The requirement 0 in analog photography

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As analog photographers, we are fortunate because manufacturers have formulated the standard process of materials to account for all factors. – After all, they want their products to go down well with consumers, and for this reason they have a natural interest in ensuring that they meet requirement 0. Therefore, consumer products may not necessarily depict motifs realistically, but they do so in a way that aligns with the preferences of the majority of viewers. In other words, in such a way that they correspond to people’s preferences. The contrast behavior and dynamic range of silver films and papers are therefore directly attributable to the fulfillment of the requirement formulated at the beginning. This is why their characteristic curves resemble those found in standard tuning. The producers achieve this by fine-tuning two factors: a) the size and distribution of the silver halide crystals in the emulsion, and b) the type and duration of development. They essentially determine the core parameters of exposure range (what level of contrast a piece of silver film or AgX-based photographic paper can handle) and contrast behavior (how the image carrier handles contrast over the usable range).

The size and distribution of the AgX crystals: It is critical because it determines how sensitive the emulsion is to light. For an exposure of strength x, the probability that a large silver halide crystal will capture the four photons necessary to form an exposure seed is greater than for a small one. For this reason, an emulsion composed of comparatively large crystals is more sensitive to light than one composed of relatively small ones.

An emulsion with identically sized silver halide crystals can only represent the two states of no density or maximum density, as the chance of forming an exposure nucleus at a given exposure strength x is evenly distributed across all crystals due to their identical size. One could say that they all switch to exposed at the same time due to their identical nature. This behavior is desirable for lithographic films used to reproduce line drawings. However, photography typically reproduces scenes with far more varied tonal values than just white and black. To enable the reproduction of such contrasts in one exposure, the emulsions must have silver halide crystals of correspondingly different sizes and sensitivities. We create these size differences by slowly adding the silver nitrate to the gelatin-bromine solution, rather than adding it quickly and all at once as we do with lith films. Initially, the process forms small crystals, some of which dissolve and form new, larger structures as the process advances. The film thus comprises silver halide crystals of varying sizes and numbers, contingent on the completion of the process. The large crystals are the oldest, the small ones the youngest. When the light hits such an emulsion, the large crystals capture most of the photons and form a latent image of the low light levels, i.e., the shadows. The less sensitive small crystals require more light to form an exposure seed and therefore record the latent image of the highlights.

Ideally, the emulsion should contain an equal number of different-sized crystals to accurately reproduce the different tonal values or contrast. Despite great efforts, however, this requirement is technically impossible to fulfill in practice because crystal formation proceeds according to partly chaotic rules. For this reason, a typical bell-shaped frequency distribution (histogram) results between large and small crystals, whose peak lies in a medium size. It is the drop in frequency to either side that gives the characteristic curve its distinctive areas of sag and shoulder. This is because the flattening of the histogram means that the crystal sizes are more evenly distributed, and this results in a lower contrast in each case, or a nonlinear relationship between input tone value and output density. Because manufacturers invest a lot of time and effort, and therefore money, in working out the manufacturing process of their emulsions, they keep the exact procedures strictly secret.

Type and Duration of Development: After exposing the silver halide crystals, the developer liquid transforms the latent image into a visible and permanent image. It does this by reducing the silver halide crystals to elemental silver, which, due to its black color, transforms the previously small density differences into clearly perceptible differences. The developer begins this work with the exposed crystals. If we gave him enough time, he would reduce all silver halide crystals to elemental silver. Then there would be no more density differences and consequently no image. Therefore, the duration of the development process greatly influences the shape of the characteristic curve of films and paper, which in turn affects their contrast behavior. It is responsible for shaping the sag and shoulder and determining the slope of the linear part in between.

The size and distribution of the silver halide crystals in an AgX image carrier’s emulsion, as well as the method or duration of its development, essentially dictate the brightness differences it can record.

Figure 30 illustrates these relationships. At the shortest development time (T1), the sag on the lower left foot is already present. It arises due to the size dispersion of the silver halide crystals present in the layer. Large crystals are more sensitive to light than small ones and are more likely to form developable nuclei when exposed to light. At the lower end of the characteristic curve, the exposure is weak, resulting in an uneven number of crystals capturing the theoretical minimum amount of four photons required for the formation of a development nucleus. This results in a nonlinear relationship between the incident intensity and the resulting density, which becomes more pronounced the longer the development takes. With longer development, increased temperature or concentration, you can push the sag to lower exposures, but you will increase the fog (the equivalent of electronic noise).

The shoulder at the upper right end does not form until T3, at which point the linear portion of the curve steepens significantly. Here we are dealing with strong exposure and many development nuclei, which the developer prefers to reduce to pure silver. Only when he has finished with them does he also turn his attention to the non-exposed crystals, thus ensuring maximum density. However, because we stop the development process after a relatively short time, we observe a non-linear relationship between the reduction of exposed crystals to silver and the reduction of non-exposed crystals. Furthermore, the size dispersion of the silver halide crystals also plays a part in this process.

When we extend the development time again, as shown in T5, the two continue to converge and the base density significantly increases. At full development, the curve would become a straight line extending from Dmax through all exposure levels. The process would then completely blacken the film.

Manufacturers choose a development time for their materials processed according to the standard process that forms both sag and shoulder and results in a linear section that is as long as possible. In summary, the relationship can be expressed as follows: Longer development time = greater contrast. The reduction of the silver halide crystals produces more silver over a longer development time, resulting in a higher maximum density and a steeper characteristic curve. However, we can only apply the change in development time in the B/W area, as we are dealing with a single light-sensitive layer there. The development of color negatives allows changes in the development time only within very narrow limits because, in contrast to black-and-white film, we are dealing with more than one light-sensitive layer, and these at least three layers for the subtractive primary colors cyan, yellow and magenta react very irritably to such deviations. The outcome would be virtually unfilterable color casts. Therefore, we must rely on the three color density curves provided by the manufacturer, which are produced through the standard process. Photographic paper is always fully developed because its contrast behavior is a property specified by the manufacturer, and only in this way can the greatest possible density be achieved.

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