Contrast Index, often abbreviated as CI, is a measure of the contrast present in a photographic negative. It is used in the darkroom to determine the correct grade of photographic paper for printing, especially when using a diffuser enlarger. By evaluating the CI, photographers can ensure that a negative with a full tonal range-from deep shadows to bright highlights-produces a balanced print with optimal contrast and tonal separation. Understanding and applying the contrast index is essential for precise darkroom printing, allowing photographers to select the right paper grade and achieve consistent, high-quality results in black-and-white photography.