing a subject against a bright or dark background. On some cameras you can use this mode with exposure lock.
Meter weighting can cause a few problems. For instance, a dark object located off center against a very light background may not be exposed properly because it is not located in the area the meter is emphasizing. Or, in some cases, holding the camera vertically may give undue emphasis to one side of the scene. These occasions are uncommon, but when they occur you can use exposure lock or exposure compensation to get a good exposure.


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