The dynamic range

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The dynamic range indicates the logarithmic exposure range in which the image carrier can convert the exposure into sufficiently separated density values. Its size depends on the minimum slope of the characteristic curve that is assumed. This is because with increasing or decreasing exposure, the tonal values move further into the sag or shoulder of the curve and at some point reach a point at which it runs almost flat and fine details are consequently no longer distinguishable from one another.

We find the smallest necessary curve slope in the sag at the minimum point. The point is located by default where the curve slope is not less than 0.20. Since this slope cannot be determined at a point, the value of 0.20 practically refers to a tangent that just touches the curve in the sag. For most common films, this tangent point falls at a density value of at least 0.10 above the base density. The minimum slope of 0.20 and the determination by means of a tangent line are equally valid for the maximum point in the shoulder. But since the shaping of this part of the curve depends very much on the type and duration of development, a general statement as a reference point is not possible here.

Between the minimum and maximum points, the dynamic range spans along the horizontal exposure axis in Log₁₀ steps (calculation in exposure steps = division by 0.3, because the logarithm of 2 [the doubling of the amount of light, one exposure stop difference] is equal to 0.3). As long as the tonal values of the subject are placed in this range, the image will have sufficient detail in all areas. If, on the other hand, the exposure range of the subject is greater than the dynamic range of the image carrier, then inevitably either the shadows or the highlights will no longer show sufficient detail.

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