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The goal of exposure determination for AgX image carriers is to distribute the tonal values to the characteristic curve in such a way that they appear as correct brightness values in the finished image without much correction. – Of course, there are manipulation options in this area, such as push/pull development or postexposure and dodging. But they are inconvenient and laborious, and few photographers have the opportunity to use them.
To meet the requirement, the medium-light tonal values of the subject must come to rest on the center of the characteristic curve. Thus, Point D on the curve serves as the fulcrum of the exposure measurement, marking the center of its linear range. Depending on the type of image carrier, everything is underexposed and black two to five exposure stops below it, and everything is overexposed and white two to five stops above it. How do we „meter“ point D? Quite simply, the light meter does it for us. No matter what we measure with it, it will always be imaged with medium brightness and thus falls quite naturally on point D. – Taking into account the above-described connection of the problem between the calibration values 18% and 12%.
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