A stop bath is a chemical solution used in photographic processing to halt the action of the developer on film or photographic paper. After an image has been exposed to a developer, immersing it in a stop bath neutralizes any remaining developer, preventing further chemical reaction and ensuring that development stops precisely at the desired point.
Using a stop bath also helps prevent the carry-over of developer into subsequent chemical baths, such as the fixer, which can otherwise alter the effectiveness of those solutions. Stop baths are typically acidic, and their use improves consistency, tonal control, and overall image quality. In both black-and-white and colour processing, a stop bath is an essential step for achieving reliable and predictable results, giving photographers better control over exposure, contrast, and detail in the final image.