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Digital Camera Know-Hows
metering modes and light meters
To take a photo of a subject exactly as it is, or as you envision it, accurate measurement of the light on and around the subject is vital. The measurement of this light is called "photometry" or "metering." Cameras use built-in light meters, which base their auto exposure values on meter information. Some light meters can even switch to different metering modes to select the optimal metering for a variety of scenes.

metering modes

Most light meters have three metering modes, which select how metering is performed on the subject. Additionally, some camera manufacturers have their
own "intelligent" metering systems.
Center-weighted metering Spot metering Multi-zone metering
While measuring the overall brightness of a scene, this method placesgreatest importance on the central section of the image. This method ispopular since the subject is often placed in the center of the scene. Uses a small point in the center of the scene to meter exposure. This modeis useful when a subject and background have a large difference in levels oflight and shadow, or when you wish to capture only a portion of the subject. Using the camera's light receptors for metering, the camera calculates thelight in each of the divided areas and determines the average exposure for the overall scene. Even when light and shadow sections are mixed in the subject, this method can produce an optimal exposure.
Useful scenes
Useful when the subject is positioned in the center of the scene or when the overall scene is evenly lit.
Useful scenes
Useful when a subject and background have extreme differences in light and shadow or when you wish to capture only a section of the subject.
Useful scenes
Useful in scenes with bright backgrounds or with a large amount of contrast such as at sunrise, sunset or at nighttime.
metering modes
Built-in light meter External light meter
Ordinarily, a camera uses a built-in light meter. Singlelens reflex cameras equipped with TTL (Through The Lens) metering can perform highly accurate exposure control since they measure the amount of light recorded on the film directly through the lens. Also, some compact cameras have a light meter window designed to calculate exposure. In both types of cameras, reflective light meters are used. External light meters have both light metering and guide functions, which determine exposure by giving priority to either aperture or shutter speed. External light meters are often necessary for medium- and large-format  cameras since many of them are not equipped with a built-in light meter.
External light meters are also essential for professional lighting. External light meters that can be used for both incidental light and reflected light are the most popular type of meters.

measuring with an incident light nad a reflective light meter
Incident light meter Reflective light meter
Obtains a more precise exposure and is less susceptible to the reflective ratio of the subject's color, since the light striking the subject is metered directly. Measures the light reflected from the subject and, regardless of whether the subject is white or black, selects an exposure that will turn it gray. This metering is convenient because metering can be performed from the camera.

digital camera know-hows