Belitski's reducer is a chemical solution used to reduce the density of photographic negatives. It typically contains ferric potassium citrate or ferric oxalate dissolved in an acidic fixing solution, which selectively lightens overexposed or dense areas of a negative without affecting the rest of the image. Photographers and darkroom professionals use Belitski's reducer to correct exposure imbalances and refine tonal range, making it a valuable tool in fine art and traditional film photography for achieving more precise and balanced prints.