ABSTRACT
Most attributes are signal-dependent and, therefore, vary as a function of the visual image density (or, equivalently, lightness) in the final image. For example, film granularity varies with exposure, partly because different exposures lead to the development of different size populations of silver halide grains. Sensors in digital cameras also produce noise that varies with exposure because of the fundamental nature of some of the contributing sources of noise; e.g., shot noise varies with the square root of signal, and fixed pattern noise is proportional to signal (see Sect. 23.3). In comparison, the amplitude of banding produced by a digital printer writing onto silver halide paper is usually signalindependent, but when this constant exposure modulation is rendered by the nonlinear tone scale of the photographic paper, it is scaled by the sensitometric gradient (objective contrast) of the paper, which is highest in the mid-tones but tends to zero as the minimum or maximum densities of the material are approached (analogously, noise power is scaled by the square of the gradient; see Sect. 22.7). Thus, even an artifact that is independent of signal at its source may vary with final image density.
