Editing Zone System (section)

=== Exposure metering ===

Any scene of photographic interest contains elements of different [[luminance]]; consequently, the "exposure" actually is many different exposures. The exposure time is the same for all elements, but the image [[illuminance]] varies with the luminance of each subject element.

Exposure is often determined using a reflected-light<ref>Adams (1981, 30) considered the [[Light meter|incident-light meter]], which measures light falling ''on'' the subject, to be of limited usefulness because it takes no account of the specific subject luminances that actually produce the image.
</ref> [[Light meter|exposure meter]]. The earliest meters measured overall average luminance; [[Light meter#Exposure meter calibration|meter calibration]] was established to give satisfactory exposures for typical outdoor scenes. However, if the part of a scene that is metered includes large areas of unusually high or low reflectance, or unusually large areas of highlight or shadow, the "effective" average reflectance<ref>
A typical scene includes areas of highlight and shadow and has scene elements at various angles to the light source, so it usually is possible to use the term "average" reflectance only loosely. Here, "effective" average reflectance is used to include these additional effects.
</ref>
may differ substantially from that of a "typical" scene, and the rendering may not be as desired.

An averaging meter cannot distinguish between a subject of uniform luminance and one that consists of light and dark elements. When exposure is determined from average luminance measurements, the exposure of any given scene element depends on the relationship of its reflectance to the effective average reflectance. For example, a dark object of 4% reflectance would be given a different exposure in a scene of 20% effective average reflectance than it would be given in a scene of 12% reflectance. In a sunlit outdoor scene, the exposure for the dark object would also depend on whether the object was in sunlight or shade. Depending on the scene and the photographer's objective, any of the previous exposures might be acceptable. However, in some situations, the photographer might wish to specifically control the rendering of the dark object; with overall average metering, this is difficult if not impossible. When it is important to control the rendering of specific scene elements, alternative metering techniques may be required.

It is possible to make a meter reading of an individual scene element, but the exposure indicated by the meter will render that element as a medium gray; in the case of a dark object, that result is usually not what is desired. Even when metering individual scene elements, some adjustment of the indicated exposure is often needed if the metered scene element is to be rendered as visualized.