A straight black border on a film is a result of what processing error?

The presence of a straight black border on a dental radiographic film is indicative of a processing error known as fixer cutoff. This occurs when the film is not fully immersed in the fixer solution during the processing phase. The fixer is essential for removing unexposed and undeveloped silver halide crystals from the film. If the film is partially submerged in the fixer, the portion not in contact with the solution remains unaffected, resulting in that area retaining the unexposed black appearance once the film is fully developed. This leads to a distinct straight edge on the film where the fixer did not adequately cover. It's crucial during the developing process to ensure that the entire film is properly processed to avoid such artifacts.