Greater edge sharpness through „sharp development“

You are here: Nature Science Photography – Visual acuity – Image sharpness III

Black-and-white films must be developed exactly for a time specified by the manufacturer and with a method that is also prescribed in order to achieve a blackening that is defined as „correct“. Only under these conditions can we ensure the reduction of a portion of the silver halide crystals to silver in accordance with the specifications. Deviation from these conditions will lead to a change in the result. For instance, reducing or eliminating movement during the development phase can result in the edge effect or neighborhood effect, also referred to as the Eberhard effect or Kostinsky effect. The analog darkroom has long used this technique to develop film to maximum edge sharpness. In many cases, this can produce results that are equal to the next larger format in terms of sharpness.

The approach leverages the fact that a more exposed part of the film, rather than a less exposed one, consumes the developer when there is no movement or insufficient movement. Thus, more and more developer migrates across this cutoff point and promotes development on the already more exposed side. Conversely, its spent components migrate across the boundary in the opposite direction, slowing down development on the previously exposed side. This process results in the creation of bright fringes surrounding dark objects and, concurrently, the creation of dark fringes around bright objects, also known as Mackie lines. The resulting increase in contrast around the edges is visually perceived as improved sharpness. However, the uneven development caused by the lack of movement can lead to cloudiness in large-area subject parts. In addition, the overly strong promotion of the edge effect can negatively impact tonal reproduction, often resulting in a sharp but enlarged grain.

In principle, the edge effect occurs with almost all film/developer combinations, but Agfa Rodinal is particularly suitable for achieving it because of the p-aminophenol it contains. Many users claim that this developer virtually eliminates the need for agitation during the process. The optimum dilution factor, however, depends strongly on the magnification scale aimed at. And the subject also plays a role. An approach that produces visually sharp portraits may not be appropriate for high-contrast nature shots. Therefore, conducting tests based on these measures is crucial. However, Rodinal leads to greater graininess.

In color film, there is the related interimage effect (also called Vertical Eberhard Effect). In this case, DIR couplers stop the formation of more silver around areas of the film that are heavily exposed during development. They do this by balancing the off-color densities by letting two adjacent film layers interact with each other. In this way, color saturation is increased without compromising overall contrast and exposure latitude. By the nature of DIR couplers, the edge effect can be customized from film to film.

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