Adjustment For Lighting Distribution
The earliest reflected-light exposure meters were wide-angle, averaging types, measuring the average scene luminance. Exposure meter calibration was chosen to result in the “best” exposures for typical outdoor scenes; when measuring a single scene element (such as the side of a building in open shade), the indicated exposure is in the approximate middle of the film or electronic sensor's exposure range. When measuring a scene with atypical distribution of light and dark elements, or a single element that is lighter or darker than a middle tone, the indicated exposure may not be optimal. For example, a scene with predominantly light tones (e.g., a white horse) often will be underexposed, while a scene with predominantly dark tones (e.g., a black horse) often will be overexposed. That both scenes require the same exposure, regardless of the meter indication, becomes obvious from a scene that includes both a white horse and a black horse. A photographer usually can recognize the difference between a white horse and a black horse; a meter usually cannot. When metering a white horse, a photographer can apply exposure compensation so that the white horse is rendered as white.
Many modern cameras incorporate metering systems that measure scene contrast as well as average luminance, and employ sophisticated algorithms to infer the appropriate exposure from these data. In scenes with very unusual lighting, however, these metering systems sometimes cannot match the judgment of a skilled photographer, so exposure compensation still may be needed.
Read more about this topic: Exposure Compensation
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