Therefore, a good lab adheres closely to the exact processing specifications for the particular equipment and materials. If exposed photographic materials are placed in a developer and allowed to develop without movement, the action slows down because the developing chemicals in contact with the film surface become exhausted. If the film or the solution is agitated, however, fresh solution is continually brought to the emulsion surface, and the development continues. An equally important effect of agitation is prevention of uneven development that may result in mottle, a nonuniform density in the print that makes it look blotchy. If there is no agitation, the exhausted solution, loaded with development by- products, may flow slowly across the emulsion from dense areas to less dense areas and produce uneven streaks.
Agitation keeps the solution uniform throughout and avoids uneven development.
In color processing, proper agitation is especially critical during the initial development step. The recommended agitation techniques will vary, depending upon the process and equipment being used. The film movement, as it passes through the developer solution is not always sufficient to create adequate agitation. If film is to be processed satisfactorily as it moves through the machine, it must be immersed in solutions of the correct temperature for the proper length of time. In addition, processing solutions must be adequately replenished and filtered, and sufficiendy agitated.