Factors affecting the degree of development

The density (blackness) achieved in a developed image depends on the type of emulsion, the exposure it received, and the degree of development.
For each individual emulsion, the degree of development depends on the development time, the temperature of the developer, the degree of agitation of the medium or developer solution, and the chemical activity of the developer, which is determined by the composition and dilution of the solution and the degree of depletion.

Time of development

When the exposed material is placed in the developer, the solution penetrates the emulsion and begins to reduce the exposed crystals to metallic silver. The longer development continues, the more silver forms and the denser the image becomes. The difference in density between light and dark areas also increases; this difference is called the density range.
For negative materials, the degree of development is measured by the average slope of a portion of the material characteristic curve. This value is called the “contrast index.”

If development is pushed too far, the density may become too high, and the developer may begin to act on the unexposed silver bromide crystals; this then causes a “developer haze” that tends to obscure the details in the shadows.

Development should therefore be stopped when the desired contrast index is reached. Since the desired degree of development may vary somewhat in different cases, contrast index curves are usually given in the film specifications so that the proper development time can be selected for the particular circumstances of film use.

Temperature of the developer solution

The speed of development is affected by the temperature of the solution:
As the temperature increases, the speed of development increases.

• Thus, at (too) low a developer temperature, the reaction is slow and the development time recommended for the normal temperature would result in underdevelopment
• At high temperature, the reaction is fast and at the same time would lead to overdevelopment.

Within certain limits, these changes in the rate of development can be compensated by increasing or decreasing the development time.
In addition to these changes, there may be other temperature-dependent effects:
At high temperatures, the gelatin of the emulsion swells and becomes so sensitive that it is easily damaged and can even detach from the substrate or fold into a fine line network, which is called retoculation. In particular, this danger exists when a high-temperature solution is quickly followed by a low-temperature solution. Therefore, the temperatures of the developer and other solutions (including the water for washing) should be kept sufficiently similar (plus/minus 5 C) low to avoid the risk of damage.

Agitation during development

When exposed photographic materials are placed in a developer solution and allowed to develop without any agitation, the process soon slows down because the developing power of the solution in the emulsion and in contact with its surface is exhausted. However, if the material is agitated, new solution components are constantly brought to the emulsion surface, and the rate of development remains constant. Therefore, agitation has an important effect on the degree of development achieved.

Another more important effect of agitation is the prevention of uneven development or spotting. If agitation does not occur, the exhausted solution, loaded with by-products of development, can flow slowly from the dense areas across the emulsion and create uneven streaks. Agitation keeps the solution uniform throughout and prevents uneven development.

Since agitation affects the rate of development, specifying a development time would mean little if agitation is not included. (See tilting, stagnant development, etc.).

Activity of the developer solution

The speed of development is also influenced by the chemical activity of the developer solution. This depends on its composition and primarily on the type and concentration of the developing agent and the effective activity of the solution.
For example, Kodak developer D-76 with its mild alkali, borax, has a slower development speed than developer D-11, which contains sodium carbonate. In addition, the activity of the developer is affected by any dilution of the solution.
In addition, the depletion of the developer affects the activity of the solution. When a developer is used, its developing power decreases, partly because of the oxidation of the developer when the exposed silver halide is reduced to metallic silver, but mainly because of the inhibiting effect of the accumulated reaction products of development. Even if the developer is not used, its activity may decrease due to oxidation of the developer from the air.