In the Zone System, measurements are made of individual scene elements, and exposure is adjusted based on the photographer's knowledge of what is being metered: a photographer knows the difference between freshly fallen snow and a black horse, while a meter does not. 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. 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. When it is important to control the rendering of specific scene elements, alternative metering techniques may be required. 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. Depending on the scene and the photographer's objective, any of the previous exposures might be acceptable. In a sunlit outdoor scene, the exposure for the dark object would also depend on whether the object was in sunlight or shade. 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. 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. May differ substantially from that of a "typical" scene, and the rendering may not be as desired.Īn averaging meter cannot distinguish between a subject of uniform luminance and one that consists of light and dark elements. 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 The earliest meters measured overall average luminance meter calibration was established to give satisfactory exposures for typical outdoor scenes. The exposure time is the same for all elements, but the image illuminance varies with the luminance of each subject element.Įxposure is often determined using a reflected-light exposure meter. Anticipation of the final result before making the exposure is known as visualization.Īny scene of photographic interest contains elements of different luminance consequently, the "exposure" actually is many different exposures. The Zone System is concerned with control of image values, ensuring that light and dark values are rendered as desired.
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Achieving the desired image involves image management (placement of the camera, choice of lens, and possibly the use of camera movements) and control of image values. Principles Visualization Īn expressive image involves the arrangement and rendering of various scene elements according to the photographer's desire.
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